Term Paper on Plants: Top 12 Papers | Biology

Here is a compilation of term papers on ‘Plants’ for class 8, 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Plants’ especially written for school and college students.

Term Paper on Plants

Term Paper Contents:

1. Term Paper on the Definition of Plants
2. Term Paper on the Plant Body and Evolution
3. Term Paper on the Leaves
4. Term Paper on the Stem
5. Term Paper on the Roots
6. Term Paper on Reproduction in Plants
7. Term Paper on Pollination
8. Term Paper on Fertilization
9. Term Paper on the Fruit
10. Term Paper on Seed Dormancy
11. Term Paper on Plant Growth
12. Term Paper on the Adaptations to Climate Change

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1. Term Paper on the Definition of Plants:

Plants, as defined in this text, are multicellular photosynthetic organisms adapted for life on land. In this article, we shall focus on the class of plants that evolved the most recently, the angiosperms. This class is characterised by specialised structures-flowers-in which sexual reproduction takes place, in which the seed is formed, and from which the fruit develops.

The angiosperms are by far the most abundant class of plants, with about 235,000 species. The group is divided into two large subclasses, the dicots (170,000 species) and the monocots (65,000 species).

According to the fossil record, plants first began to invade the land a mere half billion years ago. Not until then did the earth’s surface truly come to life. As a film of green spread from the edges of the waters, other forms of life, the heterotrophs, were able to follow.

For most of earth’s history, the land was bare. A billion years ago, seaweeds clung to the shores at low tide and perhaps some gray-green lichens patched a few inland rocks, but had anyone been there to observe it, the earth’s surface would generally have appeared as barren and forbidding as the surface of Mars does today.

The shapes of these new forms and the ways in which they lived were determined by the plant life that preceded them. Plants supplied not only their food-their chemical energy-but also their nesting, hiding, stalking, and breeding places.

And so it is today. In all terrestrial communities except those of human creation, the character of the plants still determines the character of the animals and other forms of life that inhabit a particular area. Even, we members of the human species, who have seemingly freed ourselves from the life of the land and even, on occasion, from the surface of the earth, are still dependent on the photosynthetic events that take place in the green leaves of plants.

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2. Term Paper on the Plant Body and Evolution:

The ancestor of the plants was probably a single-celled alga that floated on or just below the water’s surface. Like modern plants, its photosynthetic pigments were chlorophylls a and b and carotenoids, especially beta-carotene, all of which were contained in chloroplasts.

It had a membrane-bound nucleus and mitochondria and other cellular organelles, as well as an external cell wall of cellulose. Its energy source was sunlight, and it obtained oxygen, carbon dioxide, and the minerals it required from the waters in which it lived.

Plants have the same few and relatively simple requirements:

i. Light,

ii. Water,

iii.Oxygen,

iv. Carbon dioxide, and

v. Certain minerals.

From these simple materials they, like their ancestors, make the sugars, amino acids, and all other organic substances on which all plant and animal life depends. But there is an important difference.

In the simplest photosynthesizing organism-the single green cell or filament of cells each of the needed materials is immediately available to every cell. In a plant, however, the individual cells can no longer function autonomously but can survive only through cooperation and division of labor among the many cells and tissues forming the plant body.

The reasons for this division of labor are easy to understand. The water and minerals needed by plants are found mostly below the surface of the earth; the development of a complex and extensive root system can be seen as a response to this selection pressure. Sunlight cannot reach these belowground structures, however, and photosynthesis is relegated to another part of the plant body.

As the plants began to crowd each other and compete for light, selection pressures favoured those with more extensive and efficient light-collecting surfaces-leaves.

The stem raises these photosynthesizing surfaces into the sunlight. Through the specialised vascular tissues of the stem, water and minerals from the ground travel to the leaves, and the products of photosynthesis formed in the leaves are transported to flowers, roots, and other non-photosynthetic parts of the plant.

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3. Term Paper on the Leaves:

Leaf Shapes and Adaptations:

Leaves come in all shapes and sizes, ranging from broad fronds to tiny scales. Some of these differences can be correlated with the environments in which the plants live. Large leaves with broad surfaces are often found in plants that grow under the canopy in a tropical rain forest, where water is plentiful but where there is intense competition for light.

Leaves of such plants sometimes have “drip tips,” which facilitate the runoff of rainwater. Leaves with small photosynthetic surfaces are associated with dry climates. In conifers, for example, the photosynthetic surfaces are greatly reduced and there is an extra-thick layer of epidermis and cuticle.

Similarly, angiosperms found in dry habitats often have small, leathery leaves. This reduction in leaf surface reaches its extreme in desert cacti, which have no leaves at all. In these plants, photosynthesis takes place in the fleshy stems, which are also water-storage organs.

Leaf Abscission:

Another type of adaptation related to the conservation of water is found in the leaves of deciduous trees. Deciduous trees, which drop their leaves annually, are found in regions where there is a marked seasonal variation in available water. The fall of the leaf comes about when special enzymes dissolve the middle lamellae in a layer of weak, thin-walled cells in the petiole.

These cells compose the abscission layer, and the process is known as abscission. Deciduous trees are able, unlike the conifers, to present a broad, efficient light-collecting surface to the sun during favourable periods, while minimising their water loss during dry periods. Thus deciduous trees are far better energy collectors. On the other hand, they must pay the relatively high price, energetically speaking, of replacing all their leaves annually.

Some Leaf Modifications:

In some plants, leaves are modified as spines, which are hard, dry, and non-photosynthetic. (The terms “spine” and “thorn” are often used interchangeably; however, thorns are technically modified branches.) In other plants, such as the garden pea, leaves are modified as tendrils. In many plants, leaves are succulent; that is, they are adapted for water storage.

Other leaves are specialised for food storage. A bulb, such as the onion, is a large bud consisting of a short stem with many leaves modified for food storage. (In the onion, food is stored as sugar rather than as starch.) The “head” of a cabbage also consists of a compressed stem bearing numerous thick, overlapping leaves. In some plants, the petioles become thick and fleshy- Celery and rhubarb are two familiar examples.

In the flower-pot plant, which is an epiphyte (a plant that grows on other plants but is not parasitic on them), some of the leaves form tubes. Rainwater and debris collect in these tubes. Roots of the plant grow into the leaf tubes and collect water and minerals from them.

In addition, ants colonise the tubes, adding to the nitrogen supply with their excreta and dead bodies. Among the most spectacular modified leaves are those of the carnivorous plants, such as the pitcher plant, Venus flytrap, and sundew. These plants capture insects by various means and then digest them with enzymes secreted by the leaf cells. The insects are an additional source of nitrogen for the plant.

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4. Term Paper on the Stem:

The Structure of the Stem:

The stem holds the leaves up to the light and provides for the transportation of substances to and from the leaves. The outer surface of a young green stem, like that of the leaf and the root, is made of epidermal cells. Like the leaf, the green stem is covered with a fatty cuticle, contains stomata, and is photosynthetic.

Ground Tissue:

The bulk of the tissue of a young stem is known as the ground tissue. Like the mesophyll of the leaf, ground tissue is composed mostly of parenchyma cells. The turgor of these cells provides the chief support for young green stems.

Stems also may contain specialised supporting tissues formed from collenchyma or sclerenchyma cells. Collenchyma cells differ from the thin-walled parenchyma cells of the stem in having primary cellulose walls that are thickened at the corners or in some other uneven fashion. Their name derives from the Greek word colla, meaning “glue,” which refers to their characteristic thick, glistening walls.

Collenchyma cells are often located just inside the epidermis, forming either a continuous cylinder or distinct vertical strips of supporting tissue.

Sclerenchyma cells are of two types – fibers and sclereids. The name is derived from the Greek skleros, meaning “hard.” Fibers, which are elongated, somewhat elastic cells, typically occur in strands or bundles arranged in patterns characteristic of the plant. These supporting cells are often associated with the vascular tissues.

Plant fibers such as flax, hemp, jute, sisal, and raffia have long been used in human artifacts, including baskets, rope and cloth. Sclereids, or stone cells, which are variable in form, are less common in stems than are fibers. Layers of sclereids are found more frequently in seeds, nuts, and fruit stones, where they form the hard outer coverings.

Sderenchyma cells differ from collenchyma in three respects:

(1) They have secondary walls;

(2) The walls often contain lignin, a complex macromolecule that impregnates the cellulose and toughens and hardens it; and

(3) The cells are sometimes dead at maturity, with only their cell walls remaining.

Conducting Tissues:

Phloem, conducts the products of photosynthesis, chiefly in the form of sucrose, from the leaves to the non-photosynthetic cells of the plant. In angiosperms, the conducting cells of the phloem are called sieve-tube elements.

A sieve tube is a vertical column of sieve-tube elements joined by their end walls. These end walls, called sieve plates, have openings leading from one sieve-tube element to the next.

The sieve-tube elements, which are alive at maturity, are filled largely with a watery cytoplasm. They are characterised by the presence of a protein-containing substance called P-protein (or sometimes, slime). In mature sieve-tube elements, P-protein lies along the inner surfaces of the cell. It is continuous from one cell to the next through the sieve plates. The nucleus of a sieve-tube element disintegrates as the cell matures, as do many of the organelles.

Sieve-tube elements are characteristically associated with companion cells, which are thought to provide nuclear control and energy for the sieve tubes. A sieve-tube element can function only if its cell membrane is intact, and it is likely that the companion cell helps to maintain the membrane.

Xylem conducts water and minerals from the roots to other parts of the plant body. It is customary to think of xylem as transporting water up and phloem as transporting sugars down, but if you think of the various shapes of plants you can see that water must also often be transported laterally, as along a tendril, or even down, as to the branches of a weeping willow. Conversely, sugars must often go upward, as into a flower or fruit.

In angiosperms, the conducting cells of the xylem are tracheids and vessel elements. Both of these cell types have thick secondary walls impregnated with lignin, and both are dead at maturity. Tracheids are long, thin cells that overlap one another on their tapered ends.

These overlapping surfaces contain thin areas, pits, where no secondary wall has been deposited. Water passes from one tracheid to the next through the pits. Vessels, which are much larger, also differ from tracheids in that their end walls contain one or more perforations or are broken down entirely.

Thus, the vessel elements form a continuous vessel, which is a more effective conduit than a series of tracheids. Lower vascular plants and most gymnosperms have only tracheids. Most angiosperms have both tracheids and vessels.

Both phloem and xylem tissue contain parenchyma cells, which store food and water, and supporting fibers.

Stem Patterns:

In green stems, the xylem and the phloem are usually arranged in longitudinal parallel strands, the vascular bundles, which are embedded in the ground tissue. In young dicot stems, the vascular bundles form a ring, the vascular cylinder, around a central area of ground tissue called the pith.

The cylinder of ground tissue outside the vascular bundles is called the cortex. Within each bundle, the xylem is characteristically on the inside, adjacent to the pith, and the phloem is on the outside, adjacent to the cortex. In monocots the vascular bundles are scattered throughout the ground tissue.

Special Adaptations of the Stem:

The stems of some climbing plants coil themselves around the structures on which they are growing. Others produce modified branches in the form of tendrils. (Tendrils may also be modified leaves.) The tendrils of English ivy, grape, and Virginia creeper are all modified stems.

Runners, such as those found in most varieties of strawberry, are long, slender stems that grow along the surface of the soil. Rhizomes are underground stems that often, as in the grasses, form buds and from these buds produce upright stems bearing leaves and flowers.

Stems also may be adapted for food storage. White potatoes are enlarged rhizomes known as tubers. In some species of plants that grow in arid environments, stems are adapted for water storage. The water- storing tissues consist of large parenchyma cells that lack chloroplasts; the stem of a cactus may be 98 percent water by weight.

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5. Term Paper on the Roots:

Roots anchor the plant and are specialised for taking up water and essential minerals. In an older plant, the root system may make up more than half of the plant body. The lateral spread of tree roots is usually greater than the spread of the crown of the tree.

In a study made on a four-month-old rye plant, the total surface area of the root system, including root hairs, was calculated by extrapolation to be 639 square meters, 130 times the surface area of the leaves and stem. Root growth is affected by soil conditions and availability of water. The deepest known root was that of a pine growing on sandy, porous soil. It had penetrated to a depth of about 6.5 meters.

The Structure of the Root:

The internal structure of the root is comparatively simple.

There are three concentric layers:

1. The epidermis,

2. The cortex, and

3. The vascular cylinder.

1. The Epidermis:

The epidermal cells of the root, which enclose the entire surface of the root, absorb water and minerals from the soil. As you would expect, they lack the cuticle found on the surface of the epidermal cells of the leaf.

They are also characterised by fine, threadlike outgrowths, known as root hairs. Root hairs are slender extensions of the epidermal cells themselves; the nucleus of the epidermal cell is often found within the root hair. In the study of the rye plant, the roots were estimated to have some 14 billion root hairs; placed end to end, they would have extended more than 10,000 kilometers.

Much of the water and minerals that enter the root are taken up through these delicate outgrowths of the epidermal cells. (However, in many species mycorrhizal associations seem to substitute for root hairs.

2. The Cortex:

The cortex occupies by far the greatest volume of the young root. The cells of the cortex are parenchyma cells, like those of the mesophyll of the leaf and the ground tissue of the stem; however, as you would expect, they lack chloroplasts.

They store starch and other organic substances. The tissue of the cortex contains many air spaces. Oxygen-containing air enters these spaces through the epidermal cells and is used by the cortical cells in respiration.

Unlike the rest of the cortex, the cells of the innermost layer, the endodermis, are compact and have no spaces between them. Each endodermal cell is encircled by a Casparian strip, a fatty band within the cell wall.

The strip is continuous and is not permeable to water. Therefore, water and dissolved substances, which pass freely around the other cortical cells and through their cell walls, must pass through the cell walls and, more important, the cell membranes of endodermal cells.

Water, oxygen, and carbon dioxide pass freely through cell membranes, but many ions and other substances do not. Therefore, the membranes of the endodermal cells regulate the passage of such substances into the vascular tissues of the root and so determine what enters the plant body.

3. The Vascular Cylinder:

The vascular cylinder of the root consists of the vascular tissues (xylem and phloem) completely surrounded by one or more layers of cells, the pericycle. Branch roots arise from the pericycle. In most species, the vascular tissues are grouped in a solid cylinder, as shown in figure 13.8.

In some, however, they form a hollow cylinder around pith, a central core of ground tissue. Figure 13.5 shows the details of the vascular cylinder of a buttercup.

Patterns of Root Growth:

The first root of a plant, which originates in the embryo, is called the radicle. In many dicots, this root develops into a taproot that, in turn, gives rise to lateral or branch roots. In monocots, the primary root is usually short-lived and the final root system develops from the base of the stem; such roots are called adventitious roots. (Adventitious describes any structure growing from an unusual place.) The adventitious roots and their branches develop into a fibrous root system.

Special Adaptations of Roots:

Aerial roots are adventitious roots produced from aboveground structures. Some aerial roots, such as those of English ivy, cling to vertical surfaces and thus provide support for the climbing stem. In some plants, such as corn, aerial roots serve as prop roots.

Trees that grow in swamps, such as the red mangrove and the bald cypress, often have prop roots. In swampy areas the soil is low in oxygen, and, in some species, the prop roots are believed not only to anchor the plant but also to supply the root cells with the oxygen needed for respiration.

Black mangroves have roots whose tips grow upward out of the mud and serve this aerating function. Many special root adaptations are found among epiphytes, such as the flowerpot plant. In some species of orchids, for instance, the root is the only photosynthetic organ.

Most roots are storage organs, and in some species the roots are highly specialised for this function, with an abundance of storage parenchyma. Beets, carrots, and sweet potatoes are examples of such roots.

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6. Term Paper on Reproduction in Plants:

Unlike the higher animals, whose reproduction is almost exclusively sexual, many plants reproduce both sexually and asexually. (Asexual reproduction is often referred to as vegetative reproduction.) Organisms produced by asexual reproduction are genetically identical to their single parent, whereas organisms produced by sexual reproduction, which involves meiosis and fertilization, are different from both parents.

Asexual Reproduction among Plants:

There are many forms of asexual reproduction among plants. One of the most familiar occurs by means of runners or rhizomes. Strawberries are a common example of plants that propagate by runners, as are spider plants, commonly grown as hanging plants.

Plants that reproduce by rhizomes include potatoes, many flowering garden perennials, such as lilies-of-the-valley, irises, and dahlias, and the sod-forming grasses of lawns and pastures. Both runners and rhizomes develop adventitious roots.

Many members of the lily family, which includes onions and tulips as well as lilies, reproduce asexually by bulbs. Some species of plants with arching stems, such as raspberries, develop new roots from stem tips that touch the soil.

New plants may form if the stem is subsequently broken, separating it from the parent plant. The leaves of some plants, such as African violets, develop adventitious roots when they are detached from the parent plant and may give rise to new individuals in that way. Some species of Kalanchoe produce plantlets in the margins of leaves, which later drop to the ground and develop into separate plants. If the taproot of a dandelion is injured or broken near the soil line, a callus (a mass of undifferentiated cells) forms, plugging the wound.

Eventually several new plants grow from this callus tissue. Thus both lawn mowers and grazing animals have highly beneficial effects on the dandelion population.

The capacity of many species of plants to reproduce asexually has been exploited in the development of cultivated varieties of plants for food or ornamental use. Such plants are, of course, genetically identical to the parent stock, and so vegetative reproduction is a way of preserving uniformity.

Many plants are reproduced by stem cuttings which simply involve sticking young stems in the ground and protecting them from drying out until adventitious roots appear. Rooting can often be facilitated by hormone treatment.

Another artificial form of plant propagation is grafting, in which a stem cutting is attached to the main stem of a rooted woody plant. Most fruit trees and roses are propagated in this way. Many economically important plants are sterile and can only be propagated vegetative; these include pineapples, bananas, seedless grapes, navel oranges, and numerous ornamental plants.

Sexual Reproduction:

The flower is the structure of sexual reproduction of the angiosperms. Unlike the gonads of animals, which are permanent organs that develop in the embryo, flowers are transitory, developing seasonally. A perfect flower, as it is called, consists of four sets of floral appendages, which grow in spirals or whorls; each floral part, evolutionarily speaking, is a modified leaf.

The outermost parts of the flower are the sepals, which are commonly green and obviously leaf like in structure. The sepals, collectively known as the calyx, enclose and protect the flower bud. Next are the petals collectively called the corolla; these are also usually leaf-shaped but are often brightly coloured.

They advertise the presence of the flower among the green leaves, attracting insects or other animals that visit flowers for their nectar or for other edible substances and so carry pollen from flower to flower. Calyx and corolla together are known as the perianth.

Within the corolla are the stamens. Each stamen consists of a single elongated stalk, the filament, and at the end of the filament, the anther. The pollen grains, when they are ripe, are released-often in large numbers-from the anther, usually through narrow slits or pores. The pollen grains are immature male gametophytes.

The centermost appendages of the flower are the carpels, which contain the female gametophytes. Typically a single carpel consists of a stigma, which is a sticky surface specialised to receive the pollen; a slender stalk, the style, down which the pollen tube grows; and a base, the ovary.

Within the ovary are the ovules, each of which encloses a female gametophyte with a single egg cell. When the egg cell is fertilised, the ovule develops into a seed. A single flower may have one carpel or several carpels. The carpels may be separate or fused together.

In some species, flowers are either male (staminate) or female (carpellate). Male and female flowers may be present on the same plant, as in corn, squash, oaks, and birches, or on different plants, such as the tree of heaven (Ailanthus), the date palm, and the American mistletoe.

Species in which both male and female flowers are borne on the same plant are known as monoecious (“in one house”); species in which the male and female flowers are on separate plants are known as dioecious (“in two houses”).

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7. Term Paper on Pollination:

For flowering plants, a new cycle of life begins when a grain of pollen-often brushed from the body of a foraging insect-comes into contact with the stigma of a flower of the same species. By the time this pollen grain is released from its parent flower, it usually consists of three haploid cells (two sperm cells and a “vegetative,” or tube, cell), enclosed by the thickened outer wall of the pollen grain.

Pollen is commonly produced in great quantities; the probability of any particular pollen grain reaching the stigma of an appropriate flower is very small. The pollen grain contains its own nutrients and has so tough an outer coating that intact grain thousands of years old have been found in peat bogs.

In lower plants, you will recall, there is a distinct cycle of alternation of generations in which the sporophyte produces spores that produce gametophytes that produce gametes, with gametophyte and sporophyte having separate existences.

In the course of plant evolution, the gametophyte stage has been steadily reduced and, in the angiosperms, all that remains of the male gametophyte is the tough, tiny pollen grain and the pollen tube. The sperm cells are the gametes.

Once on the stigma, the pollen grain germinates, and, under the influence of the tube nucleus, a pollen tube grows down through the style into an ovule. The ovule contains the female gametophyte, which has also become reduced in size in the course of evolution.

In many species it consists of seven cells, with a total of eight haploid nuclei. One of the seven cells is the egg, containing a single haploid nucleus.

On either side of the egg are two small cells known as synergids. At the opposite end of the gametophyte are three small cells, the antipodal cells, whose function, if any, is unknown.

The large central cell contains two haploid nuclei, called the polar nuclei because they move to the center from each end, or pole, of the gametophyte.

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8. Term Paper on Fertilization:

One of the sperm cells carried by the pollen tube unites with the egg. This fertilised cell, the zygote, develops into the young sporophyte, or embryo. The nucleus of the second sperm cell unites with the two nuclei of the central cell. From the resultant 3n cell, a specialised tissue called the endosperm develops. It completely surrounds and nourishes the embryo. These extraordinary phenomena of fertilization and triple fusion-together called “double fertilization takes place, in all the natural world, only among the flowering plants.

The seed consists of the embryo, which develops from the fertilised egg; the stored food, which consists of or derives from the endosperm; and the seed coat, which develops from the outermost layer or layers (integuments) of the ovule.

The ovule or ovules are contained within the ovary, which is the enlarged base of either a single carpel or fused carpels.

9. Term Paper on the Fruit:

As the embryo develops, the ovary wall (pericarp) develops into the fruit. As you know from your own observations, fruits have many forms. They are generally classified as simple, aggregate, or multiple, depending on the arrangement of the carpels from which they develop.

Simple fruits develop from one carpel or the united carpels of a single flower. Aggregate fruits, such as magnolia, raspberry, and strawberry, are formed from a number of separate carpels of one flower. Multiple fruits consist of the carpels of more than one flower.

A pineapple, for example, is a multiple fruit formed from a flower cluster of many separate flowers. The ovaries of these flowers become fused as they mature.

Simple fruits are by far the most diverse of the three groups. When ripe, they may be soft and fleshy or dry. There are three main types of fleshy fruit-the berry, the drupe, and the pome.

In the berry type, examples of which are tomatoes, dates, and grapes, there are one to several carpels, each of which may have one or many seeds. The inner layer of the pericarp is usually fleshy.

In the drupe, there are also one to several carpels, but each usually contains only a single seed. The inner wall of the fruit is stony and usually tightly adherent to the seed.

Some familiar drupes are the peach, cherry, olive, and plum. The peach is a typical drupe; the skin, the succulent, edible portion of the fruit, and the stone are three layers of the mature ovary wall. The almond-shaped structure within the stone is the seed. The coconut is a drupe whose outer covering is fibrous rather than fleshy.

A highly specialised sort of fleshy fruit is the pome, which is characteristic of the subfamily of roses that produces rose hips. The pome is derived from an inferior ovary in which the fleshy portion comes largely from the perianth. Apples and pears are pomes.

Dry fruits are classified into two groups, dehiscent and indehiscent. In dehiscent fruits, the tissues of the mature ovary wall break open, releasing the seeds. In indehiscent fruits, the seeds remain in the fruit after the fruit is shed from the parent plant.

There are several sorts of dehiscent fruits. A fruit derived from a single carpel in which the ovary wall splits down one side is known as a follicle; the fruits of columbines and milkweeds are examples. In the pea family (legumes), the ovary splits down two sides; the pod is the mature pericarp and the peas are the seeds. Other examples of dehiscent fruit are shown in figure 14.5.

Indehiscent fruits are characteristic of a great variety of plant families. The most common is the achene, a small, single-seeded fruit. Winged achenes, such as those found in the elm and the ash, are called samaras. The most familiar kind of indehiscent fruit is the nut, which resembles the achene but has a stony coat and is derived from a compound ovary (an ovary formed from fused carpels).

Examples of nuts are acorns and hazelnuts. Note that the word nut is used very indiscriminately in common speech: Peanuts are legumes; pine nuts are seeds; almonds and coconuts are drupes.

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10. Term Paper on Seed Dormancy:

The seeds of most wild plants require a period of dormancy before they will germinate. This genetic requirement ensures that the seed will “wait” at least until the next favourable growth period. Seeds can remain dormant and yet viable with the embryo in a state of suspended animation-for hundreds of years.

The record for dormancy, as far as is known, has been set by some seeds of Arctic tundra lupine found in a lemming burrow in the Yukon. Deeply buried in the permanently frozen silt, they are estimated to be at least 10,000 years old. But when a sample was planted, the seeds germinated within 48 hours.

The seed coat apparently plays a major role in maintaining dormancy. In some species, the seed coat seems to act primarily as a mechanical barrier, preventing the entry of water and gases, without which growth is not possible.

In these cases, growth is initiated by the seed coat’s being worn away in various ways—such as being washed by rainfall, abraded by sand or soil, burned away by a forest fire, decomposed by microbial action, or partially digested as it passes through the digestive tract of a bird or other animal.

In other species, dormancy seems to be maintained chiefly by chemical inhibitors in the seed coat. These inhibitors undergo chemical changes in response to various environmental factors, such as light or prolonged cold or a sudden rise in temperature, which neutralise their effects, or they may be washed or eroded away. Eventually, the embryo resumes growth.

The dormancy requirement in seeds apparently evolved only recently, geologically speaking, among groups of plants subjected to the environm­ental stress of increasing winter cold characteristic of the most recent Ice Age.

By this time-only 1 to 2 million years ago-the angiosperms were already a highly diversified group, and different populations responded to these pressures in different ways. This explains why even closely related plants have different mechanisms for maintaining and breaking dormancy.

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11. Term Paper on Plant Growth:

The “double fertilization” of angiosperms produces a 3n cell and the zygote (the fertilised egg cell). The 3n cell divides mitotically to produce endosperm, the tissue that nourishes the developing embryo and, in many cases, the young seedling. The fertilised egg cell also divides mitotically, and as the embryo grows, its cells begin to differentiate—become different from one another-and the embryo begins to take on a characteristic form, a process known as morphogenesis.

In its earliest stages, the embryo proper consists of a globular mass of cells. Suspensor cells, also formed from divisions of the fertilised egg cell, are believed to push the developing embryo into the endosperm and to be actively involved in the absorption of nutrients from the endosperm.

As development of the embryo proceeds, changes in its internal structure result in the beginnings of the tissue systems of the plant. At the same time, or slightly later, emergence of the one cotyledon in monocots, or the two in dicots, occurs. The stages in the development of a dicot embryo are shown in figure 14.6.

In the earliest stages of embryonic growth, cell division takes place throughout the body of the young plant. As the embryo grows older, however, the addition of new cells becomes gradually restricted to certain parts of the plant body – the apical meristems of the root and the shoot.

During the rest of the life of the plant, all the primary growth- which chiefly involves the elongation of the plant body-originates in these meristems.

The existence of such meristematic areas, which add to the plant body throughout the life of the plant, is one of the principal differences between plants and animals. Higher animals stop growing when they reach maturity, although the cells of certain “turnover” tissues, such as nails, hair, skin, or the lining of the intestine, continue to divide.

Plants, however, continue to grow during their entire life span. Growth in plants is the counterpart, to some extent, of mobility in animals. Plants “move” by extending their roots and shoots, both of which involve changes in size and form.

By growth, a plant modifies its relationship with the environment, turning toward the light and extending its roots. The sequence of growth stages in plants thus corresponds to a whole series of motor acts in animals, especially those concerned with the search for food and water. In fact, growth in plants serves many of the functions that we group under the term “behaviour” in animals.

Primary Growth in Plants:

A seed characteristically contains very little moisture (only about 5 to 10 percent of its total weight). Germination begins with a massive entry of water into the seed.

The seed coat ruptures and the young sporophyte emerges.

Primary growth, which begins immediately, involves the elongation of stems and roots, the formation of branches, and the differentiation of the conducting tissues and other specialised tissues of the young shoot and root. All primary growth originates in the apical meristems of the shoot and root.

Primary Growth of the Root:

The first part of the embryo to break through the seed coat, in nearly all seed plants, is the embryonic root, the radicle. Figure 14.7 diagrams the growing zone of the young root of a dicot. At the very tip is the root cap, which protects the apical meristem as the root is pushed through the soil.

The cells of the root cap wear away and are constantly replaced by new cells from the meristem. The cells in the meristem designated as apical initials are those that produce new cells. All the other cells in the root are the progeny of these relatively few meristematic cells. The apical initials are always capable of dividing.

Some of the daughter cells remain in the tip of the meristem to maintain the apical initial population. Others differentiate as they divide, some becoming cells of the root cap and others forming the complex tissues of the root. The maximum rate of cell division occurs at a point well above the tip of the meristem. Then, just above the point where cell division ceases, the cells gradually elongate, growing to 10 or more times their previous length, often within the span of a few hours.

This process of elongation without cell division is the principal cause of root growth, although, of course, growth is ultimately dependent on the production of the new cells that become part of the zone of elongation.

As the cells stop dividing, some begin to differentiate, forming first the sieve-tube elements, the conducting cells of the phloem, and then vessel elements, the conducting cells of xylem. At the level of xylem differentiation, the endodermis takes shape, and to the inside of the endodermis, the pericycle forms.

This tissue gives rise to branch roots. At about this same stage, the epidermal cells differentiate and begin to extend root hairs into the crevices between the soil grains. This same basic pattern of growth is seen in the first root of a seedling and is repeated over and over again in the growing root tips of a tree 50 meters tall.

Primary Growth of the Shoot:

The organisation of the developing shoot tip is somewhat similar to that seen in the root-first, a zone in which most of the cell division takes place; next, a zone of cell elongation; and finally, a zone of differentiation.

These zones are not as distinct in the shoot as they are in the root, however, because of the regular occurrence of nodes and their appendages. Also, no covering analogous to the root cap is produced over the shoot tip.

As in the root, the outermost layer of cells develops into the epidermis. In the shoot, these cells have an outer covering, the cuticle. Other cells differentiate to form ground tissue, which in dicots includes the cortex and the pith, and the primary vascular tissues-the primary xylem and the primary phloem.

The pattern of development is more complicated, however, than in the root tip since the apical meristem of the shoot is the source of tissues that give rise to new leaves, branches, and flowers. (At the time of flowering the apical meristem form the floral parts and ceases to exist.)

Here you can see the apical meristem, which is very small, and the beginnings-primordia-of leaves. Leaves are formed in an orderly sequence at the shoot tip. The leaf originates by the division of cells in a localised area along the side of the shoot apex.

The vascular tissue differentiates upward, becoming part of the general vascular system that connects the plant from root to leaf tip. In some species, leaves arise simultaneously in pairs opposite one another, as in our figure. In other species, the leaves occur spirally or in circles (whorls) at the nodes.

As the internodes elongate, the young leaves become separated so that the leaves clustered so tightly together around the apex will eventually be spaced out along the stem of the plant.

Buds, Flowers, and Branches:

As the growing tip of the shoot elongates, small masses of meristematic tissue are left just above the point at which the leaf joins the stem (the leaf axil). These new meristematic regions, the buds, remain dormant until after the growth of the adjacent leaf and internode is complete.

In many species, the buds do not develop at all unless the apical meristem of the shoot is damaged or removed.

In some species, some buds are destined to become lateral branches, or a specialised shoot, such as a rhizome or a tuber or a flower. In other species, whether or not a particular bud will become a flower or a branch is determined by environmental conditions, particularly day length.

Secondary Growth in Plants:

Secondary growth is the process by which woody plants increase the thickness of trunks, stems, branches, and roots after primary growth has ceased. The so-called “secondary tissues” are not derived from the apical meristems; they are the result of the production of new cells by the vascular cambium and cork cambium. The cambiums are called lateral meristems.

The vascular cambium is a thin, cylindrical sheath of tissue in between the xylem and the phloem. In plants with secondary growth, the cambium cells divide continually during the growing season, adding new xylem cells-that is, secondary xylem—on the outside of the primary xylem, and secondary phloem on the inside of the primary phloem.

Some daughter cells remain as a cylinder of undifferentiated cambium. As the tree grows older, the living parenchyma cells of the xylem in the center of the trunk die, and those vessels cease to function. This nonfunctional wood is called heartwood, as distinct from sapwood, which consists of living parenchyma cells and functional vessels.

As the girth of stems and roots increases by secondary growth, the epidermis becomes stretched and torn. In, response to this tearing process, a new type of cambium, the cork cambium, and forms from the cortex.

Cork (phellem), which is a dead tissue at maturity, is produced from the cork cambium. The cork cambium forms anew each year, moving further inward until; finally, there is no cortex left. The tissues outside the vascular cambium, including the phloem, constitute the bark.

Figure 14.7 shows the cross section of the stem of a young dicot, in which some secondary growth has taken place. In the center of the stem is the pith, composed of loosely packed parenchyma cells.

The first cylinders of tissues around the pith are layers of xylem, composed of vessels and other cell types. Around the outermost layer of xylem is a layer of meristematic tissue, the vascular cambium. Each year, during the growing season, the cambium undergoes a number of mitotic divisions, forming new xylem (secondary xylem) on its inner surface and new phloem (secondary phloem) on its outer surface. By this continuous formation of layers of xylem and (to a small extent) phloem, tree trunks increase their diameter as primary growth increases their height.

Season after season the new xylem forms visible growth layers, or rings. Each growing season leaves its trace, so that the age of a tree can be estimated by counting the number of growth rings in a section near its base. Since the rate of growth of a tree depends on climatic conditions, it is possible to determine from the width of the annual growth layers of ancient trees fluctuations in temperature and rainfall that occurred hundreds of years ago.

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12. Term Paper on the Adaptations to Climate Change:

The angiosperms evolved during a relatively mild period in the earth’s history. As the climate became colder, water became locked in snow and ice for part of the year. As a consequence, the angiosperms, which already possessed some adaptations to drought (perhaps because of highland origins), were placed under new environmental stress. Some did not survive, and some were pushed toward the Equator.

Those that did survive in colder, drier areas did so because of selection for existing characteristics that offered advantages in these relatively unfavourable environments. Chief among such characteristics is the capacity to remain dormant during periods when water is in short supply and when climatic conditions are unfavourable for delicate growing buds, shoots, new leaves, and root tips.

Depending on their characteristic patterns of active growth, dormancy, and death, modern plants are classified as annuals, biennials, and perennials.

Annuals, Biennials, and Perennials:

Among annual plants, the entire cycle from seed to vegetative plant to flower tossed again takes place within a single growing season. Annuals include many of our weeds, wild flowers, garden flowers, vegetables, and grasses and most other monocots.

Such plants usually show little or no secondary growth. All vegetative organs (roots, stems, and leaves) die, and only the dormant seed bridges the gap between one generation and the next. Plants with non-woody stems, such as most annuals, are known as herbs.

In biennial plants, the period from seed germination to seed formation spans two growing seasons. The first season of growth often results in a rosette of leaves near the soil surface, a short stem, and a root, sometimes a storage root, such as a carrot.

In the second growing season, extensive stem elongation, flowering, fruiting, and seed formation occur. This completes the life cycle and the vegetative organs die. Biennials may be herbaceous or woody.

Perennials are characteristically woody plants that live for many seasons and whose vegetative structures persist from year to year. In a woody plant, the stem increases in diameter and becomes firm due to the xylem inside. On the outside, a protective layer of dead tissue, the cork, accumulates.

The ancestral angiosperm is believed to have been a woody plant. Modern plants in favourable climates, such as a tropical rain forest, may live year after year with little change during the annual cycle, just as did the ancestral plants.

Perennials that live in areas where part of the year is unfavourable to growth show a variety of adaptations. Some, such as cacti and most conifers, undergo little apparent change, although their rates of metabolism, and therefore of growth, change with the seasons.

Most dicots living in the temperate zones undergo both structural and functional changes that allow them to take maximum advantage of the growing season and still survive the rest of the year. In some, all of the aboveground structures die, and the plants overwinter as underground roots, stems (rhizomes or tubers), or buds (bulbs). Others, including many of the common vines, shrubs, and trees, are deciduous.