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Home , Embryo, Ovary (botany), Ovule

HOW ARE FORMED II transfer the from the anther to the to fertilize the . Sometimes the reproductive organs are formed in separate on the How Seeds same . Such are said to be monoecious. The , 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 , and the flowers of another plant of the species may have only pistils. To such A FLOWTLR exists to producc . plants the term dioecious is applied. For that, two organs are essential. The holly is an example. The produces pollen grains that later form the male ceils, or . A SEED is a ripened containing The stamen has a stalk, or filament, an . at the tip of which is the pollen sac, or A , in terms of origin, is a ripened anther. containing the seeds. The pistil, usually in the center of Both ovule and ovary are in the pis- the , is the female organ. Nor- til of the flower. To identify seeds and mally it has three fairly distinct parts: correctly, one has to follow the The ovary, which contains one or more development of these parts to . 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— and pet- contains ovules so small as to be scarcely als—are not directly involved in sexual visible without a magnifying glass. . 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, . Their main function is to pro- and the mature grain, or fruit, is single tect the 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 are arranged Above and inside the calyx are the together on a common , , known collectively as the co- which later becomes the cob of the 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 have glands—nectaries—in which a grows to reach the ovule. A grain of sweet liquid, called , 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 , 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. , the ''naked-seeded" plants, bear cones and produce seeds on the surface of cone scales. Among them are such 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; /, cones, and the pollen is produced in nuclei; m, ; 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 —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 , or 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 , the 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. and allies; and the seed Mosses have sporophytcs that are plants, the . 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 , 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. independent sporophyte with true makes its appearance in this group, roots, stems, and leaves. but the sex organs and -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 is usually quite small, which it divides to form a mass of cells, though still independent and self- HOW SEEDS ARE FORMED 13 sustaining. 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 . cells—pollen grains and embryo sacs— In seed plants, the last big step in thus have nuclei with the reduced or , 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 —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 ; 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- , 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; 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. nuclei by the sperm nuclei from the Special cells of the ovule and anther pollen tube; differentiate into embryo-sac-- growth of the fertilized egg and its cells and pollen-mother-cells, all hav- differentiation into an embryo plus a ing the diploid or sporophytic number surrounding coat—the seed; and of chromosomes. maturing of the seed, usually with The mother cells divide by a process an accumulation of stored food. called , which results in the Pollen grains are carried from the daughter cells receiving half the num- stamens to the stigma of the pistil by 14 YEARBOOK OF AGRICULTURE 1961

Watermelon from Flower to Fruit

GERMINATION

A, male flower; J^, female flower; O y pollinating {honey bee); a, stamens; b, stigma; c, style; dy petals {corolla); i, ovary {develops hito mature watermelon fruit); fj cross section of ovary; g, ovules; k, ovule at time of fertilization; iy egg; j, sperm nucleus; K, mature seeds {Honey Cream variety); /, portion of mature plant {) bearing male and female flowers and tendrils; my mature . HOW SEEDS ARE FORMED 15 insects, wind, or gravity. This crucial difl'erentiated cells in its early stages. step may be seriously impeded if As enlargement continues, three well- conditions are not right. defined structures are formed: The epi- An example: Honey bees pollinate cotyl, or young ; the , clover flowers in fields grown for seed or young ; and the one or two production. If the clover flowers dur- , or seed leaves. ing a long rainy period, the bees can- Usually the cotyledons of the embryo not fly, and the seed crop may fall far become thickened to permit storage of short of the normal because of inade- food materials, such as , , quate pollination. oil, or protein. The accumulation of Insects are attracted to flowers by stored food in the embryo or other odor, color, or nectar. Some of the parts of the seed usually signals ma- clovers produce great amounts of turity. The period of "filling" of the nectar. Much of the commercial honey embryo, or , is one of stress available in markets is made by bees on the mother plant, because large working in clover fields. This is an amounts of organic food materials example of partnership in Nature, in must be manufactured by the leaves which the bees, by pollinating the and transported to the developing clover blossoms, play a necessary part seeds. in seed production and in return re- Finally, enlargement of the embryo ceive nectar for their services. ceases, the parts become dry, and the The pollen grain germinates on the seed becomes a dormant living organ- surface of the stigma and produces ism prepared to withstand adverse con- a long, slender tube, which grows ditions. through of the style to the ovule. Two male nuclei, or sperms, move THE MATURE SEED we have discussed down each pollen tube to the ovule. so far from the standpoint of origin as One unites with the egg in the embryo a ripened ovule. Structurally, though, sac of the ovule; the other with the the seed is a resting embryo plant, two polar nuclei. This is called double which is surrounded by a seedcoat and fertilization. may have an endosperm. The fertilized egg develops into a The embryo has one or more cotyle- rudimentary plant, the embryo of the dons, which in many instances will seed, the starting point of the next serve as foliage leaves when the seed plant generation. The fertilized polar has germinated. nuclei develop into a tissue called the Between the cotyledons are located endosperm, which surrounds and nour- two growing points—the hypocotyl, ishes the growing embryo. which will produce the root, and the The endosperm in most seeds is ab- epicotyl, which will give rise to the sorbed completely by the embryo by shoot. the time the seed matures. Among the One of the wonders of the seed is plants whose seeds contain no endo- that the entire aboveground part of sperm are bean, watermelon, garden the plant develops from the tiny epi- pea, and pumpkin. The edible part of cotyl, and the elaborate root system the is endosperm. In corn, originates in the small hypocotyl. wheat, and other cereals, the endo- Because the early growth and en- sperm makes up a large part of the largement of epicotyl and hypocotyl nutriment in the seed. after germination of the seed will de- pend on the food supplies stored in AFTER FERTILIZATION, the embryo, the cotyledons and other parts of the which starts as a single cell, grows seed, the seed is important as a storage rapidly, and the ovule expands to ac- organ. commodate the enlarging structures The seedcoat, or testa, is developed within. The embrvo is a mass of un- from one or two outer layers or integu- i6 YEARBOOK OF AGRICULTURE 1961 Soaked (A) and Germinating (B) Seeds of Dent Corn (Left) and Great Northern Bean (Right)

em en / em

B ..^h

hi, flat side of grain; A2, longitudinal section of grain^ narrow side showing starchy endosperm stained dark with iodine—embryo not stained; A3, longitudinal section of grain, narrow side, showing embryo stained dark with IV, a dye that is specific for fats and oils. The starchy endosperm is uiistained; A4, side view of soaked bean; A5, unstained of bean with attached portion of seedcoat; Aß, cotyledon of bean stained with iodine to show high starch content—epicotyl and hypocotyl low in starch; A?, íongitu- dirial section of soaked bean showing two fleshy cotyledons. Bi and B2, germinating grains; B3 and B4, ger- minating beans; c, cotyledon; d, dent in grain; e, epicotyl; en, endosperm; em, embryo; h, hypocotyl^ hi, hilum; p, primary root; s, seedcoat; sr, secondary root. merits of the ovule. These layers form long fibers produced by epidermal cells a covering, whose function is to protect of the cotton seed cling to passing ob- the embryo against drying out, me- jects, and the seeds thus are carried chanical injury, and attacks by insects, from place to place. fungi, and bacteria. Usually the outer coat is hard and durable, and the inner THE SEED usually matures at the one is thin and membranous. Often same time the fruit ripens. the two seem to be fused into one layer. Fruits are classified as simple, aggre- The seedcoat and fruit wall may de- gate, or multiple. velop appendages or special structures The simple fruits are further classi- that adapt the seed to certain ways of fied as dry and fleshy. dissemination. In cotton, for example, In certain fruits, the wall, or peri- HOW SEEDS ARE FORMED 17 carp, which sometimes is called the to replace overmature individuals in seed vessel, is composed of three the forest. layers—exocarp, mesocarp, and endo- Many examples may be cited of the carp. interdependence of plants and The peach is a one-seeded fleshy based upon stored food in the seed. fruit whose pericarp has three layers— Notable is 's use of seeds for food. the outer skin (exocarp); the fleshy This point leads to a related one. edible part (mesocarp); and the hard, Despite the marvelous mechanisms bony pit (endocarp), which surrounds that produce and disperse seed, many the seed. of the improved varieties of crop plants One has to have some knowledge of would perish under natural conditions. fruit classification if he wants to iden- People, who need the seeds for food, tify seeds correctly as to origin and have bred and selected crop plants structure. The fact that a blackberry is adapted to their needs. Such improve- not a but an and ment and specialization have been watermelon is a hard-rinded berry possible only because the seed is a indicates that common terms do not product of sexual reproduction in the always indicate the true structure of a flower. The sexual reproduction means fruit. that the plant, represented by the We should remember that the fruit embryo of the seed, may show traits is a device for seed distribution and inherited from either or both parents that its structure is related to the way and consequently be different from in which seeds of each species are either. dispersed. That is the key to the improvement of useful, beautiful plants. FROM THE STANDPOINT of function, a seed is a device for the reproduction, THUS, seeds serve us in at least three preservation, increase, and dissemina- ways. A large part of agriculture has tion of the plant species. to do with producing seeds that are The many different ways in which used for food and as materials for many seeds are dispersed illustrate the com- other uses. Many seeds are grown for plexity of Nature's plan in providing planting, so farmers can have the best for the perpetuation of plant species. seed for the next crop. Seeds are the Dryness is a factor of great importance basis of all procedures to improve plants in this connection. At low moisture through breeding. content, the living embryo respires Plant explorers search all parts of very slowly, and some seeds remain the world for unusual plants that viable for many years, even if they are might furnish valuable characters for subjected to harsh conditions. the plant breeder. Resistance to dis- The food stored in seeds is also im- ease has been bred into many crop portant. The reserves supply energy to plants by use of a wild seed that in the embryo as it resumes growth dur- itself was worthless. Desert plants ing germination. The young frequently provide drought-resistance plant is thus given a start in its new characters valuable in breeding dry- location, often a long distance from land crops. the mother plant that produced the Man's dependence on the crops he seed. has developed through work with seeds Furthermore, the stored food has a behooves us to learn more about their part in Nature's plan of seed distribu- origin, structure, and function. tion, for it is attractive to animals. Squirrels bury many more acorns and JOHN W. MCKAY is a horticulturist at nuts in the soil than they ever consume the Plant Industry Station^ Beltsville, Md. as food. Some of these seeds sprout and He is a graduate of the University of Texas grow into young oak and hickory trees and University of California at Berkeley. 55GS8S°—Gl 3