about 260,000 known species of mosses, liverworts, ferns, herbaceous and woody plants, bushes, vines, trees, and various other forms that mantle the earth and are also found in its waters. Plants range in size and complexity from small, nonvascular mosses, which depend on direct contact with surface water, to giant redwood trees, the largest living organisms, which can draw water and minerals through their vascular systems to elevations of more than 100 m (more than 330 ft).
Only a tiny percentage of plant species are directly used by humans for food, shelter, fiber, and drugs. At the head of the list are rice, wheat, corn, legumes, cotton, conifers, and tobacco, on which whole economies and nations depend. Of even greater importance to humans are the indirect benefits reaped from the entire plant kingdom and its more than 3 billion years of carrying out photosynthesis. Today the world’s biomass is composed overwhelmingly of plants, which not only underpin all food webs but also modify climates and create and hold down soil, making what would otherwise be stony, sandy masses habitable for life.
Cell Structure and Function The tremendous variety of plant species is, in part, a reflection of the many distinct cell types that make up individual plants. Fundamental similarities exist among all these cell types, however, and these similarities indicate the common origin and the interrelationships of the different plant species. Each individual plant cell is at least partly self-sufficient, being isolated from its neighbors by a cell membrane, or plasma membrane, and a cell wall. The membrane and wall allow the individual cell to carry out its functions.
There are many variants of the generalized plant cell and its parts. Similar kinds of cells are organized into structural and functional units, or tissues, which make up the plant as a whole, and new cells (and tissues) are formed at growing points of actively dividing cells. These growing points, called meristems, are located either at the stem and root tips, where they are responsible for the primary growth of plants, or laterally in stems and roots, where they are responsible for secondary plant growth. Three tissue systems are recognized in vascular plants: dermal, vascular, and ground.
The dermal system consists of the epidermis, or outermost layer, of the plant body. It forms the skin of the plant, covering the leaves, flowers, roots, fruits, and seeds. Epidermal cells vary greatly in function and structure.
The epidermis may contain stomata, openings through which gases are exchanged with the atmosphere. Specialized cells called guard cells, which, through changes in their size and shape, alter the size of the stomatal openings and in effect, regulate the gas exchange, surround these openings. The epidermis is covered with a waxy coating called the cuticle, which functions as a waterproofing layer and thus reduces water loss from the plant surface through evaporation.
The vascular tissue system consists of two kinds of conducting tissues: the xylem, responsible for conduction of water and dissolved mineral nutrients, and the phloem, responsible for conduction of food. The xylem also stores food and helps support the plant.
The xylem consists of two types of conducting cells: tracheids and vessels. Elongated cells, with tapered ends and secondary walls, both types lack cytoplasm and are dead at maturity. The walls have pitsareas in which secondary thickening does not occurthrough which water moves from cell to cell. Vessels usually are shorter and broader than tracheids, and in addition to pits they have perforation.
The phloem, or food-conducting tissue, consists of cells that are living at maturity. The principal cells of phloem, the sieve elements, are so called because of the clusters of pores in their walls through which the protoplasts of adjoining cells are connected. Two types of sieve elements occur: sieve cells, with narrow pores in rather uniform clusters on the cell walls, and sieve-tube members, with larger pores on some walls of the cell than on others. Although the sieve elements contain cytoplasm at maturity, the nucleus and other organelles are lacking. Associated with the sieve elements are companion cells that do contain nuclei and that are responsible for manufacturing and secreting substances into the sieve elements and removing waste products from them.
The ground, or fundamental, tissue systems of plants consist of three types of tissue. The first, called parenchyma, is found throughout the plant and is living and capable of cell division at maturity. Usually only primary walls are present, and these are uniformly thickened. The cells of parenchyma tissue carry out many specialized physiological functionsfor example, photosynthesis, storage, secretion, and wound healing. They also occur in the xylem and phloem tissues.
The body of a vascular plant is organized into three general kinds of organs: roots, stems, and leaves. These organs all contain the three kinds of tissue systems mentioned above, but they differ in the way the cells are specialized to carry out different functions.
The function of roots is to anchor the plant to its substrate and to absorb water and minerals. Thus, roots are generally found underground and grow downward, or in the direction of gravity. Unlike stems, they have no leaves or nodes. The epidermis is just behind the growing tip of roots and is covered with root hairs, which are outgrowths of the epidermal cells. The root hairs increase the surface area of the roots and serve as the surface through which water and nutrients are absorbed.
Internally, roots consist largely of xylem and phloem, although many are highly modified to carry out specialized functions. Thus, some roots are important food and storage organsfor example, beets, carrots, and radishes. Many tropical trees have aerial prop roots that serve to hold the stem in an upright position.
Roots increase in length through the activity of apical meristems and in diameter through the activity of lateral meristems.
Stems usually are above ground, grow upward, and bear leaves, which are attached in a regular pattern at nodes along the stem. The portions of the stem between nodes are called internodes. Stems increase in length through the activity of an apical meristem at the stem tip. This growing point also gives rise to new leaves, which surround and protect the stem tip, or apical bud, before they expand. Modified leaves called bud scales usually protect apical buds of deciduous trees, which lose their leaves during part of the year.
Stems are more variable in external appearance and internal structure than are roots, but they also consist of the three tissue systems and have several features in common. Vascular tissue is present in bundles that run the length of the stem, forming a continuous network with the vascular tissue in the leaves and the roots. The vascular tissue of herbaceous plants is surrounded by parenchyma tissue, whereas the stems of woody plants consist mostly of hard xylem tissue. Stems increase in diameter through the activity of lateral meristems, which produce the bark and wood in woody plants. The bark, which also contains the phloem, serves as a protective outer covering, preventing damage and water loss.
Within the plant kingdom are many modifications of the basic stem, such as the thorns of hawthorns. Climbing stems, such as the tendrils of grapes and Boston ivy, have special modifications that allow them to grow up and attach to their substrate. Many plants have reduced leaves or no leaves at all, and their stems act as the photosynthetic surface. Some stems creep along the surface of the ground and serve to reproduce the plants through vegetative means; many grasses reproduce in this way. Other stems are borne underground and serve as food-storage organs, often allowing the plant to survive through the winter; the so-called bulbs of tulips and crocus are examples.
Leaves are the primary photosynthetic organs of most plants. They usually are flattened blades that consist, internally, mostly of parenchyma tissue called the mesophyll, which is made up of loosely arranged cells with spaces between them. The spaces are filled with air, from which the cells absorb carbon dioxide and into which they expel oxygen. The mesophyll is bounded by the upper and lower surface of the leaf blade, which is covered by epidermal tissue. A vascular network runs through the mesophyll, providing the cell walls with water and removing the food products of photosynthesis to other parts of the plants.
The leaf blade is connected to the stem through a narrowed portion called the petiole, or stalk, which consists mostly of vascular tissue. Appendages called stipules are often present at the base of the petiole.
Many specialized forms of leaves occur. Some are modified as spines, which help protect plants from predators. Certain groups of plants possess highly modified leaves that trap and digest insects, providing needed nutrients. Some leaves are brightly colored and petal-like, serving to attract pollinators to otherwise small, unattractive flowers. Perhaps the most highly modified leaves are flowers themselves. The individual parts of flowersstamens, petals, and sepalsare all modified leaves that have taken on reproductive functions.
Growth and Differentiation
The growth and differentiation of the various plant tissue and organ systems are controlled by various internal and external factors.
Rooted as they are in the ground, plants are commonly thought of as leading sedentary, vegetative, passive lives. A look, however, at the ingeniously developed interactions that plants have with their biological surroundings quickly corrects this notion.
Cooperation and Competition
Many plant species exist as separate male and female plants, and pollen from male flowers must reach the female flowers in order for pollination and seed development to take place. The agent of pollination is sometimes the wind (a part of the physical environment), but in many cases it is an insect, bat, or bird. Plants may also rely on agents for dispersing their seed. Thus, after pollination, cherry trees develop cherries that attract birds, which ingest the fruit and excrete the cherrystones in more distant terrain.
Plants have evolved many other mutually beneficial relationships, such as the nitrogen-fixing bacteria that occur in the nodules on the roots of legumes. Many prairie grasses and other plants that flourish on open land depend on various herbivores to keep forests from closing in and shading them.
In the competition among plants for light, many species have evolved such mechanisms as leaf shape, crown shape, and increased height in order to intercept the sun’s rays. In addition, many plants produce chemical substances that inhibit the germination or establishment of seeds of other species near them, thus excluding competing species from mineral resources as well as light.