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HOW SEEDS ARE FORMED II Transfer the Pollen from the Anther to the Stigma to Fertilize the Ovules

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HOW SEEDS ARE FORMED II Transfer the Pollen from the Anther to the Stigma to Fertilize the Ovules HOW SEEDS ARE FORMED II transfer the pollen from the anther to the stigma to fertilize the ovules. Sometimes the reproductive organs are formed in separate flowers on the How Seeds same plant. Such plants are said to be monoecious. The watermelon, cu- cumber, and other members of the Are Formed gourd family are examples. Individual plants of some species have one-sexed flowers—that is, flow- JOHN W. MCKAY ers on a plant may have only stamens, and the flowers of another plant of the species may have only pistils. To such A FLOWTLR exists to producc seed. plants the term dioecious is applied. For that, two organs are essential. The holly is an example. The stamen produces pollen grains that later form the male ceils, or sperms. A SEED is a ripened ovule containing The stamen has a stalk, or filament, an embryo. at the tip of which is the pollen sac, or A fruit, in terms of origin, is a ripened anther. ovary containing the seeds. The pistil, usually in the center of Both ovule and ovary are in the pis- the flower, is the female organ. Nor- til of the flower. To identify seeds and mally it has three fairly distinct parts: fruits correctly, one has to follow the The ovary, which contains one or more development of these parts to maturity. immature seeds, called ovules; above The botanical definition of a fruit is the ovary, a slender style, or tube; at much broader than the popular mean- the tip of the style, the stigma, on ing of the word. For example, the ma- which the pollen is deposited. ture bean pod is the fruit of the bean The stamen and pistil are called plant, and bean seeds are ripened essential organs because they are neces- ovules. The bean pod originates in the sary if there is to be seed. bean flower as a minute ovary, which Two other organs—sepals and pet- contains ovules so small as to be scarcely als—are not directly involved in sexual visible without a magnifying glass. reproduction. Some flowers do not Fruit and seeds are present in mini- have them. We call them accessory ature form in the flower as ovary and organs. A flower that has all four or- ovules, hence the importance of the gans is a complete flower. flower in the development of the seed. The sepals are the lowermost of the Each female flower in the corn plant four organs. Usually they look like has an ovary containing a single ovule, leaves. Their main function is to pro- and the mature grain, or fruit, is single tect the bud until it has developed seeded, and the ovary wall and the into a flower. The sepals collectively seedcoat are united to form a single are called the calyx. covering. Many ovaries are arranged Above and inside the calyx are the together on a common receptacle, petals, known collectively as the co- which later becomes the cob of the ear rolla. In many flowers they are brightly of corn. The long, slender silk is the colored. The petals of many flowers style through which the pollen tube have glands—nectaries—in which a grows to reach the ovule. A grain of sweet liquid, called nectar, is secreted. corn is a familiar example of a single- The colors, the nectar, and the odors seeded fruit that is commonly called a of essential oils produced by the petals seed. of many flowers attract insects, hum- All the important cereals are mem- mingbirds, people, and other creatures, bers of the grass family, and have one- which, with wind and gravity, may seeded fruits. 12 YEARBOOK OF AGRICULTURE T961 The large sunflower and other mem- A Typical Flower bers of the aster family, such as lettuce, dandelion, and aster, produce one- seeded fruits. By contrast, the ovary of the water- melon flower contains many ovules, which mature to produce the many- seeded watermelon (fruit). The plants that produce seed are in two natural divisions. Gymnosperms, the ''naked-seeded" plants, bear cones and produce seeds on the surface of cone scales. Among them are such trees as pine, fir, cypress, a-a-ûj petals (corolla); b~b¡ sepals {calyx); c, cedar, and redwood. In the gymno- receptacle; dy pistil {composed of: e, stigma; /, sperms, ovules are produced in female style; gj ovary); h, stamen (composed of: i, anther; j, filament); ¿, pollen^ kk, pollen tube; /, sperm cones, and the pollen is produced in nuclei; m, egg cell; n, ovule. male cones. Pollen is carried to the ovule-bearing female cones by the wind, and each ovule matures into a "naked" seed. This group has no struc- the young sporophyte—or asexual ture comparable with the fruit of the phase of the plantas life cycle. During angiosperms, the other division, which this period, the sporophyte gets its are the true flowering plants. food, water, and other essentials of life from the garnetophyte—the part of the THE SEED-PRODUCING plants—sper- plant which produces gametes, or sex matophytes—originated from lower cells—and thus is parasitic on it. forms of the plant kingdom through a The many-celled and differentiated long series of evolutionary changes in sporophyte is structurally adapted to both the reproductive and vegetative withstand drying out and is linked structures. with the beginning of the existence of The four great divisions of the plant plants on land. In water plants, such kingdom, beginning with the simplest as algae, the zygote is protected against and ending with the most complex, are drying, since it is bathed in water at all algae and fungi; liverworts and mosses; times. ferns and fern allies; and the seed Mosses have sporophytcs that are plants, the spermatophytes. about equal in size to the gameto- Algae and fungi lack difl'erentiation phyte but depend on the latter for of the plant body into true roots, nourishment. stems, and leaves, although some of the The ferns and fern allies are the first forms may have structures that resem- great group of plants that develop an ble those organs. Sexual reproduction independent sporophyte with true makes its appearance in this group, roots, stems, and leaves. but the sex organs and spore-produc- This is one of the most important ing structures are usually one celled steps in the evolution of the plant and primitive. Most of the algae live kingdom. The development of a vessel in water, and the simple, reproductive system, which allows water and food structures seem to depend on water for to be conducted rapidly through stems, functioning and distribution. roots, and leaves, was mainly respon- Liverworts and mosses are essen- sible for this advance. tially land plants. The zygote—the The sporophyte is the dominant fertilized egg—is retained in the fe- plant body in this group, and the male sex organs for some time, during gametophyte is usually quite small, which it divides to form a mass of cells, though still independent and self- HOW SEEDS ARE FORMED 13 sustaining. Spores are disseminated as l:>er of chromosomes characteristic of asexual todies when conditions are the species. Male and female game- favorable and develop into small, tophytes derived from the daughter separate gametophytes. cells—pollen grains and embryo sacs— In seed plants, the last big step in thus have nuclei with the reduced or plant evolution, the sporophyte is com- gametophytic number of chromo- pletely dominant over the gameto- somes. Fusion of sperm and egg at phyte. Spores are retained in special fertilization restores the double, or organs of the sporophyte (essential diploid, chromosome number in the parts of the flower), and male and fe- new embryo or sporophyte. male gametophytes arc formed within The goal of plant evolution seems to these organs. have been the vegetative development The fertilized egg in all lower plants and specialization of the sporophyte, develops immediately into the mature with consequent reduction in the ga- sporophyte, but in seed plants it grows metophyte. for a time and then goes into a dor- Sexual reproduction in seed plants, mant condition to form the seed. resulting in the formation of a young Some of the lower algae rriultiply resting sporophyte, the embryo of the only by mitotic cell division—a process seed, is highly efficient. The success of of exact splitting of the chromosomes, the seed plants in dominating the resulting in two identical cells where vegetation of the earth undoubtedly one existed before. can be ascribed to development of the Beginning with the higher algae and seed, which makes possible the dis- extending through the rest of the plant persal of plant species over wide areas kingdom, a process known as alterna- and their survival through periods of tion of generations is the framework of unfavorable environmental conditions. evolutionary change. That means that a sexual generation THE FORMATION of seed in the higher (gametophyte) alternates with an plants depends on processes of sexual asexual phase (sporophyte). In algae reproduction in the flower. One should and fungi, the gametophyte generation know the nature of these processes and is the dominant plant body, and the where they occur. sporophyte, usually quite small, is Six steps in the development of parasitic on the gametophyte. reproductive plant structures leading Evolutionary differentiation from to fomiation of seed are: this point on involved a gradual reduc- The formation of stamens and pistils tion in size of the gametophyte and in flower buds; consequent increase of the sporophyte. the opening of the flower, which In the seed plants, reduction of the signals the sexual maturity of these gametophyte generation has reached organs; the point at which the male gameto- pollination, which consists in trans- phyte (pollen tube) and the female fer of the pollen from the stamen to the gametophyte (embryo sac) are much pistil; germination of the pollen; and reduced and are parasitic on the sporo- formation of the pollen tube; phyte—an exact reversal of the rela- fertilization of the egg and polar tion of the two phases in algae.
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