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The first plants to colonize land were most likely closely related to modern-day mosses (bryophytes) and are thought to have appeared about 500 million years ago. They were followed by liverworts (also bryophytes) and primitive vascular plants, the pterophytes, from which modern ferns are derived. The life cycle of bryophytes and pterophytes is characterized by the alternation of generations. The completion of the life cycle requires water, as the male gametes must swim to the female gametes. The male gametophyte releases sperm, which must swim—propelled by their flagella—to reach and fertilize the female gamete or egg. After fertilization, the zygote matures and grows into a sporophyte, which in turn will form sporangia, or "spore vessels,” in which mother cells undergo meiosis and produce haploid spores. The release of spores in a suitable environment will lead to germination and a new generation of gametophytes.
The Evolution of Seed Plants
In seed plants, the evolutionary trend led to a dominant sporophyte generation, in which the larger and more ecologically significant generation for a species is the diploid plant. At the same time, the trend led to a reduction in the size of the gametophyte, from a conspicuous structure to a microscopic cluster of cells enclosed in the tissues of the sporophyte. Lower vascular plants, such as club mosses and ferns, are mostly homosporous (produce only one type of spore). In contrast, all seed plants, or spermatophytes, are heterosporous, forming two types of spores: megaspores (female) and microspores (male). Megaspores develop into female gametophytes that produce eggs, and microspores mature into male gametophytes that generate sperm. Because the gametophytes mature within the spores, they are not free-living, as are the gametophytes of other seedless vascular plants. Heterosporous seedless plants are seen as the evolutionary forerunners of seed plants.
Seeds and pollen—two adaptations to drought—distinguish seed plants from other (seedless) vascular plants. Both adaptations were critical to the colonization of land. Fossils place the earliest distinct seed plants at about 350 million years ago. The earliest reliable record of gymnosperms dates their appearance to the Carboniferous period (359–299 million years ago). Gymnosperms were preceded by the progymnosperms (“first naked seed plants”). This was a transitional group of plants that superficially resembled conifers (“cone bearers”) because they produced wood from the secondary growth of the vascular tissues; however, they still reproduced like ferns, releasing spores to the environment. In the Mesozoic era (251–65.5 million years ago), gymnosperms dominated the landscape. Angiosperms took over by the middle of the Cretaceous period (145.5–65.5 million years ago) in the late Mesozoic era, and have since become the most abundant plant group in most terrestrial biomes.
The two innovative structures of pollen and seed allowed seed plants to break their dependence on water for reproduction and development of the embryo, and to conquer dry land. The pollen grains carry the male gametes of the plant. The small haploid (1n) cells are encased in a protective coat that prevents desiccation (drying out) and mechanical damage. Pollen can travel far from the sporophyte that bore it, spreading the plant’s genes and avoiding competition with other plants. The seed offers the embryo protection, nourishment and a mechanism to maintain dormancy for tens or even thousands of years, allowing it to survive in a harsh environment and ensuring germination when growth conditions are optimal. Seeds allow plants to disperse the next generation through both space and time. With such evolutionary advantages, seed plants have become the most successful and familiar group of plants.
Gymnosperms (“naked seed”) are a diverse group of seed plants and are paraphyletic. Paraphyletic groups do not include descendants of a single common ancestor. Gymnosperm characteristics include naked seeds, separate female and male gametes, pollination by wind, and tracheids, which transport water and solutes in the vascular system.
Life Cycle of a Conifer
Pine trees are conifers and carry both male and female sporophylls on the same plant. Like all gymnosperms, pines are heterosporous and produce male microspores and female megaspores. In the male cones, or staminate cones, the microsporocytes give rise to microspores by meiosis. The microspores then develop into pollen grains. Each pollen grain contains two cells: one generative cell that will divide into two sperm, and a second cell that will become the pollen tube cell. In the spring, pine trees release large amounts of yellow pollen, which is carried by the wind. Some gametophytes will land on a female cone. The pollen tube grows from the pollen grain slowly, and the generative cell in the pollen grain divides into two sperm cells by mitosis. One of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of an egg cell in the process of fertilization.
Female cones, or ovulate cones, contain two ovules per scale. One megasporocyteundergoes meiosis in each ovule. Only a single surviving haploid cell will develop into a female multicellular gametophyte that encloses an egg. On fertilization, the zygote will give rise to the embryo, which is enclosed in a seed coat of tissue from the parent plant. Fertilization and seed development is a long process in pine trees—it may take up to two years after pollination. The seed that is formed contains three generations of tissues: the seed coat that originates from the parent plant tissue, the female gametophyte that will provide nutrients, and the embryo itself. Figure 14.3.1 illustrates the life cycle of a conifer.
Figure 14.3.1: This image shows the lifecycle of a conifer.
At what stage does the diploid zygote form?
- when the female cone begins to bud from the tree
- when the sperm nucleus and the egg nucleus fuse
- when the seeds drop from the tree
- when the pollen tube begins to grow
CONCEPT IN ACTION
Watch this video to see the process of seed production in gymnosperms.
Diversity of Gymnosperms
Modern gymnosperms are classified into four major divisions and comprise about 1,000 described species. Coniferophyta, Cycadophyta, and Ginkgophyta are similar in their production of secondary cambium (cells that generate the vascular system of the trunk or stem) and their pattern of seed development, but are not closely related phylogenetically to each other. Gnetophyta are considered the closest group to angiosperms because they produce true xylem tissue that contains both tracheids and vessel elements.
Conifers are the dominant phylum of gymnosperms, with the most variety of species. Most are tall trees that usually bear scale-like or needle-like leaves. The thin shape of the needles and their waxy cuticle limits water loss through transpiration. Snow slides easily off needle-shaped leaves, keeping the load light and decreasing breaking of branches. These adaptations to cold and dry weather explain the predominance of conifers at high altitudes and in cold climates. Conifers include familiar evergreen trees, such as pines, spruces, firs, cedars, sequoias, and yews (Figure 14.3.2). A few species are deciduous and lose their leaves all at once in fall. The European larch and the tamarack are examples of deciduous conifers. Many coniferous trees are harvested for paper pulp and timber. The wood of conifers is more primitive than the wood of angiosperms; it contains tracheids, but no vessel elements, and is referred to as “soft wood.”
Cycads thrive in mild climates and are often mistaken for palms because of the shape of their large, compound leaves. They bear large cones, and unusually for gymnosperms, may be pollinated by beetles, rather than wind. They dominated the landscape during the age of dinosaurs in the Mesozoic era (251–65.5 million years ago). Only a hundred or so cycad species persisted to modern times. They face possible extinction, and several species are protected through international conventions. Because of their attractive shape, they are often used as ornamental plants in gardens (Figure 14.3.3).
Figure 14.3.3: This Encephalartos ferox cycad exhibits large cones. (credit: Wendy Cutler)
The single surviving species of ginkgophyte is the Ginkgo biloba (Figure 14.3.4). Its fan-shaped leaves, unique among seed plants because they feature a dichotomous venation pattern, turn yellow in autumn and fall from the plant. For centuries, Buddhist monks cultivated Ginkgo biloba, ensuring its preservation. It is planted in public spaces because it is unusually resistant to pollution. Male and female organs are found on separate plants. Usually, only male trees are planted by gardeners because the seeds produced by the female plant have an off-putting smell of rancid butter.
Gnetophytes are the closest relatives to modern angiosperms, and include three dissimilar genera of plants. Like angiosperms, they have broad leaves. Gnetum species are mostly vines in tropical and subtropical zones. The single species of Welwitschia is an unusual, low-growing plant found in the deserts of Namibia and Angola. It may live for up to 2000 years. The genus Ephedra is represented in North America in dry areas of the southwestern United States and Mexico (Figure 13.4.5). Ephedra’s small, scale-like leaves are the source of the compound ephedrine, which is used in medicine as a potent decongestant. Because ephedrine is similar to amphetamines, both in chemical structure and neurological effects, its use is restricted to prescription drugs. Like angiosperms, but unlike other gymnosperms, all gnetophytes possess vessel elements in their xylem.
CONCEPT IN ACTION
Watch this BBC video describing the amazing strangeness of Welwitschia.
Gymnosperms are heterosporous seed plants that produce naked seeds. They appeared in the Carboniferous period (359–299 million years ago) and were the dominant plant life during the Mesozoic era (251–65.5 million years ago). Modern-day gymnosperms belong to four divisions. The division Coniferophyta—the conifers—are the predominant woody plants at high altitudes and latitudes. Cycads resemble palm trees and grow in tropical climates. Gingko biloba is the only species of the division Gingkophyta. The last division, the Gnetophytes, is a diverse group of species that produce vessel elements in their wood.
Figure 14.3.1 At what stage does the diploid zygote form?
A. When the female cone begins to bud from the tree
B. When the sperm nucleus and the egg nucleus fuse
C. When the seeds drop from the tree
D. When the pollen tube begins to grow
B. The diploid zygote forms after the pollen tube has finished forming so that the male generative nucleus (sperm) can fuse with the female egg.
Which of the following traits characterizes gymnosperms?
A. The plants carry exposed seeds on modified leaves.
B. Reproductive structures are located in a flower.
C. After fertilization, the ovary thickens and forms a fruit.
D. The gametophyte is longest phase of the life cycle.
What adaptation do seed plants have in addition to the seed that is not found in seedless plants?
B. vascular tissue
What are the four modern-day groups of gymnosperms?
The four modern-day groups of gymnosperms are Coniferophyta, Cycadophyta, Gingkophyta, and Gnetophyta.
- the ovulate strobilus on gymnosperms that contains ovules
- the dominant division of gymnosperms with the most variety of species
- a division of gymnosperms that grow in tropical climates and resemble palm trees
- a division of gymnosperm with one living species, the Gingko biloba, a tree with fan-shaped leaves
- a division of gymnosperms with varied morphological features that produce vessel elements in their woody tissues
- a seed plant with naked seeds (seeds exposed on modified leaves or in cones)
- a megaspore mother cell; larger spore that germinates into a female gametophyte in a heterosporous plant
- smaller spore that produces a male gametophyte in a heterosporous plant
13.3: Seed Plants - Gymnosperms - Biology
By the end of this section, you will be able to do the following:
- Describe the two major innovations that allowed seed plants to reproduce in the absence of water
- Explain when seed plants first appeared and when gymnosperms became the dominant plant group
- Discuss the purpose of pollen grains and seeds
- Describe the significance of angiosperms bearing both flowers and fruit
The first plants to colonize land were most likely related to the ancestors of modern day mosses (bryophytes), which are thought to have appeared about 500 million years ago. They were followed by liverworts (also bryophytes) and primitive vascular plants—the pterophytes—from which modern ferns are descended. The life cycle of bryophytes and pterophytes is characterized by the alternation of generations, which is also exhibited in the gymnosperms and angiosperms. However, what sets bryophytes and pterophytes apart from gymnosperms and angiosperms is their reproductive requirement for water. The completion of the bryophyte and pterophyte life cycle requires water because the male gametophyte releases flagellated sperm, which must swim to reach and fertilize the female gamete or egg. After fertilization, the zygote undergoes cellular division and grows into a diploid sporophyte, which in turn will form sporangia or “spore vessels.” In the sporangia, mother cells undergo meiosis and produce the haploid spores. Release of spores in a suitable environment will lead to germination and a new generation of gametophytes.
In seed plants, the evolutionary trend led to a dominant sporophyte generation accompanied by a corresponding reduction in the size of the gametophyte from a conspicuous structure to a microscopic cluster of cells enclosed in the tissues of the sporophyte. Whereas lower vascular plants, such as club mosses and ferns, are mostly homosporous (producing only one type of spore), all seed plants, or spermatophytes, are heterosporous, producing two types of spores: megaspores (female) and microspores (male). Megaspores develop into female gametophytes that produce eggs, and microspores mature into male gametophytes that generate sperm. Because the gametophytes mature within the spores, they are not free-living, as are the gametophytes of other seedless vascular plants.
Ancestral heterosporous seedless plants, represented by modern-day plants such as the spike moss Selaginella, are seen as the evolutionary forerunners of seed plants. In the life cycle of Selaginella, both male and female sporangia develop within the same stem-like strobilus. In each male sporangium, multiple microspores are produced by meiosis. Each microspore produces a small antheridium contained within a spore case. As it develops it is released from the strobilus, and a number of flagellated sperm are produced that then leave the spore case. In the female sporangium, a single megaspore mother cell undergoes meiosis to produce four megaspores. Gametophytes develop within each megaspore, consisting of a mass of tissue that will later nourish the embryo and a few archegonia. The female gametophyte may remain within remnants of the spore wall in the megasporangium until after fertilization has occurred and the embryo begins to develop. This combination of an embryo and nutritional cells is a little different from the organization of a seed, since the nutritive endosperm in a seed is formed from a single cell rather than multiple cells.
Both seeds and pollen distinguish seed plants from seedless vascular plants. These innovative structures allowed seed plants to reduce or eliminate their dependence on water for gamete fertilization and development of the embryo, and to conquer dry land. Pollen grains are male gametophytes, which contain the sperm (gametes) of the plant. The small haploid (1n) cells are encased in a protective coat that prevents desiccation (drying out) and mechanical damage. Pollen grains can travel far from their original sporophyte, spreading the plant’s genes. Seeds offer the embryo protection, nourishment, and a mechanism to maintain dormancy for tens or even thousands of years, ensuring that germination can occur when growth conditions are optimal. Seeds therefore allow plants to disperse the next generation through both space and time. With such evolutionary advantages, seed plants have become the most successful and familiar group of plants.
Both adaptations expanded the colonization of land begun by the bryophytes and their ancestors. Fossils place the earliest distinct seed plants at about 350 million years ago. The first reliable record of gymnosperms dates their appearance to the Pennsylvanian period, about 319 million years ago ((Figure)). Gymnosperms were preceded by progymnosperms, the first naked seed plants, which arose about 380 million years ago. Progymnosperms were a transitional group of plants that superficially resembled conifers (cone bearers) because they produced wood from the secondary growth of the vascular tissues however, they still reproduced like ferns, releasing spores into the environment. At least some species were heterosporous. Progymnosperms, like the extinct Archaeopteris (not to be confused with the ancient bird Archaeopteryx), dominated the forests of the late Devonian period. However, by the early (Triassic, c. 240 MYA) and middle (Jurassic, c. 205 MYA) Mesozoic era, the landscape was dominated by the true gymnosperms. Angiosperms surpassed gymnosperms by the middle of the Cretaceous (c. 100 MYA) in the late Mesozoic era, and today are the most abundant and biologically diverse plant group in most terrestrial biomes.
Figure 1. Plant timeline. Various plant species evolved in different eras. (credit: United States Geological Survey) Figure modified from source.
Evolution of Gymnosperms
The fossil plant Elkinsia polymorpha, a “seed fern” from the Devonian period—about 400 million years ago—is considered the earliest seed plant known to date. Seed ferns ((Figure)) produced their seeds along their branches, in structures called cupules that enclosed and protected the ovule—the female gametophyte and associated tissues—which develops into a seed upon fertilization. Seed plants resembling modern tree ferns became more numerous and diverse in the coal swamps of the Carboniferous period.
Figure 2. Seed fern leaf. This fossilized leaf is from Glossopteris, a seed fern that thrived during the Permian age (290–240 million years ago). (credit: D.L. Schmidt, USGS)
Fossil records indicate the first gymnosperms (progymnosperms) most likely originated in the Paleozoic era, during the middle Devonian period: about 390 million years ago. The previous Mississippian and Pennsylvanian periods, were wet and dominated by giant fern trees. But the following Permian period was dry, which gave a reproductive edge to seed plants, which are better adapted to survive dry spells. The Ginkgoales, a group of gymnosperms with only one surviving species—the Ginkgo biloba—were the first gymnosperms to appear during the lower Jurassic. Gymnosperms expanded in the Mesozoic era (about 240 million years ago), supplanting ferns in the landscape, and reaching their greatest diversity during this time. The Jurassic period was as much the age of the cycads (palm-tree-like gymnosperms) as the age of the dinosaurs. Ginkgoales and the more familiar conifers also dotted the landscape. Although angiosperms (flowering plants) are the major form of plant life in most biomes, gymnosperms still dominate some ecosystems, such as the taiga (boreal forests) and the alpine forests at higher mountain elevations ((Figure)) because of their adaptation to cold and dry growth conditions.
Figure 3. Conifers. This boreal forest (taiga) has low-lying plants and conifer trees. (credit: L.B. Brubaker, NOAA)
Seeds and Pollen as an Evolutionary Adaptation to Dry Land
Bryophyte and fern spores are haploid cells dependent on moisture for rapid development of multicellular gametophytes. In the seed plants, the female gametophyte consists of just a few cells: the egg and some supportive cells, including the endosperm-producing cell that will support the growth of the embryo. After fertilization of the egg, the diploid zygote produces an embryo that will grow into the sporophyte when the seed germinates. Storage tissue to sustain growth of the embryo and a protective coat give seeds their superior evolutionary advantage. Several layers of hardened tissue prevent desiccation, and free the embryo from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy—induced by desiccation and the hormone abscisic acid—until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.
Pollen grains ((Figure)) are male gametophytes containing just a few cells and are distributed by wind, water, or an animal pollinator. The whole structure is protected from desiccation and can reach the female organs without depending on water. After reaching a female gametophyte, the pollen grain grows a tube that will deliver a male nucleus to the egg cell. The sperm of modern gymnosperms and all angiosperms lack flagella, but in cycads, Ginkgo, and other primitive gymnosperms, the sperm are still motile, and use flagella to swim to the female gamete however, they are delivered to the female gametophyte enclosed in a pollen grain. The pollen grows or is taken into a fertilization chamber, where the motile sperm are released and swim a short distance to an egg.
Figure 4. Pollen fossils. This fossilized pollen is from a Buckbean fen core found in Yellowstone National Park, Wyoming. The pollen is magnified 1,054 times. (credit: R.G. Baker, USGS scale-bar data from Matt Russell)
Evolution of Angiosperms
The roughly 200 million years between the appearance of the gymnosperms and the flowering plants gives us some appreciation for the evolutionary experimentation that ultimately produced flowers and fruit. Angiosperms (“seed in a vessel”) produce a flower containing male and/or female reproductive structures. Fossil evidence ((Figure)) indicates that flowering plants first appeared about 125 million years ago in the Lower Cretaceous (late in the Mesozoic era), and were rapidly diversifying by about 100 million years ago in the Middle Cretaceous. Earlier traces of angiosperms are scarce. Fossilized pollen recovered from Jurassic geological material has been attributed to angiosperms. A few early Cretaceous rocks show clear imprints of leaves resembling angiosperm leaves. By the mid-Cretaceous, a staggering number of diverse flowering plants crowd the fossil record. The same geological period is also marked by the appearance of many modern groups of insects, suggesting that pollinating insects played a key role in the evolution of flowering plants.
New data in comparative genomics and paleobotany (the study of ancient plants) have shed some light on the evolution of angiosperms. Although the angiosperms appeared after the gymnosperms, they are probably not derived from gymnosperm ancestors. Instead, the angiosperms form a sister clade (a species and its descendents) that developed in parallel with the gymnosperms. The two innovative structures of flowers and fruit represent an improved reproductive strategy that served to protect the embryo, while increasing genetic variability and range. There is no current consensus on the origin of the angiosperms. Paleobotanists debate whether angiosperms evolved from small woody bushes, or were related to the ancestors of tropical grasses. Both views draw support from cladistics, and the so-called woody magnoliid hypothesis—which proposes that the early ancestors of angiosperms were shrubs like modern magnolia—also offers molecular biological evidence.
The most primitive living angiosperm is considered to be Amborella trichopoda, a small plant native to the rainforest of New Caledonia, an island in the South Pacific. Analysis of the genome of A. trichopoda has shown that it is related to all existing flowering plants and belongs to the oldest confirmed branch of the angiosperm family tree. The nuclear genome shows evidence of an ancient whole-genome duplication. The mitochondrial genome is large and multichromosomal, containing elements from the mitochondrial genomes of several other species, including algae and a moss. A few other angiosperm groups, called basal angiosperms, are viewed as having ancestral traits because they branched off early from the phylogenetic tree. Most modern angiosperms are classified as either monocots or eudicots, based on the structure of their leaves and embryos. Basal angiosperms, such as water lilies, are considered more ancestral in nature because they share morphological traits with both monocots and eudicots.
Figure 5. Ficus imprint. This leaf imprint shows a Ficus speciosissima, an angiosperm that flourished during the Cretaceous period. (credit: W. T. Lee, USGS)
Flowers and Fruits as an Evolutionary Adaptation
Angiosperms produce their gametes in separate organs, which are usually housed in a flower. Both fertilization and embryo development take place inside an anatomical structure that provides a stable system of sexual reproduction largely sheltered from environmental fluctuations. With about 300,000 species, flowering plants are the most diverse phylum on Earth after insects, which number about 1,200,000 species. Flowers come in a bewildering array of sizes, shapes, colors, smells, and arrangements. Most flowers have a mutualistic pollinator, with the distinctive features of flowers reflecting the nature of the pollination agent. The relationship between pollinator and flower characteristics is one of the great examples of coevolution.
Following fertilization of the egg, the ovule grows into a seed. The surrounding tissues of the ovary thicken, developing into a fruit that will protect the seed and often ensure its dispersal over a wide geographic range. Not all fruits develop completely from an ovary such “false fruits” or pseudocarps, develop from tissues adjacent to the ovary. Like flowers, fruit can vary tremendously in appearance, size, smell, and taste. Tomatoes, green peppers, corn, and avocados are all examples of fruits. Along with pollen and seeds, fruits also act as agents of dispersal. Some may be carried away by the wind. Many attract animals that will eat the fruit and pass the seeds through their digestive systems, then deposit the seeds in another location. Cockleburs are covered with stiff, hooked spines that can hook into fur (or clothing) and hitch a ride on an animal for long distances. The cockleburs that clung to the velvet trousers of an enterprising Swiss hiker, George de Mestral, inspired his invention of the loop and hook fastener he named Velcro.
Building Phylogenetic Trees with Analysis of DNA Sequence Alignments
All living organisms display patterns of relationships derived from their evolutionary history. Phylogeny is the science that describes the relative connections between organisms, in terms of ancestral and descendant species. Phylogenetic trees, such as the plant evolutionary history shown in (Figure), are tree-like branching diagrams that depict these relationships. Species are found at the tips of the branches. Each branching point, called a node, is the point at which a single taxonomic group (taxon), such as a species, separates into two or more species.
Figure 6. Plant phylogeny. This phylogenetic tree shows the evolutionary relationships of plants.
Phylogenetic trees have been built to describe the relationships between species since the first sketch of a tree that appeared in Darwin’s Origin of Species. Traditional methods involve comparison of homologous anatomical structures and embryonic development, assuming that closely related organisms share anatomical features that emerge during embryo development. Some traits that disappear in the adult are present in the embryo for example, an early human embryo has a postanal tail, as do all members of the Phylum Chordata. The study of fossil records shows the intermediate stages that link an ancestral form to its descendants. However, many of the approaches to classification based on the fossil record alone are imprecise and lend themselves to multiple interpretations. As the tools of molecular biology and computational analysis have been developed and perfected in recent years, a new generation of tree-building methods has taken shape. The key assumption is that genes for essential proteins or RNA structures, such as the ribosomal RNAs, are inherently conserved because mutations (changes in the DNA sequence) could possibly compromise the survival of the organism. DNA from minute samples of living organisms or fossils can be amplified by polymerase chain reaction (PCR) and sequenced, targeting the regions of the genome that are most likely to be conserved between species. The genes encoding the 18S ribosomal RNA from the small subunit and plastid genes are frequently chosen for DNA alignment analysis.
Once the sequences of interest are obtained, they are compared with existing sequences in databases such as GenBank, which is maintained by The National Center for Biotechnology Information. A number of computational tools are available to align and analyze sequences. Sophisticated computer analysis programs determine the percentage of sequence identity or homology. Sequence homology can be used to estimate the evolutionary distance between two DNA sequences and reflect the time elapsed since the genes separated from a common ancestor. Molecular analysis has revolutionized phylogenetic trees. In some cases, prior results from morphological studies have been confirmed: for example, confirming Amborella trichopoda as the most primitive angiosperm known. However, some groups and relationships have been rearranged as a result of DNA analysis.
Seed plants appeared about one million years ago, during the Carboniferous period. Two major innovations were seeds and pollen. Seeds protect the embryo from desiccation and provide it with a store of nutrients to support the early growth of the sporophyte. Seeds are also equipped to delay germination until growth conditions are optimal. Pollen allows seed plants to reproduce in the absence of water. The gametophytes of seed plants shrank, while the sporophytes became prominent structures and the diploid stage became the longest phase of the life cycle.
In the gymnosperms, which appeared during the drier Permian period and became the dominant group during the Triassic, pollen was dispersed by wind, and their naked seeds developed in the sporophylls of a strobilus. Angiosperms bear both flowers and fruit. Flowers expand the possibilities for pollination, especially by insects, who have coevolved with the flowering plants. Fruits offer additional protection to the embryo during its development, and also assist with seed dispersal. Angiosperms appeared during the Mesozoic era and have become the dominant plant life in terrestrial habitats.
Introduction to the Spermatophytes
The spermatophytes, which means "seed plants", are some of the most important organisms on Earth. Life on land as we know it is shaped largely by the activities of seed plants. Soils, forests, and food are three of the most apparent products of this group.
Seed-producing plants are probably the most familiar plants to most people, unlike mosses, liverworts, horsetails, and most other seedless plants which are overlooked because of their size or inconspicuous appearance. Many seedplants are large or showy. Conifers are seed plants they include pines, firs, yew, redwood, and many other large trees. The other major group of seed-plants are the flowering plants, including plants whose flowers are showy, but also many plants with reduced flowers, such as the oaks, grasses, and palms.
Click on the buttons below to find out more about the Spermatophytes.
You can navigate deeper into the Spermatophyte groups by selecting Systematics!
For information about collections on plants cataloged on-line, or for images, checklists, and databases, try our list of Botanical Collection Catalogs.
13.3: Seed Plants - Gymnosperms - Biology
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The most successful major group - the seed plants - includes almost 400,000 species. Seed plants themselves are divided into two groups - the first being the gymnosperms, which are predominantly conifers, but also includes plants like cycads or ginkgos.
The second group is the angiosperms, which includes over 90 percent of known plant species, including magnoliids and other flowering trees, all fruits, and flowering plants.
As seed plants, both gymnosperms and angiosperms share several key features. First, their life cycles are dominated by the sporophyte stage. Second, they have microscopic gametophytes which form haploid gametes - including female gametes called ovules which are typically housed in structures like cones or ovaries to protect and nourish them.
The male counterpart gametes called pollen grains are formed in separate structures, and are easily dispersed by environmental methods like wind, or animals, to hopefully encounter and fertilize the female gametes.
Finally, once fertilized, these now diploid structures form seeds - structures which contain nourishment for the future seedling.
Here, the differences between the lineages begins - while gymnosperms&rsquo seeds are typically housed in cones or on scales which are open to the air or animals for dispersal, angiosperms may produce fruits surrounding or attached to their seeds.
Fruits contain one or more seeds, and facilitate seed dispersal. For example, animals may eat the fruit and then leave the area, before passing the seeds which often remain intact.
Alternatively, the fruits can help the seeds to float, fly, or hitch a ride on an animal to their final destinations.
Whatever the method of dispersal, when the seeds reach their germination spot, another classification distinguishes the different seed plant lineages - cotyledon number. Cotyledons are a part of the seed that forms the embryonic leaf or leaves upon germination.
Gymnosperms are referred to as multi-cotyledenous and will most typically have anywhere from 8 to 20 plus of these embryonic leaves, which may grow in a whorl shape around the embryonic stem.
Most angiosperms on the other hand have either one or two cotyledons, and are broadly categorized as monocots or eudicots based on this number and this difference is seen right from the initial seed.
Examples of eudicots include oaks and roses, while monocots include grasses, orchids, and corn.
Aside from cotyledons, several other features distinguish monocots from eudicots. For example, the arrangement of roots, which are typically tap root or fibrous root systems, or the vascular tissue in stems, which occurs in a ringed arrangement in eudicots and is scattered in monocots.
Additionally, leaf veins form a net-like structure in eudicots and a parallel arrangement in monocots and flower organ development occurs in segments of four to five in eudicots, versus threes in monocots.
Overall, these many and varied adaptations of seed plants have allowed them to become the dominant vegetation on Earth.
34.4: Introduction to Seed Plants
Most plants are seed plants&mdashcharacterized by seeds, pollen, and reduced gametophytes. Seed plants include gymnosperms and angiosperms.
Gymnosperms&mdashcycads, ginkgo biloba, gnetophytes, and conifers&mdashtypically form cones. The pollen cones contain male gametophytes. The ovulate cones contain female gametophytes and form exposed seeds when fertilized.
Angiosperms, the most diverse and ubiquitous group of land plants, form flowers, and fruit. Like the cones of gymnosperms, the flowers and fruit of angiosperms enable sexual reproduction.
Flowers facilitate pollen dispersal. The fertile flower structures&mdashstamens and carpels&mdashcontain male and female gametophytes, respectively. Fruits facilitate seed dispersal, often forming after flowers have released pollen. As seeds develop from a flower&rsquos fertilized ovules, the ovary wall thickens, forming a fruit containing seeds.
Angiosperms were historically categorized as monocots or dicots based on their number of cotyledons&mdashor seed leaves. However, based on genetic evidence, most species classically considered dicots are now called eudicots. Legumes (e.g., beans) and most well-known flowering trees (e.g., oaks) are eudicots.
The other former dicots belong to one of four small lineages. Three of these&mdashAmborella, water lilies, and star anise and its relatives&mdashare considered basal angiosperms due to their early divergence from ancestral angiosperms. The fourth group&mdashthe magnoliids&mdashcontains thousands of species, including magnolias.
Examples of monocots include orchids, grasses, palms, corn, rice, and wheat. Aside from cotyledon number, other characteristics distinguish monocots from eudicots. Leaf veins are typically parallel in monocots and netlike in eudicots. In stems, the vascular tissue is often scattered in monocots and ring-like in eudicots. Unlike eudicots, monocots generally lack a primary root. Pollen grains typically have one opening in monocots and three openings in eudicots. Finally, flower organs are often found in multiples of three in monocots and multiples of four or five in eudicots.
Coen, Olivier, and Enrico Magnani. 2018. &ldquoSeed Coat Thickness in the Evolution of Angiosperms.&rdquo Cellular and Molecular Life Sciences 75 (14): 2509&ndash18. [Source]
Linkies, Ada, Kai Graeber, Charles Knight, and Gerhard Leubner-Metzger. 2010. &ldquoThe Evolution of Seeds.&rdquo New Phytologist 186 (4): 817&ndash31. [Source]
6.4 View Prepared Slides of Angiosperms
- Lily anther (Figure 6.9)
- Identify: anther, microsporangium, pollen grains with tube and generative nuclei. The structure in the middle of the slide is the Lily ovary. The anthers are around the ovary.
- Lily anther with mature pollen (Figure 6.10)
- Identify: pollen grain, tube cell with nucleus, generative cell with nucleus
- Lily pollen tubes (Figure 6.11)
- Identify: sigma tissue, pollen tubes
- Lily ovary (Figure 6.12)
- Identify: ovary, ovules, female gametophytes (embryo sac). If this slides is not available, you can observe the lily ovary in the “Lily anther x.s.” slide.
- Tilia 2 year old stem (Figure 6.13)
- Capsella seeds (Figure 6.14)
- Identify: embryo, cotyledons, root tip, shoot tip
Figure 6.10: Lily anther with mature pollen.
Figure 6.11: Lily pollen tubes.
Figure 6.13: Tilia 2 year old stem
Figure 6.14: Capsella seeds.
13.3: Seed Plants - Gymnosperms - Biology
Mechel Golenberke: My First Website
Biology II &ndash Chapter 22 Plan: Introduction to Plants
22.1 What is a Plant & 22.2 Seedless Plants
Describe what plants need to survive
Describe how the first plants evolve
Explain the process of alternation of generation
Identify the characteristics of green algae
Describe the adaptations of bryophytes
Explain the importance of vascular tissue
answer questions on video &ndash move to lab table for answers
Labs: Comparing Algae & Plants Bryophytes & Ferns
Distinguish among unicellular, filamentous, and colonial forms of organization.
Locate and identify chloroplasts, holdfasts, and pyrenoids of various species of algae.
Examine an example of nonvascular plants: liverworts
Identify and study the structure and function of liverwort parts
Examine an example of nonvascular plants: mosses
Identify and study the structure and function of liverwort parts
Observe reproductive structures in a moss
Trace the life cycle of a moss
Demonstrate the absorptive ability of sphagnum moss
Examine an example of vascular plants: ferns
Identify and study the structure and function of fern parts
Explain the reproductive pattern of ferns
22.3 Seed Plants &ndash Gymnosperms
1. Biology Junction Gymnosperms & Angiosperms (NO QUESTION GUIDE) (124 slides) (1-41 Gymnosperms only)
answer questions on video &ndash move to lab table for answers
Describe the reproductive adaptations of seed plants
Identify the reproductive structures of gymnosperms
Gymnosperm Reproduction Lab
1. Reproductive Structures of Gymnosperms Lab
To describe the general features of gymnosperms
To understand the life cycles of gymnosperms
To identify the significant features of the life cycles for various gymnosperms and state the particular evolutionary importance
To be able to differentiate between representative organisms in each group: pine, cycad, ginkgo, Ephedra and Welwitschia.
22.4 Flowering Plants & Reproductive Structures of Angiosperms
1. Biology Junction Gymnosperms & Angiosperms (NO QUESTION GUIDE) (124 slides) (42-124 Angiosperms only)
Ethnobotanist The relatively new field of ethnobotany studies the interaction between a particular culture and the plants native to the region. Seed plants have a large influence on day-to-day human life. Not only are plants the major source of food and medicine, they also influence many other aspects of society, from clothing to industry. The medicinal properties of plants were recognized early on in human cultures. From the mid-1900s, synthetic chemicals began to supplant plant-based remedies.
Pharmacognosy is the branch of pharmacology that focuses on medicines derived from natural sources. With massive globalization and industrialization, it is possible that much human knowledge of plants and their medicinal purposes will disappear with the cultures that fostered them. This is where ethnobotanists come in. To learn about and understand the use of plants in a particular culture, an ethnobotanist must bring in knowledge of plant life and an understanding and appreciation of diverse cultures and traditions. The Amazon forest is home to an incredible diversity of vegetation and is considered an untapped resource of medicinal plants yet, both the ecosystem and its indigenous cultures are threatened with extinction.
To become an ethnobotanist, a person must acquire a broad knowledge of plant biology, ecology, and sociology. Not only are the plant specimens studied and collected, but also the stories, recipes, and traditions that are linked to them. For ethnobotanists, plants are not viewed solely as biological organisms to be studied in a laboratory, but as an integral part of human culture. The convergence of molecular biology, anthropology, and ecology make the field of ethnobotany a truly multidisciplinary science.
Gnetum: Distribution, Habitat and Relationships | Gnetales
Gnetum, represented by about 40 species is confined to the tropical and humid regions of the world. Nearly all species, except G. microcarpum, occur below an altitude of 1500 metres. Five species (Gnetum contractum, G. gnemon, G. montanum, G. ula and G. latifolium) have been reported from India (Fig. 13.1). Gnetum ula is the most commonly occurring species of India.
According to Bhardwaj (1957) various species of Gnetum occur in India in the following regions:
It is a woody climber having branches with swollen nodes. It is found in Western Ghats near Khandala, forests of Kerala, Nilgiris, Godawari district of Andhra Pradesh and Orissa.
A scandent shrub growing in Kerala, Nilgiri Hills and Coonoor in Tamil Nadu.
A shrubby plant found in Assam (Naga-Hills, Golaghat and Sibsagar).
A climber with smooth, slender branches, swollen at the nodes. It is found in Assam, Sikkim and parts of Orissa.
A climber found in Andaman and Nicobar Islands.
2. Habit of Gnetum:
Majority of the Gnetum species are climbers except a few shrubs and trees. G. trinerve is apparently parasitic. Two types of branches are present on the main stem of the plant, i.e. branches of limited growth and branches of unlimited growth. Each branch contains nodes and intemodes Stem of several species of Gnetum is articulated
In climbing species the branches of limited growth or short shoots are generally un-branched and bear the foliage leaves. The leaves (9-10) are arranged in decussate pairs (Fig. 13.2). They often lie in one plane giving the appearance of a pinnate leaf to the branch. The leaves are large and oval with entire margin and reticulate venation as also seen in dicotyledons. Some scaly leaves are also present.
3. Anatomy of Gnetum:
Young root (Fig. 13.3) has several layers of starch-filled parenchymatous cortex, the cells of which are large and polygonal in outline. An endodermal layer is distinguishable. Casparian strips are seen in the cells of the endodermis. The endodermis follows 4-6 layered pericycle. Roots are diarch and exarch. Small amount of primary xylem, visible in young roots, becomes indistinguishable after secondary growth.
The secondary growth is of normal type. A continuous zone of wood is present in the old roots (Fig. 13.4). It consists of tracheids, vessels and xylem parenchyma. The tracheids have uniseriate bordered pits along with bars of Sanio.
Vessels have simple or small multiseriate bordered pits. Some of the xylem elements have starch grains. Bars of Sanio are generally absent in the vessels. Phloem consists of sieve cells and phloem parenchyma.
(ii) Young Stem:
The young stem in transverse section is roughly circular in outline, and resembles with a typical dicotyledonous stem. It remains surrounded by a single-layered epidermis, which is thickly circularized and consists of rectangular cells. Some of the epidermal cells show papillate outgrowths. Sunken stomata are present.
The cortex consists of outer 5-7 cells thick chlorenchymatous region, middle few-cells thick parenchymatous region and inner 2-4 cells thick sclerenchymatous region. Endodermis and pericycle regions are not very clearly distinguishable. Several conjoint, collateral, open and endarch vascular bundles are arranged in a ring (Fig. 13.5) in the young stem.
Xylem consists of tracheitis and vessels. Presence of vessels is an angiospermic character. Protoxylem elements are spiral or annular while the metaxylem shows bordered pits which are circular in outline. The phloem consists of sieve cells and phloem parenchyma.
An extensive pith, consisting of polygonal, parenchymatous cells, is present in the centre of the young stem.
(iii) Old Stem:
Old stems in Gnetum show secondary growth. In G. gnemon the secondary growth is normal, as seen also in the dicotyledons. But in majority of the species (e.g., G. ula, G. africanum, etc.) the anamolous secondary growth is present.
The primary cambium is ephemeral, i.e., short-lived. The secondary cambium in different parts of cortex develops in the form of successive rings, one after the other (Fig. 13.6). The first cambium cuts off secondary xylem towards inside and secondary phloem towards outside. This cambium ceases to function after some time.
Another cambium gets differentiated along the outermost secondary phloem region, and the same process is repeated. In the later stages, more secondary xylem is produced on one side and less on the other side, and thus the eccentric rings of xylem and phloem are formed in the wood.
This type of eccentric wood is the characteristic feature of angiospermic lianes. The periderm is thin and develops from the outer cortex. It also possesses lenticels. The cortex also contains chlorenchvmatous and parenchymatous tissues along with many sclereids.
In old stems the secondary wood consists of tracheids and vessels. Tracheids contain bordered pits on their radial walls while vessels contain simple pits. Transitional stages (Fig. 13.7), containing one to many perforations in the terminal part of the vessels, are also seen commonly.
In tangential longitudinal section (T.L.S) of the stem (Fig. 13.8), the wood xylem and medullary rays are visible. Bordered pits on both the radial and tangential walls are present. Medullary rays are either uniseriate or multiseriate and consist of polygonal parenchymatous cells. They are boat-shaped (Fig. 13.8) and their breadth varies from 2 to many cells. Sieve cells of the phloem contain oblique and perforated sieve plates.
Internally, Gnetum leaves also resemble with a dicot leaf. It is bounded by a layer of thickly circularized epidermis on both the surfaces. Stomata are distributed all over the lower surface except on the veins. The mesophyll is differentiated generally into a single-layered palisade and a well-developed spongy parenchyma.
The latter consists of many loosely-packed cells. Many stellately branched sclereids are present near the lower epidermis in the spongy parenchyma. Many stone cells and latex tubes are present in the midrib region of the leaf.
Several vascular bundles in the form of an arch or curve are present in the prominent midrib region (Fig. 13.9). A ring of thick-walled stone cells is present just outside the phloem. Each vascular bundle is conjoint and collateral.
The xylem of each vascular bundle faces towards the upper surface while the phloem faces towards the lower surface. The xylem consists of tracheids, vessels and xylem parenchyma while the phloem consists of sieve cells and phloem parenchyma.
4. Reproduction of Gnetum:
Gnetum is dioecious. The reproductive organs are organised into well-developed cones or strobili. These cones are organised into inflorescences, generally of panicle type. Sometimes the cones are terminal in position.
A cone consists of a cone axis, at the base of which are present two opposite and connate bracts. Nodes and internodes are present in the cone axis. Whorls of circular bracts are present on the nodes. These are arranged one above the other to form cupulas or collars (Fig. 13.10). Flowers are present in these collars. Upper few collars may be reduced and are sterile in nature in G. gnemon.
Male Cone and Male Flower:
The male flowers are arranged in definite rings above each collar on the nodes of the axis of male cone. The number of rings varies between 3-6. The male flowers in the rings are arranged alternately. There is a ring of abortive ovules or imperfect female flowers above the rings of male flowers.
Each male flower contains two coherent bracts which form the perianth (Fig. 13.11). Two unilocular anthers remain attached on a short stalk enclosed within the perianth. At maturity, when the anthers are ready for dehiscence, the stalk elongates and the anthers come out of the perianth sheath. In Gnetum gnemon a few (2-3) flowers are sometimes seen fusing each other (Fig. 13.12).
Development of Male Flower (Figs. 13.13, 13.14):
In very young cones, certain cells below each collar become meristematic. They divide repeatedly and form a small hump-like outgrowth. Certain cells on the upper side of this annular outgrowth start to differentiate into the initials of the ovules. They develop into abortive ovules which form the uppermost ring. The cells of the lower side of this annular outgrowth form the primordium of male flower.
A central cushion of cells develops by the repeated divisions in the male flower primordium. This cushion gets surrounded by a circular sheath called perianth. The sheath-like perianth encloses the central cushion-like mass only partially. With the development of a depression or notch in the central mass two lobes differentiate and later on develop into two anther lobes.
With the help of many divisions the basal portion of this central mass of cells starts to differentiate into a stalk. This stalk elongates and pushes the anther lobes towards the outer side. Each anther lobe remains surrounded by an epidermal layer and a few wall layers which enclose a microsporangium.The innermost wall layer enclosing the sporogenous tissue is known as tapetum.
The sporogenous cells become loose, contract, round up and change into the spore mother cells. In the process of microspore formation the tapetum and two wall layers are used for the developing microspores. The spore mother cells undergo meiosis and ultimately the spore tetrads are formed.
The characteristic radial thickenings develop in the epidermal cells. They help in the dehiscence of microsporangium. The microspores are ornamented.
The female cones resemble with the male cones except in some definite aspects. A single ring of 4-10 female flowers or ovules is present just above each collar (Fig. 13.15). Only a few of the ovules develop into mature seeds (Fig. 13.15B).
In the young condition, there is hardly any external difference between female and male cones. All the ovules are of the same size when young but later on a few of them enlarge and develop into mature seeds. All the ovules never mature into seeds.
Ovule or Female Flower:
Each ovule (Fig. 13 16) consists of a nucellus surrounded of three envelopes. The nucellus consists of central mass of cells. The inner envelope elongates beyond the middle envelope to form the micropylar tube or style. The nucellus contains the female gametophyte. There is no nucellar beak in the ovule of Gnetum.
Stomata, sclereids and laticiferous cells are present in the two outer envelopes. Madhulata (1960) observed the formation of a circular rim from the outer epidermis of the inner integument in G. gnemon. Thoday (1921), however, observed the formation of a second such rim at a higher level. The ovules in G. ula are stalked.
More than one rings of ovules in the male cones in Gnetum gnemon have been reported by Thompson (1960) and Madhulata (1960). Collars, arranged spirally in the female cones of G. gnemon and G. ula have been observed by several workers including Maheshwari (1953).
Pearson (1912) reported some cones bearing only two collars in G. buchholzianum. Rarely, the lower collars in the male cones bear one or two fertile ovules whereas normal male flowers are present in the upper collars of the same cone.
Morphological Nature of Three Envelopes:
Several different views have been given by many different workers regarding the morphological nature of the three envelopes surrounding the nucellus.
A few of them are under mentioned:
(i) According to Strasburger (1872) three envelopes of nucellus are integuments developing from the differentiation of single integument.
(ii) Baccari (1877) opined that the outer envelope is a perianth while the inner two envelopes are integuments.
(iii) Van Tieghem (1869) considered the two inner envelopes as the integuments while the outer envelope as an ovary or analogous to it.
(iv) According to Lignier and Tison (1912), however, the outer two envelopes form a perianth while the inner envelope is equivalent to an angiospermic ovary. Vasil (1959) also supported the view of Lignier and Tison (1912) in case of Gnetum ula.
Mega-Sporangium, Mega-Sporogenesis and Female Gametophyte:
Four to ten ovular primordia differentiate on the annular meristematic ring. This ring develops below each collar of the female cone in the same manner as that of the male cone. The ovular primordium divides and re-divides several times to form a mass of cells.
All the three envelopes of the female flower develop around this mass of cells The innermost third envelope remains fused with the nucellus at the base while its upper portion remains free and form the long micropylar tube or ‘style’.
In the young conditions, an outer epidermal layer is distinguishable in the nucellus. Two to four archesporial cells develop below the epidermis at a later stage. The archesporial cells divide periclinally to form outer primary’ parietal cells and inner sporogenous cells. The primary parietal cells and the epidermal layer divide periclinally and anticlinally several times resulting into a massive nucellus.
The sporogenous cells divide and re-divide to form megaspore mother cells which remain arranged in linear rows. All the megaspore mother cells may divide reductionally and form tetrasporic embryo-sacs but ultimately all, except one, degenerate.
As many as 256 (Gnetum gnemon) to 1500 (G. ula) free-nuclei are formed in the female gametophyte leaving a vacuole in the centre (Fig. 13.18). The female gametophyte is tetrasporic in development. It is broader towards the micropylar end and it tapers towards the chalazal end.
The nuclei near the chalazal end get surrounded by cell walls while those towards micropylar end remain free. Gametophyte is thus partly cellular and partly-nuclear. The archegonia are absent in Gnetum.
Certain nuclei near the micropylar end start to function as egg nuclei. According to Swamy (1973) the only nucleus in a uninucleate cell or one of the nuclei in a multinucleate cell enlarges and functions as the egg in G. ula. The nucellar beak is absent in Gnetum.
The megaspore mother cell divides reductionally and forms four free haploid nuclei in the mother cell. Megaspore tetrads are never formed in Gnetum.
Microsporangium and Micro-Sporogenesis:
Development of the microsporangium (Fig. 13.19) can be studied only in young anthers. Two archesporial cells are distinguished below the epidermal layer (Fig. 13.19A). Archesporial cells divide and re-divide to form many-celled archesporium (Fig. 13.19B). The outermost layer of the archesporium divide periclinally to form an outer layer of parietal cells and inner layers of sporogenous cells (Fig. 13 .19C).
The parietal cells form the wall layers and tapetal layer by periclinal divisions (Fig. 13.19D). The sporogenous cells develop into microspore mother cells by some irregular divisions. Tapetal cells later on become bi-nucleate (Fig. 13.19D, E). Microspore mother cells divide reductionally to form haploid microspores.
The microspores may be arranged in isobilateral, decussate or tetrahedral manner in their earlier stages. Side by side the wall cells and the tapetal cells degenerate and ultimately dis-organise. The epidermal cells become thick, cutinized and radially elongated.
Many fibrous thickenings also develop in these cells (Fig. 13.19H). Small globular structures are present on the inner surface of the epidermis in Gnetum ula and G. gnemon. Anthers dehisce along a double row of small cells which extends from the tip towards the base.
Pollen grains or microspores are roughly spherical in outline. They are uninucleate and remain surrounded by a thick and spiny exine and thin intine. Mature pollen grains are shed at three-nucleate stage. These include prothallial nucleus, tube nucleus and generative nucleus (Fig. 13.20, Upper) in Gnetum Africanism and G. gnemon according to Pearson (1912, 1914).
This three-nucleate stage is reached by first dividing the microspore nucleus mitotically into two and then one of them again gets divided. Further development is affected only in the pollen chamber. The intine comes out by rupturing the exine and forms a pollen tube.
The tube nucleus migrates into the pollen tube. The generative nucleus also adopts the same course and divides into two unequal male gametes in the tube. Prothallial nucleus does not enter the pollen tube.
Thompson (1916) opined that the prothallial cell does not form at all in the male gametophyte (Fig. 13.20, Middle). The microspore nucleus divides into a tube nucleus and a generative cell. The latter divides into a stalk cell and body cell. The tube nucleus and body cell enter in the pollen tube where the body cell divides into two equal male gametes.
According to Negi and Madhulata (1957) the microspore nucleus in Gnetum gnemon and G. ula divides into a small lenticular cell and a large cell (Fig. 13.20, Lower). The lenticular cell does not take part in the further development and ultimately disappears.
The other large nucleus divides into a tube nucleus and a generative cell, both of which pass into the tube. The generative cell divides into two equal male gametes in the tube. A stalk cell is never formed in these species.
Wind helps in carrying the pollen grains up to the micropylar tube of the ovule. The micropylar tube secretes a drop of fluid in which certain pollen grains get entangled and reach up to the pollen chamber. The nucellus cells below the pollen chamber are full of starch.
The fertilization in Gnetum has been studied only by a few workers. Vasil (1959) studied this phenomenon in G. ula. At the time of fertilization, the pollen tube pierces through the membrane of the female gametophyte just near to a group of densely cytoplasmic cells. The tip of pollen tube bursts and the male cells are released. One of the male cells enters the egg cell.
The male and female nuclei, after lying side by side for some time, fuse with each other and form the zygote. According to Swamy (1973), the only identifying features of the zygote are its spherical shape and dense cytoplasm. Both the male cells of a pollen tube may remain functional if two eggs are present close to the pollen tube.
In all gymnosperms, except Gnetum, a cellular endosperm (Fig. 13.21) develops before fertilization. In Gnetum, the cell formation, although starts before fertilization, a part of the gametophyte remains free-nuclear at the time of fertilization.
After fertilization the wall formation in the female gametophyte starts in such a way that the cytoplasm gets divided into many compartments. Each of these compartments contains many nuclei (Fig. 13.21C).
All the nuclei of one compartment fuse and form a single nucleus. The wall formation starts from the base and proceeds upwards. The wall formation varies greatly in Gnetum. Only the lower portion of the gametophyte may become cellular leaving the remaining upper portion free-nuclear. Sometimes the entire gametophyte may become cellular.
In some cases the upper portion may become cellular instead of the lower portion. Sometimes only the middle portion may become cellular and in still other cases there may not be any wall formation at all. The characteristic triple fusion of the angiosperms is, however, absent in Gnetum.
The embryo development in several species of Gnetum has been studied by many different workers including Lotsy (1899), Coulter (1908) and Thompson (1916), but the details put forward by these wokers are highly variable.
Maheshwari and Vasil (1961) have stated that in all the angiosperms the first division of the zygote is accompanied by a wall formation but in all gymnosperms, except Sequoia sempervirens, these are free-nuclear divisions in the zygote. Gnetum in this respect forms a link in between gymnosperms and angiosperms by showing both free-nuclear divisions as well as cell divisions.
Thompson (1916) opined that a two-celled pro-embryo is formed (Fig. 13.22 A). From each of these two cells develops a tube called suspensor (Fig. 13.22B). Now the nucleus divides and one of the two nuclei undergoes free-nuclear divisions forming four nuclei. The embryo gets organised by these four nuclei (Fig. 13.22C, D). There is no division in the other larger nucleus..
Madhulata (1960) has worked on the zygote development in Gnetum gnemon. According to her 2-4 or sometimes up to 12 zygotes may develop in a gametophyte, of which normally one remains functional. From the zygote develops generally one or sometimes 2-3 small tubular outgrowths.
Only one of these tubes receives the nucleus and survives while the remaining tubes disintegrate and soon die. The surviving outgrowth elongates, becomes branched and grows into different directions through the intercellular spaces of the endosperm. All the primary suspensor tubes usually remain coiled round each other.
A small cell is cut off at the tip of the primary suspensor tube in Gnetum gnemon. It soon divides first transversely and then longitudinally resulting into four cells. Now irregular divisions take place forming a group of cells. Some of these cells divide and elongate to form secondary suspensor (Fig. 13.23). The remaining cells at the tip form the embryonal mass.
In Gnetum ula a small cell is cut off at the tip of the tube called peculiar cell. This peculiar cell soon divides and forms a group of cells. The secondary suspensor and embryonal mass are differentiated (Fig. 13.24) from this group of cells. By this time, the wall of the tube starts to become thick.
What so ever may be the pattern of formation of the embryonal mass and secondary suspensor, the cells of the former are small, compact, dense in cytoplasm and develop into embryo-proper while that of the latter (i. e. secondary suspensor) are thin-walled, uninucleate and highly vacuolated.
The primary and secondary suspensors help in pushing the embryo into the endosperm. Soon a stem tip with two lateral cotyledons form in the tip region of the embryonal mass. On the opposite side develop the root tip with a root cap.
A feeder develops after the formation of stem and root tips (Fig. 13.25). The feeder is a protuberance-like structure present in between root and stem tips. Thus, the stem tip, two cotyledons, feeder, root tip and root cap are the parts of a mature embryo.
Gnetum seeds (Fig. 13.26) are oval to elongated in shape and green to red in colour. It remains surrounded by a three-layered envelope which encloses the embryo and the endosperm. Outer envelope is fleshy, and consists of parenchymatous cells. It imparts colour to the seed.
The middle envelope is hard, protective and made up to three layers, i.e., outer layer of parenchymatous cells, middle of palisade cells and innermost fibrous region. The inner envelope is parenchymatous. Branched vascular bundles traverse through all the three envelopes.
Germination of Seed:
Germination is of epigeal type (Fig. 13.27). The cotyledons are pushed out of the seed. The hypocotyl elongates, and this brings the cotyledons out of the soil. The first green leaves of the plant are formed by the cotyledons. The first pair of foliage leaves is produced by the development of plumule. A persistent feeder is present up to a very late stage in the seed.
5. Relationships of Gnetum:
Gnetum and Other Gymnosperms:
Gnetum shows several resemblances with gymnosperms and has, therefore, been finally included under this group.
Some of the characteristics common in both Gnetum and other gymnosperms are under mentioned:
1. Wood having tracheids with bordered pits.
2. No sieve tubes and companion cells are present.
3. Presence of naked ovules.
4. Absence of fruit formation because of the absence of ovary.
5. Anemophilous type of pollination.
6. Development of prothallial cell.
8. Resemblance of the vascular supply of the peduncle of the cone of Cycadeoidea wielandii with that of a single flower of Gnetum.
9. Resemblance of the structure of basal part of the ovule in Gnetum and Bennettites.
Gnetum and Angiosperms:
A key position to Gnetum has been assigned by scientists while discussing the origin of angiosperms. Both Gnetales and angiosperms originated from a common stalk called “Hemi-angiosperm”.
Thompson (1916) opined that the ancestors of both Gnetum and angiosperms were close relatives. Some other workers have gone up to the extent in stating that Gnetum actually belongs to angiosperms. Hagerup (1934) has shown a close relationship between Gnetales and Piperaceae.
In a beautiful monograph on Gnetum, Maheshwari and Vasil (1961) have stated that “Gnetum remains largely a phylogenetic puzzle. It is gymnospermous, but possesses some strong angiospermic features”.
Some of the resemblances between Gnetum and angiosperms are under mentioned:
1. The general habit of the sporophyte of many species of Gnetum resembles with angiosperms.
2. Reticulate venation in the leaves of Gnetum is an angiospermic character.
3. Presence of vessels in xylem is again an angiospermic character.
4. Clear tunica and corpus configuration of shoot apices is a character of both Gnetum and angiosperms.
5. Strobili of Gnetum resemble much more with angiosperms than any of the gymnosperms
6. Micropylar tube of Gnetales can be compared with the style of the angiosperms because both perform more or less similar functions.
7. Tetrasporic development of the female gametophyte is again a character which brings Gnetum close to angiosperms.
8. Absence of archegonia again brings Gnetum and angiosperms much closer.
9. Dicotyledonous nature of the embryo of Gnetum brings it quite close to the dicotyledons.
Resemblances Between Gnetum, Ephedra and Welwitschia:
All the three genera of Gnetales show following resemblances:
(2) Vessels in their secondary wood,
(3) Similar structure and development of perforation plates in their vessels
(4) Similar Gnetalean mode of development of their vessels i.e. by the dissolution of torus and middle lamella of the bordered pits
(5) Almost similar structure of their sieve cells and phloem parenchyma
(6) Spiral or annular elements in their protoxylem
(7) Arrangement of their flowers in compound strobili
(10) Stalked male flowers bearing synangia made of 1-6 or more sporangia
(11) Almost consistent structure of the wall of their microsporangia
(12) Wingless pollen grains
(14) Ovules surrounded by several envelopes which are interpreted variously as integuments or perianth
Ch.30 - Seed Plants Flashcards Preview
In addition to seeds, what else is common to all seed plants?
A ______ is an embryo and its food supply, surrounded by a protective coating.
A seed is an embryo and its food supply, surrounded by a protective coating.
_____ million years ago, while cultivation of seed plants for food (wheat, maize, bananas) occurred
360 million years ago, while cultivation of seed plants for food (wheat, maize, bananas) occurred
13 thousand years ago.
A seed is a multicellular structure made of wh 3 structures?
A seed is a multicellular structure made of:
Embryo (2n new generation)
Nutritive tissue (haploid in gymno, triploid in angio)
Seed coat (from ovule covering -> maternal tissue = 2n)
In a seed, the food supply comes fr the _______ (gametophyte/sporophyte) and is ______ (haploid/diploid)
In a seed, the food supply comes fr the gametophyte and is haploid.
Note: angiosperms have triploid (3n) food source.
T/F: most gametophytes of seed plants are microscopic
most gametophytes of seed plants are microscopic
Sporophyte is dominant generation
Reduced gametophytes in seed plants develop from ______ inside ______, contained in parent sporophyte.
Reduced gametophytes in seed plants develop from spores inside sporangia, contained in parent sporophyte.
Seed plant gametophytes are heterosporous. What does this indicate about where they're found and how they look?
gametophyte of each sex look different/found in different places
megasporangia → _______ → ______ (fe/male) gametophyte
microsporangia → ______ → _______ (fe/male) gametophyte
megasporangia → megaspores → female gametophyte
microsporangia → microspores → male gametophyte
The _________ is a layer of sporophyte tissue that envelops and protects megasporangium.
The integument is a layer of sporophyte tissue that envelops and protects megasporangium.
An unfertilized gymnosperm ovule is made up of ________, _________, and _________. The female gametophyte develops from the ________ and produces egg(s).
An unfertilized gymnosperm ovule is made up of integument, megasporanium, and megaspore. The female gametophyte develops from the megaspore and produces egg(s).
Label the indicated structures in the attached figure of an unfertilized ovule.
A microspore is also called a ___________, and has a wall made out of __________, which encloses and protects the _______ (fe/male) gametophyte.
A microspore is also called a pollen grain, and has a wall made out of sporopollenin, which encloses and protects the male gametophyte.
Pollination is the transfer of ______ to the part of the seed plant that contains the ______.
Pollination is the transfer of pollen to the part of the seed plant that contains the ovule.
After germination and fertilization, a gymnosperm ovule develops into a _____ which consists of the _______ (gameto/sporophyte) embryo, its food source, and a ___________.
After germination and fertilization, a gymnosperm ovule develops into a seed which consists of the sporophyte embryo, its food source, and a protective coat
Label the structures in the fertilized ovule.
Label the structures in the attached gymnosperm seed.
Compare and contrast spore vs seeds.
Seed coating is multicellular, provides more protection
Seeds have supply of stored food, can survive longer than spores
Gymnosperms, aka "naked seeds", have seeds that are not enclosed in _______. Instead, their seeds are exposed on modified leaves called _______, wh typ form cones called ________.
Gymnosperms, aka "naked seeds", have seeds that are not enclosed in ovaries. Instead, their seeds are exposed on modified leaves called sporophylls, wh typ form cones called strobili.
Pine trees are gymnosperms. The pine tree itself is _______ (sporo/gametophyte).
Pine trees are gymnosperms. The pine tree itself is sporophyte.
_________ are scale-like structures packed into cones. Ovule cones (aka _________) are large, while pollen cones (aka _________) are small.
Sporangia are scale-like structures packed into cones. Ovule cones (aka megasporangia) are large, while pollen cones (aka microsporangia) are small.
Microsporocytes divide by ______ (meiosis/mitosis) producing _______ (haploid/diploid) microspores, wh dev into ________.
Microsporocytes divide by meiosis producing haploid microspores, wh dev into pollen grains.
During gymnosperm germination, the _________ forms, growing toward the (micro/megasporangium).
The _________ (micro/megasporocyte) goes through ______ (meiosis/mitosis) producing _______ (two/four/six) haploid cells.
One cell survives, becoming the _________ (micro/megaspore), and the _______ (sporo/gametophyte) develops inside.
During gymnosperm germination, the pollen tube forms, growing toward the megasporangium.
The megasporocyte goes through meiosis producing four haploid cells.
One cell survives, becoming the megaspore, and the gametophyte develops inside.
Approx how many years ago (and in wh period) did gymnosperms first appear?
305 million years ago (Carboniferous) – First fossil gymnosperms found gymnosperms diverged fr angiosperms.
Additional file 1: Figure S1.
Interpretative line drawings showing branching pattern of Cosmosperma polyloba. Abbreviations: st, stem pr, primary rachis sr, secondary rachis tr, tertiary rachis. (a) Stem, primary and secondary rachises and basal part of a tertiary rachis in Fig. 1h. (b) Bifurcate primary rachis, two secondary rachises, and a tertiary rachis bearing ultimate pinnae and conical prickles in Fig. 6c. (c) Secondary rachis with alternate tertiary rachises, ultimate pinnae and conical prickles in Fig. 6b. (d) Bifurcated primary rachis, two secondary rachises and alternate tertiary rachises with ultimate pinnae and conical prickles in Fig. 4a. (TIFF 2282 kb)
Additional file 2: Figure S2.
Interpretative line drawings showing frond and ultimate pinnae of Cosmosperma polyloba. Abbreviations same as in Figure S1. (a) Bifurcate primary rachis, two secondary rachises, and one tertiary rachis with 11 ultimate pinnae in Fig. 6a. (b-e) Ultimate pinnae in Fig. 6(c, left arrow), Fig. 6(c, right arrow), Fig. 6(b, arrow) and Fig. 4(c), respectively. Highly dissected and planate pinnules alternately arranged along the quaternary rachis. (TIFF 2025 kb)
Additional file 3: Figure S3.
Interpretative line drawing showing synangiate pollen organs on fertile axes of Cosmosperma polyloba. (a) Anisotomous fertile rachises with terminal pollen organs in Fig. 7a. Conical prickles sparsely located along the fertile rachises sparsely. (b, c) Two stages of dégagement on pollen organs in Fig. 7d, e, respectively. (TIFF 1395 kb)