Term Paper on Vascular Plants | Plants | Biology

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Term Paper on Vascular Plants

Term Paper Contents:

1. Term Paper on the Introduction to Vascular Plants
2. Term Paper on the Trends among Vascular Plants
3. Term Paper on Class Filicinae: Ferns
4. Term Paper on Gymnosperms  
5. Term Paper on Class Angiosperms: Flowering Plants

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1. Term Paper on the Introduction to Vascular Plants: 

Phylum Tracheophyta:

Rhynia major is an example of the earliest known vascular plants, dating back some 400 million years ago. It does not look much like any modern vascular plant and, indeed, is much more primitive in appearance than a bryophyte.

However, it differs from all bryophytes in one important respect- Within its stem is a central cylinder of vascular tissue, specialised for conducting water up the plant body and products of photosynthesis down.

The vascular system in Rhynia, and in modern tracheophytes as well, consists of two distinct tissues- the phloem, which conducts sugars and other soluble organic molecules from the leaves to other parts of the plant body, and the xylem, which conducts water and minerals up from the roots.

The conducting elements of the phloem are sieve cells or sieve-tube elements, and the conducting elements of the xylem are tracheids and vessel elements. In stems, the strands of xylem and phloem are side by side, either in vascular bundles or arranged in two concentric layers (cylinders), in which one tissue (typically the phloem) appears outside the other.

Rhynia major and its relatives are now extinct, as are some other subphyla of vascular plants.

Three groups of vascular plants still have living representatives. Two of the groups are small- the Lycophytina (club mosses) and the Sphenophytina (horsetails). The third group, the Pterophytina, a very large subphylum, includes all the major groups of plants: ferns, gymnosperms, and flowering plants.

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2. Term Paper on the Trends among Vascular Plants:

Beginning with a very simple vascular plant, such as Rhynia, it is possible to trace some major evolutionary trends. One early development was the root, a structure specialised for the absorption of water. Another was the leaf. Two distinct types of leaves evolved the microphyll and the megaphyll.

The microphyll contains only a single strand of vascular tissue, whereas the megaphyll typically contains a complex system of veins. These two types of leaves seem clearly to have evolved in different ways. Of the three groups of modern plants, the Lycophytina and the Sphenophytina have microphylls and the Pterophytina have megaphylls.

A third evolutionary trend is the development of increasingly complex and efficient vascular systems. A fourth trend is seen in the reduction of the gametophyte. In all vascular plants, the gametophyte is smaller than the sporophyte.

However, in the more primitive tracheophytes, the gametophyte is separate and nutritionally independent, whereas in the more recently evolved forms, it has been reduced to microscopic size and dependency status.

Also related to the reproductive cycle is the trend toward heterospory. The earliest vascular plants produced only one kind of spore (homospory) in one kind of sporangium. Upon germination, such spores produce gametophytes on which both antheridia and archegonia form.

In plants that are heterosporous, two different kinds of gametophytes develop; one bearing archegonia and the other, antheridia. (As the gametophytes become reduced, archegonia and antheridia decrease in size until, in the most highly evolved gymnosperms and in the angiosperms, they disappear.)

The final and perhaps the most significant innovation was the seed. The seed is, in effect, the sporangium, outside of which is one or two layers of tissue, the integument, and inside of which is the spore. The spore germinates to form the female gametophyte, in which an egg cell differentiates and becomes an embryonic sporophyte-“three generations under one roof.”

The sporangium with its integument and contents is called the ovule, and it is the mature ovule that is properly known as the seed. The earliest known seeds were fossilized in late Devonian deposits some 350 million years ago.

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3. Term Paper on Class Filicinae: Ferns:

Ferns are vascular plants that can usually be distinguished from most other plants by their large, feathery leaves, which, in most species, unroll from base to tip during growth. The sporangia commonly are on the undersurface of the leaves or, sometimes, on specialised leaves. Spore-bearing leaves are called sporophylls. The carpels and stamens of flowers are also sporophylls.

According to the fossil record, ferns first appeared almost 400 million years ago, and they are still relatively abundant. Most of the 11,000 living species are found in the tropics, but some occur in temperate and even arid regions. Because they have flagellated sperm and need free water for fertilization, those species growing in arid regions exploit

The stems of ferns are anatomically simple compared with those of gymnosperms and angiosperms, and are often reduced to a creeping underground stem (rhizome). Although ferns do not exhibit secondary growth-the type of growth that results in increase of girth and formation of bark and woody tissue-some grow very tall. For instance, Cyathea australis, a tree fern found on Norfolk Island in the South Pacific sometimes reaches 28 meters in height.

The leaves of ferns are usually finely divided into pinnae; these divided leaves spread widely and so collect more light, and apparently they are thus adapted to growing on the forest floor in diffused light. The sporangia develop on the lower surface of sporophylls, which may resemble the other green leaves on the plant or may be nonphotosynthetic stalks (modified leaves).

The sporangia commonly occur in small clusters known as sori (singular, sorus). In the ferns, as in all the vascular plants, the predominant plant form is the sporophyte. The gametophyte of the homosporous ferns begins development as a small alga like filament of cells, each filled with chloroplasts, and then develops into a flat structure, often only one layer of cells in thickness.

Although this gametophyte is small, it is nutritionally independent, as is the sporophyte. All but a few genera are homosporous, and the single gametophyte produces both antheridia and archegonia. The sperm are coiled and multi-flagellate.

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4. Term Paper on Gymnosperms:

The humid Paleozoic era was the age when the earth’s coal deposits were formed from lush vegetation that sank so swiftly into the warm, marshy soil that there was no chance for much of it to decompose. At the close of this era, in the Permian period, 225 to 280 million years ago, there were worldwide changes of climate, with the advent of widespread glaciers and drought.

The amphibians gave way to the scaly-skinned reptiles, which may have been better suited to the harsher climate. Under similar selective pressures, plants began to evolve specialised structures that enabled them to survive during periods when no water was available.

It was during the Permian period that the gymnosperms-the naked- seed plants-evolved. There are four groups of gymnosperms with living representatives, three small groups (the cycads, the ginkgos, and the Gnetinae) and one large and familiar group (the conifers).

The Seed:

The seed is a protective structure in which the embryonic plant can lie dormant until conditions become favourable for its survival. Thus, in its function, it parallels the spores of bacteria or the resistant zygotes of the freshwater algae.

In structure it is far more elaborate, however. A seed includes the embryo (the young, dormant sporophyte), a store of nutritive tissue, and an outer protective coat.

To understand the evolution of the seed it is necessary to return for a moment to the life cycle, or alternation of generations, found in all the vascular plants, and foreshadowed in the green algae. In the green algae, the two generations, sporophyte and gametophyte, are independent. In most ferns, the gametophytes, although still independent, are usually smaller than the sporophytes.

In the seed plants, the gametophytic generation is reduced still further and is totally dependent on the sporophyte. All gymnosperms are heterosporous, producing both male and female gametophytes.

The female gametophyte develops on the mother sporophyte within the ovule. Within the female gametophyte, one or more egg cells are formed. Sperm cells develop in another gametophyte, also protected and nourished by a sporophyte. They are carried to the female gametophyte and fertilise the eggs developing there.

The zygote develops into an embryo. Following fertilization, the ovule enlarges and its outer surface hardens, forming a protective cover around the embryo and the gametophyte tissue in which the embryo is embedded. This complex is released as the seed.

Let us look at a specific example, the formation of a pine seed. A pine tree has two types of cones, which produce the two types of spores. The small “male” cones resemble those of club mosses, but on the large “female” cones, the scales that bear the ovules are much thicker and tougher than the sporophylls of the male cones.

In the male cones, specialised cells inside the sporangia undergo meiosis to produce haploid spores. Each spore differentiates into a microscopic, windborne pollen grain, an immature male gametophyte. The wind is an unreliable messenger, disseminating the pollen grains at random, and wind-pollinated plants characteristically produce pollen in great quantities.

Fertilization:

Within the ovules of the female cones, a second type of spore is formed by meiosis. (Spores that give rise to female gametophytes are often referred to as megaspores, as distinct from microspores, from which male gametophytes arise.) Of the four spore cells produced within the ovule by each meiotic sequence, three disintegrate and the remaining one forms a tiny gametophyte.

This haploid gametophyte grows within the cone and develops two or more archegonia, each of which contains a single egg cell. The development from the spore into the gametophyte with its egg cells may take many months-slightly more than a year, for example, in some common pines. The ripening ovule secretes a sticky liquid.

When the cones become dusted with pollen, some of the pollen sifts down between the cone scales and comes into contact with the liquid. Pollen grains, caught in the sticky liquid, are drawn to the ovule as the liquid dries.

Here the pollen grain develops into a mature gametophyte. The male gametophyte produces two non-motile cells, the male gametes, or sperm. These are carried toward the egg within the pollen tube, which is produced by the male gametophyte and which grows through the tissues of the ovule.

Because the drought-resistant pollen is blown to the ovule of the female cones by the wind, and the sperm are carried to the egg by the pollen tube, the pines and other conifers are not dependent on free water for fertilization. Thus they are able to reproduce sexually when (and where) ferns and bryophytes cannot.

Following fertilization, the zygote begins to divide and forms the embryo. As the ovule matures, its outer walls harden into a seed coat enclosing both the embryo and the female gametophyte (the latter provides food for the embryo when the seed germinates). After the cone matures, it releases its seeds. In most conifers the seeds are winged and are spread widely by the wind.

Figure 10.8 shows a cross section of a pine seed. The seed coat and the wing on which the seed is carried arise from the hardened outer layers of the ovule, derived from the mother sporophyte.

The next layer represents the body of the gametophyte; swollen and packed with stored food reserves, it grows and displaces the original sporophyte tissue. The inner core is the embryo with its many cotyledons, the embryonic leaves, which will appear as the first leaves of the shoot of the new sporophyte when the seed germinates. The lower part of the embryo, the radicle, will form the root.

The seed was in existence by the close of the Carboniferous period, and according to the fossil record, some of the fernlike plants and even some of the club mosses had seed-like structures.

But it was not until the close of the Permian period, at the end of the Paleozoic era (some 225 million years ago), when the land became colder and drier, that plants that had seeds gained a major evolutionary advantage and the seed plants became the dominant plants of the land.

The Conifer Leaf:

Another feature commonly associated with conifers, although not with all gymnosperms, is their needlelike leaf. A cross section of a pine leaf, which may be 10 or more centimeters long but only 1 to 3 millimeters in diameter. In the center you can see veins, the vascular transport system, which carries water in one set of conducting cells (the tracheids) and sugars in another (the sieve cells).

Outside the veins are parenchyma cells in which photosynthesis takes place. The ducts on the facing sides of the needles carry resin, a substance that is released if the plant is wounded and may serve to close the break. The outside layer of cells (epidermis) is porous but very hard. The needlelike leaf, despite its slender form, is a megaphyll. It is well adapted to long periods of low humidity, such as in northern winters, and to moisture-losing, sandy soils.

One or both of those are characteristic of many regions in which modern conifers are abundant. (In areas with more available water in the air or soil, they cannot usually compete with angiosperms.)

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5. Term Paper on Class Angiosperms: Flowering Plants:

It is believed that the angiosperms evolved from a now-extinct group of gymnosperms. They appear in the fossil record suddenly and in abundance during the Cretaceous period, about 100 million years ago, as the dinosaurs were vanishing. Numerous angiosperm genera appeared suddenly at that time, and many of these seem to have been very similar to our modern plants.

According to Daniel Axelrod, a paleobotanist at the University of California, angiosperms probably arose long before this time, during the Permian period. They are likely to have originated on the less fertile hills and uplands of tropical areas, the richer lowlands being crowded with ferns, gymnosperms, and lycopods.

Once established, they spread into the lowlands, where they soon became the dominant plant forms and were deposited as fossils. During this mid-Cretaceous period, the climate of the earth was warmer and more uniform than it is at present, and by the end of the Cretaceous period, much of the land was covered with a rich forest of angiosperms, reaching almost as far north as the Arctic Circle.

About 250,000 different species of angiosperms are known. They dominate the tropical and temperate regions of the world.

The angiosperms include not only the plants with conspicuous flowers but also most of the great trees, the oak, the willow, the elm, the maple, and the birch; all the fruits, vegetables, nuts, and herbs; the cactus and the coconut; and all the corn, wheat, rice, and other grains and grasses that are the staples of the human diet and the basis of agricultural economy all over the world.

Angiosperms, like other vascular plants, contain chlorophylls a and b and betacarotene, and have megaphylls, stomata, and a cuticle impervious to water. The modern forms have a more highly evolved vascular system than is found in other groups. They also have two new, interrelated structures that distinguish them from all other plants: the flower and the fruit.

The Flower:

Figure 10.9 is a diagram of a simple flower. The central structure is the carpel, the female reproductive structure. The carpel is believed to be a modified leaf, which, in the course of evolution, enfolded in such a way that the ovule is attached to its inner surface.

A single carpel may contain one or more ovules, and a single flower one or more carpels. The carpels may be separate (blackberry) or fused (tomato and apple). The swollen base of the carpel or carpels is the ovary. Within the ovary is the ovule, or ovules, in which the female gametophyte develops.

The tip of the carpel or carpels has become specialised as a stigma, a sticky surface to which pollen grains adhere. The stigma and ovary are connected by a slender column of tissue, the style.

The pollen grains (immature male gametophytes) develop in the stamen, which, like the carpel, is the evolutionary descendant of a sporophyll. It consists of the anther, which is a group of sporangia in which the male gametophytes develop, and a supporting filament.

The other parts of the flower are specialised to attract insects and other pollinators. Pollen grains produced in the anthers are carried to the stigma of (usually) another flower, where they germinate, developing pollen tubes that grow down through the style toward the ovule.

The sperm cells are carried by the pollen tubes to the female gametophyte. This gametophyte typically consists of only eight cells, one of which is the egg cell. It is fertilised by a sperm cell, giving rise to an embryo sporophyte.

Evolution of the Flower:

The flower evolved as a device by which plants induce animals to transport their pollen, and hence sperm, to the egg cells. The most primitive flowers are believed to have resembled the modern hepatica, which has numerous floral parts, each clearly separate from the other.

By comparing this type of flower with some of the more specialised ones, such as a composite or an orchid, it is possible to see four main trends in flower evolution, as listed below:

1. Reduction in number of floral parts. Most specialised flowers have few stamens and few carpels, and these are present in a definite number, depending on the species.

2. Fusion of floral parts. Carpels and petals, in particular, have become fused, sometimes elaborately so.

3. Elevation of free floral parts above the ovary. In the primitive flower, the floral parts arise at the base of the ovary. Such ovaries are said to be superior. In the more advanced flowers, the free portions of the floral parts are above the ovary; this is an important adaptation by which the ovules are protected from foraging insects. Such ovaries are said to be inferior.

4. Changes in symmetry. The radial symmetry of the primitive flower has given way, in more advanced flowers, to bilaterally symmetrical forms.

The Agents of Evolution:

The early gymnosperms from which the angiosperms evolved were probably wind-pollinated, as are modern gymnosperms. And, as in the modern gymnosperms, the ovule probably exuded droplets of sticky sap in which pollen grains were caught and drawn to the female gametophytes. Insects, probably beetles, feeding on plants must have come across the protein-rich pollen grains and the sticky, sugary droplets. As they began to depend on these new-found food supplies, they inadvertently carried pollen from plant to plant.

Beetle pollination must have been more efficient than wind pollination for some plant species because, clearly, selection began to favour those plants that had insect pollinators. The more attractive the plants were to the beetles, the more frequently they would be visited and the more seeds they would produce.

Any chance variations that made the visits more frequent or that made pollination more efficient thus offered immediate advantages; more seeds would be formed, and more offspring would survive. Nectaries (nectar-secreting organs) evolved, which lured the pollinators. Plants developed white or brightly coloured flowers that called attention to the nectar and other food supplies.

The carpel, originally a leaf-shaped structure, became folded on itself, enclosing and protecting the ovule from hungry pollinators. By the beginning of the Cenozoic era, some 65 million years ago, the first bees, wasps, butterflies, and moths had appeared.

These are long-tongued insects for which flowers are often the only source of nutrition for the adult forms. From this time on, flowers and certain insect groups had a profound influence on one another’s history, each shaping the other as they evolved together.

A flower that attracts only a few kinds of animal visitors and attracts them regularly has an advantage over flowers visited by more promiscuous pollinators: Its pollen is less likely to be wasted on a plant of another species.

In turn, it is an advantage for the insect to have a “private” food supply that is relatively inaccessible to competing insects. Most of the distinctive features of modern flowers are special adaptations that encourage regular visits (constancy) by particular pollinators. The varied colours and odours allow sensory recognition by pollinators. The diverse shapes such as deep nectaries and complex landing platforms that are found, for example, in orchids, snapdragons, and irises, represent ways of excluding indiscriminate pollinators.

Dispersal of Fruits and Seeds:

Following fertilization in the angiosperm, the ovary develops into the fruit. Fruits are adaptations for seed dispersal. Many edible fleshy fruits become brightly coloured as they ripen, attracting the attention of birds and mammals.

The seeds within the fruits pass through the digestive tract hours later and are often deposited some distance away. In some species the seed coat’s exposure to digestive juices or bile is a prerequisite for germination. The seeds themselves may be toxic, as in apples, discouraging animals from grinding up and digesting the seeds.

In some dry fruits, the fruit bursts open, shooting out the seeds, as in impatiens or witch-hazel. Some fruits, such as the samara of maples, carry wings; in others, the seeds themselves are winged or tufted, as in the milkweed. Some species of geranium send forth their seeds by a sort of slingshot.

Burrs are dry fruits that adhere to fur, feathers, or clothing and so are carried by involuntary porters to far-off fields and meadows. In the tumbleweed, the whole plant is blown across open country, with fruits opening and scattering the seeds as they go. The many extraordinary means of seed dispersal are another major reason for the evolutionary success of angiosperms. Angiosperms differ from the other vascular plants in their more efficient conducting systems, their flowers, which increase the efficiency of fertilization, and their fruits, which increase the efficiency of distribution.

Angiosperms represent the most successful of all plants in terms of numbers of individuals, numbers of species, and their effects on the existence of other organisms.