Plants evolved more than 430 million years ago from multicellular green algae. By 300 million years ago, trees had evolved and formed forests, within which the diversification of vertebrates, insects, and fungi occurred. Roughly 266,000 species of plants are now living. The two major groups of plants are the bryophytes and the vascular plants; the latter group consists of nine divisions that have living members. Bryophytes and ferns require free water so that sperm can swim between the male and female sex organs; most other plants do not.
Vascular plants have elaborate ater- and food conducting strands of cells, cuticles, and stomata; many of these plants are much larger that any bryophyte. Seeds evolved between the vascular plants and provided a means to protect young individuals. Flowers, which are the most obvious characteristic of angiosperms, guide the activities of insects and other pollinators so that pollen is dispersed rapidly and precisely from one flower to another of The same species, thus promoting out crossing. Many angiosperms display other modes of pollination, including self-pollination.
Evolutionary Origins Plants derived from an aquatic ancestor, but the evolution of their onducting tissues, cuticle, stomata, and seeds has made them progressively less dependent on water. The oldest plant fossils date from the Silurian Period, some 430 million years ago. The common ancestor of plants was a green alga. The similarity of the members of these two groups can be demonstrated by their photosynthetic pigments (chlorophyll a and b,) carotenoids); chief storage product (starch); cellulose- rich cell walls (in some green algae only); and cell division by means of a cell plate (in certain green algae only).
Major Groups As mentioned earlier, The two major groups of plants are The bryophytes- mosses, liverworts, and hornworts–and The vascular plants, which make up nine other divisions. Vascular plants have two kinds of well-defined conducting strands: xylem, which is specialized to conduct water and dissolved minerals, and phloem, which is specialized to conduct The food molecules The plants manufacture. Gametophytes and Sporophytes All plants have an alternation of generations, in which haploid gametophytes alternate with diploid sporophytes.
The spores that sporophytes form as a result of meiosis grow into gametophytes, which produce gametes–sperm and eggs–as a result of mitosis. The gametophytes of bryophytes are nutritionally independent and remain green. The sporophytes of bryophytes are usually nutritionally dependent on The gametophytes and mostly are brown or straw-colored at maturity. In ferns, sporophytes and gametophytes usually are nutritionally independent; both are green. Among The gymnosperms and angiosperms, The gametophytes are nutritionally dependent on the sporophytes.
In all seed plants–gymnosperms and angiosperms–and in certain lycopods and a few ferns, the gametophytes are either female (megagametophytes) or male (microgametophytes). Megagametophytes produce only eggs; microgametophytes roduce only sperm. These are produced, respectively, from megaspores, which are formed as a result of meiosis within megasporangia, and microspores, which are formed in a similar fashion within microsporangia. In gymnosperms, the ovules are exposed directly to pollen at the time of pollination; in angiosperms, the ovules are enclosed within a carpel, and a pollen tube grows through the carpel to the ovule.
The nutritive tissue in gymnosperm seeds is derived from the expanded, food-rich gametophyte. In angiosperm seeds, the nutritive tissue, endosperm, is unique and is formed from a cell that results from the fusion of the polar uclei of the embryo sac with a sperm cell. The pollen of gymnosperms is usually blown about by the wind; although some angiosperms are also wind-pollinated, in many the pollen is carried from flower to flower by various insects and other animals.
The ripened carpels of angiosperm grow into fruits, structures that are as characteristic of members of the division as flowers are. GYMNOSPERMS AND ANGIOSPERMS Gymnosperms Gymnosperms are non-flowering plants. They also make up four of the five divisions of the living seed plants, with angiosperms being the fifth. In gymnosperms, the ovules are not completely enclosed by the tissues of he sporophytic individual on which they are borne at the time of pollination. Common examples are conifers, cycads, ginkgo, and gnetophytes.
Fertilization of gymnosperms is unique. The cycad sperm, for example, swim by means of their numerous, spirally arranged flagella. Among the seed plants, only the cycads and Ginkgo have motile sperm. The sperm are transported to the vicinity of the egg within a pollen tube, which bursts, releasing them; they then swim to the egg, and fertilize it. Angiosperms The flowering plants dominate every spot on land except for the polar regions, the high mountains, and the driest deserts. Despite their overwhelming success, they are a group of relatively recent origin.
Although they may be about 150 million years old as a group, the oldest definite angiosperm fossils are from about 123 million years ago. Among the features that have contributed to the success of angiosperms are their unique reproductive features, which include the flower and the fruit. Angiosperms are characterized primarily by features of their reproductive system. The unique structure known as the carpel encloses the ovules and matures into the fruit. Since the ovules are enclosed, pollination is indirect. History
The ancestor of angiosperms was a seed-bearing plant that was probably already pollinated by insects to some degree. No living group of plants has the correct combination of characteristics to be this ancestor, but seeds have originated a number of times during the history of the vascular plant. Although angiosperms are probably at least 150 million years old as a group, the oldest definite fossil evidence of this division is pollen from the early Cretaceous Period. By 80 or 90 million years ago, angiosperms were more common worldwide that other plant groups.
They became abundant and diverse as rier habitats became widespread during the last 30 million years or so. Flowers and Fruits Flowers make possible the precise transfer of pollen, and therefore, outcrossing, even when the stationary individual plants are widely separated. Fruits, with their complex adaptations, facilitate the wide dispersal of angiosperms. The flowers are primitive angiosperms had numerous, separate, spirally arranged flower parts, as we know from the correlation of flowers of this kind with primitive pollen, wood, and other features.
Sepals are homologous with leaves, the petals of most angiosperms appear to be homologous with stamens, lthough some appear to have originated from sepals; and stamens and carpels probably are modified branch systems whose spore-producing organs were incorporated into the flower during the course of evolution. Bees are the most frequent and constant visitors of flowers. They often have morphological and physiological adaptations related to their specialization in visiting the flowers of particular plants.
Flowers visited regularly by birds must produce abundant nectar to provide the birds with enough energy so theat they will continue to be attracted to them. The nectar visited plants tends to be well protected by the tructure of the flowers. Fruits, which are characteristic of angiosperms, are extremely diverse. The evolution of structures in particular fruits that have improved their possibilities for dispersal in some special way has produced many examples of parallel evolution. Fruits and seeds are highly diverse in terms of their dispersal, often displaying wings, barbs, or other structures that aid their dispersal.
Means of fruit dispersal are especially important in the colonization of islands or other distant patches of suitable habitat. VASCULAR PLANT STRUCTURE Vegetative Organs A vascular plant is basically an axis consisting of root and shoot. The root penetrates the soil and absorbs water and various ion, which are crucial for plant nutrition, and it also anchors the plant. The shoot consists of stem and leaves. The stem serves as a framework for the positioning of the leaves, the principal places where photosynthesis takes place.
Plant Tissue The stems and roots of vascular plants differ in structure, but both grow at their apices and consist of the same three kinds of tissues: 1. Vascular tissue–conducts materials within the structure; it consists of two types: (1) xylem–conducts water and dissolved inerals (2) phloem–conducts carbohydrates, mainly sucrose, which the plant uses for food, as well as hormones, amino acids, and other substances necessary for plant growth 2. Ground tissue–performs photosynthesis and stores nutrients; the vascular tissue is embedded 3.
Dermal tissue–the outer protective covering of the plant Growth Plants grow by means of their apical meristems, zones of active cell division at the ends of the roots and the shoots. The apical meristem gives rise to three types of primary meristems, partly differentiated tissues in which ctive cell division continues to take place. These are the protoderm, which gives rise to the epidermis; the procambium, which gives rise to the vascular tissues; and the ground meristem, which becomes the ground tissue. The growth of leaves is determinate, like that of flowers; the growth of stems and roots is indeterminate.
Water reaches the leaves of a plant after entering it through the roots and passing upward via the xylem. Water vapor passes out of the leaves by entering intercellular spaces, evaporating, and moving out through stomata. Stems branch by means of buds that form externally at the point where he leaves join the stem; roots branch by forming centers where pericycle cells begin dividing. Young roots grow out through the cortex, eventually breaking through the surface of the root. Propagation An angiosperm embryo consists of an axis with one or two cotyledons, or seedling leaves.
In the embryo, the epicotyl will become the shoot, and the radicle, a portion of the hypocotyl, will become the root. Food for the developing seedling may be stored in the endosperm at maturity or in the embryo itself. NUTRITION AND TRANSPORT IN PLANTS The body of a plant is basically a tube embedded in the ground and xtending up into the light, where expanded surfaces–the leaves–capture the sun’s energy and participate is gas exchange. The warming of the leaves by sunlight increases evaporation from them, creating a suction that draws water into the plant through the roots and up the plant through the xylem to the leaves.
Transport from the leaves and other photosynthetically active structures to the rest of the plant occurs through the phloem. This transport is driven by osmotic pressure; the phloem actively picks up sugars near the places where they are produced, expanding ATP in the process, and unloads them here they are used. Most of the minerals critical to plant metabolism are accumulated by the roots, which expend ATP in the process. The mineral are subsequently transported in the water stream through the plant and distributed to the areas where they are used–another energy-requiring process.
Soil Soils are produced by the weathering of rocks in the earth’s crust; they vary according to the composition of those rocks. The crust includes about 92 naturally occurring elements. Most elements are combined into inorganic compounds called minerals; most rocks consist of several different minerals. They weather to give rise to soils, which differ according to the composition of their parent rocks. The amount of organic materials in soils affects their fertility and other properties. About half of the total soil volume is occupied by empty space, which my be filled with air or water depending on moisture conditions.
Not all of the water in soil, however, is available to plants, because of the nature of water itself. Water Movement Water flows through plants in a continuous column, driven mainly by transpiration through the stomata. The plant can control water loss primarily by closing its stomata. The cohesion of water molecules and their adhesion to the walls of the very narrow cell columns through which they pass are additional important factors in maintaining the flow of water to the tops of plants. The movement of water, with its dissolved sucrose and other substances, in the phloem does not require energy.
Sucrose is loaded into the phloem near sites of synthesis, or sources, using energy supplied by the companion cells or other nearby parenchyma cells. The sucrose is unloaded in sinks, at the places where it is required. The water potential is lowered where the sucrose is oaded into the sieve tube and raised where it is unloaded. Nutrient Movement Apparently most of the movement of ions into a plant takes place through the protoplast of the cells rather than between their walls. Ion passage through cell membranes seems to be active and carrier mediated, although the details are not well understood.
The initial movement of nutrients into the roots is an active process that requires energy and that, as a result, specific ions can be can be maintained within the plant at very different concentrations from the soil. When roots are deprived of oxygen, they lose their ability to absorb ions, a efinite indication that they require energy for this process to occur successfully. A starving plant–one from which light has been excluded–will eventually exhaust its nutrient supply and be unable to replace it.
Once the ions reach the xylem, they are distributed rapidly throughout the plant, eventually reaching all metabolical active parts. Ultimately the ions are removed from eh roots and relocated to other parts of the plant, their passage taking place in the xylem, where phosphorus, potassium, nitrogen, and sometimes iron may be abundant in certain seasons. The accumulation of ions by lants is an active process that usually takes place against a concentrations gradient and requires the expenditure of energy.
Carbohydrates Movement Carbohydrate movement is where water moves through the phloem as a result of decreased water potential in areas of active photosynthesis, where sucrose is actively being loaded into the sieve tubes, and increased water potential in those areas where sucrose is being unloaded. Energy for the loading and unloading of the sucrose and other molecules is supplied by companion cells or other parenchyma cells. However, the movement of water and issolved nutrients within the sieve tubes is a passive process that does not require the expenditure of energy. Plant Nutrients Plants require a number of inorganic nutrients.
Some of these are macronutrients, which the plants need in relatively large amounts, and others are micronutrients, those required in trace amounts. There are nine macronutrients: 1. Carbon 2. Hydrogen 3. Oxygen 4. Nitrogen 5. Potassium 6. Calcium 7. Phosphorus 8. Magnesium 9. Sulfur that approach or exceed 1% of a plant’s dry weight, whereas there are seven micronutrients: 1. Iron 2. Chlorine 3. Copper 4. Manganese 5. Zinc 6. Molybdenum 7. Boron that are present only in trace amounts. PLANT DEVELOPMENT Differentiation in Plant Plants, unlike animals, are always undergoing development.
Their cells do not move in relation to one another during the course of development, which is a continuous process. Animals undergo development according to a fixed blueprint that is followed rigidly until they are mature. Plants, in contrast, develop constantly. The course of their development is mediated by hormones, which are produced as a result of interactions with the external environment. Embryonic Development Embryo development in animals involves extensive movements of cells in elation to one another, but the same process in plants consists of an orderly production of cells, rigidly bound by their cellulose-rich cell wall.
The cells do not move in relation to one another in plant development, as they do in animal development. By the time about 40 cells have been produced in an angiosperm embryo, differentiation begins; the meristematic shoot and root apices are evident. Germination in Plants In the germination of seeds, the mobilization of the food reserves stored in the cotyledons and in the endosperm is critical. In the cereal grains, this process is mediated by hormones of the kind known as gibberellins, which ppear to activate transcription of the loci involved in to production of amylase and other hydrolase enzymes.
REGULATION OF PLANT GROWTH Plant Hormones Hormones are chemical substances produced in small quantities in one part of an organism and transported to another part of the organism, where they bring about physiological responses. The tissues in which plant hormones are produced are not specialized particularly for that purpose, nor are there usually clearly defined receptor tissues or organs. The major classes of plant hormones–auxins, cytokinins, gibberellins, thylene, and abscisic acid–interact in complex ways to produce a mature, growing plant.
Unlike the highly specific hormones of animals, plant hormones are not produced in definite organs nor do they have definite target areas. They stimulate or inhibit growth in response to environmental clues such as light, day length, temperature, touch, and gravity and thus allow plants to respond efficiently to environmental demands by growing in specific directions, producing flowers, or displaying other responses appropriate to their survival in a particular habitat. Tropisms Tropisms in plants are growth responses to external stimuli.
A phototropism is a response to light, gravvitropism is a response to gravity, and thigmotropism is a response to touch. Turgor Movement Turgor movements are reversible but important elements in adaptation of plants to their environments. By means of turgor movements, leaves, flowers, and other structures of plants track light and take full advantage of it. Dormancy Dormancy is a necessary part of plant adaptation that allows a plant to bypass unfavorable seasons, such as winter, when the water my be frozen, or periods of drought. Dormancy also allows plants to survive in many areas where they would be unable to grow otherwise.