Flowering and Fruiting of Plants |
| Some basic knowledge of the structure of seed-forming plants, and particularly of the flower, is essential to visualize the marvelous contrivances and unique requirements for the union of the sex cells which give rise to the viable seed. Although each has a basic pattern, their intricate and diverse
modifications permit plant life in some form to survive over much of the surface of our globe.
The Plant Roots, stems, leaves, and even flower parts are sometimes concerned with asexual or vegetative reproduction. Particular sections of different plants are frequently preferable for vegetative reproduction, for example, the runners or stolons of the strawberry, the tuber of the potato, the bulb of the onion, the corm of the iris, the nodes or joints of the sugar cane, and the leaf of a violet. Reproduction in garlic is by bulbils, sometimes called cloves, that form in the flower head. Bulbils also form in the inflorescence of some agaves. Asexual reproduction in plants has certain advantages. The asexual offspring of a plant, usually referred to as clones, are genetically identical. An example would be cuttings taken from a grapevine, rooted and used to create an entire orchard of a single clone. The plants would be uniform in appearance, vigor, flowering time, fruit ripening time, and fruit quality. Asexual reproduction can be made at any time, even before the plant is mature enough to produce seeds, or with plants such as the sweet potato or sugar cane that normally set no seed under our climatic conditions. Asexual reproduction has some disadvantages. If there is a degree of self-sterility in the parent plant, this cannot be overcome by cross- pollination between the plants unless another compatible cultivar is interplanted. The use of asexual parts is sometimes bulky or otherwise less convenient than the use of seeds. Diseases and insects are more likely to be transferred on asexual parts than on seeds. Some plants cannot be easily or economically reproduced asexually. Some plants reproduce both asexually and sexually, and both types of reproduction have certain advantages from the plant standpoint. Sexual reproduction, in which insects or other external agents sometimes play a part, concerns the development of seed in the flower. The external agent's contribution depends upon construction of the flower and the compatibility of the flower with its own pollen. In sexual reproduction, cross-pollination can occur, leading to higher production or quality through more complete fertilization. It can also lead to hybrid vigor, or heterosis, from the crossing of two unlike plants to produce a more vigorous one. Such mixing of genes may also enable future generations to adapt to different environmental conditions, insuring their survival, as they have apparently done in the past (Leppik 1970a, b). Almost two centuries ago, after Knight (1799) had studied the effects of self-fertilization in plants, he concluded that no plant can maintain itself with self-fertilization for an unlimited number of generations. In a figurative sense, it would seem as if Nature abhors self-fertilization and constantly strives ingeniously to achieve cross-pollination within the species. In numerous plants, selfing is permitted only after all efforts at cross-pollination have failed. Selfing is the plants final attempt to survive until favorable opportunity for crossing can occur. Again, figuratively speaking, Nature orders the plant: "Become fertilized, cross- fertilized if you can, self-fertilized if you must." The Flower The flower has a simple basic pattern, but with seemingly infinite variations. Typically, the flower is composed of the sexual organs, protected by delicate colorful petals that form a tube or crownlike corolla, and which in turn are supported and partially protected by the usually green, more durable sepals, collectively called the calyx. The calyx and corolla combined are referred to as the perianth. There may be leaflike bracts just below the sepals. The male part (or androecium) of the sexual organs are the stamens, which consist of the hairlike filaments bearing the pollen-producing anthers on the extremities. At the appropriate time, these anthers dehisce or split open and disgorge the male element, the numerous microscopic and usually yellow grains of pollen. The size of pollen grains varies from 4 to 6 microns for the little forget-me- not (Myosotis sylvatica Hoffm., family Boraginaceae) (Meeuse 1961*) to the relatively gigantic 350-micron grain of Cymbopetalum odoratissimum Rodr., family Annonaceae (Walker 1971), or the 2,550 by 3.7-micron tubelike grain of the water-pollinated eel grass (Zostera marina L., family Naiadaceae) (Wodehouse 1935). The size of the majority of pollen grains is in the 25- to 50-micron range. (1 micron = 0.001 mm). The shape and sculpturing of pollen grains is even more diverse, and their characteristics are used in the identification of the plant source of the pollen (Wodehouse 1935, Zander 1935, 1937). The amount of pollen produced per flower varies from only 32 grains in the four-o'clock (Mirabilis jolapa L., family Nyctaginaceae) (Kerner 1897*, v. 4, p. 98), to several spoonfuls in the blossom of the Abyssinian banana (Musa ensete G. Mel., family Musaceae) (Pryal 1910). The female part (or gynoecium) of the flower is the pistil, consisting of the ovary with one to numerous ovules and, extending from the ovary, the style with the receptive portion, the stigma, on or near the tip. The pistil may be composed of one or more carpels. The ovary produces the fruit and the ovules the seeds. The fruit on some plants--for example, certain citrus or bananas--may develop without viable seeds. Some flowers, like that of the coconut, produce only one seed. A watermelon may contain 1,000 seeds. The extreme example seems to be the orchid (Cyenoches chlorochilon [=C. ventricosum var. chlorochilon (Klotsch) P. H. Allen] ) with 3,770,000 sporelike seeds only 470 to 560 microns long (Ames 1946, Marden 1971). Typically, the ovary, with its style and stigma, occupies the central portion of the flower, which is surrounded by the stamens. The size of the flower varies from 1.5 to 2.0 mm for Pilostyles thurberi Gray, family Rafflesiaceae (Munz and Keck 1959) of Southwestern United States, to 1,000 mm or more for the jungle flower of Sumatra in the same family (Rafflesia arnoldii R. Br.), which weighs almost 25 pounds (Kerner 1896*, v. 1, pp. 202 - 204). Flower petals vary in color through all shades from black to white, but they are rarely green. They vary in shape from that of the simple spring beauty (Claytonia virginica L.) to the intricately ornate orchids. Likewise, flowers vary in aroma from the seemingly odorless pomegranate to the highly aromatic sweetclover or the repulsive Rafflesia arnoldii. The stalk or stem on which a cluster of flowers develop is referred to as the peduncle. In the cluster, the stalk of an individual flower or floret is called the pedicel. The end of the pedicel on which the flower parts rest is called the receptacle. Depending upon the arrangement of flowers within the floral cluster or inflorescence, they may be referred to collectively as a catkin, corymb, head, panicle, raceme, spadix, spike, or umbel. A flower with both pistil and stamens present is called a complete, perfect, or hermaphrodite flower. Frequently, one or more of the sexual parts will be missing, vestigial, or nonfunctioning. If this is the case with the male elements but the pistil is normal, the flower is referred to as pistillate or female. If the pistil is in any way nonfunctional but the stamens produce viable pollen, the flower is referred to as staminate or male. If both pistillate and staminate flowers are on the same plant but distinct from each other, the plant is said to be monoecious. Corn, with its pollen-producing stamens (the tassel) on the top of the plant and the pistils and ovaries (silks and grains) several feet below, is a common example of a monoecious plant. If some of the flowers are perfect while others on the same plant are unisexual, the plant is referred to as polygamous. If the two sexes are on separate plants within a species or variety, it is referred to as dioecious. In some plants, the stamens mature before the pistil is receptive to pollen. Such plants are referred to as protandrous. If the pistillate part matures and ceases to be receptive to pollen before the anthers of the same flower release the pollen, the flower is referred to as protogynous. Plants that are either protandrous or protogynous are referred to as dichogamous. The avocado is a dichogamous plant that has both types of flowers but on different cultivars. A few plants have complete flowers, some of which never open. The pollen is released directly onto the stigma within the closed flower and self- fertilization results. Such flowers are referred to as being fertilized in the bud or cleistogamous flowers. The lemon has both completely normal and cleistogamous flowers. Finally, within some species, there are differences in arrangement of the sexual parts, for example, one flower will have high anthers and a low stigma, whereas other flowers, sometimes in the same cluster but more often on different plants within the species, will have low anthers and a high stigma. Such plants are referred to as heterogamous, and such flowers are referred to as pin and thrum types. Some plants are receptive to their own pollen; however, within the individual flower the pollen becomes mature either before or after the stigma is receptive. For pollination to take place, the pollen must be transferred from one blossom to another. In still other plants, their own pollen is unacceptable as is pollen from other plants of the same variety. Only pollen from another variety of the same or closely related species will cause set of fruit and seed. The mode of transfer of pollen from one plant to another or within the flower depends upon the species of plants. The flower usually opens early in the morning although in some plants (for example, alfalfa, citrus) opening occurs throughout the day, in others (for example, evening primrose) opening occurs late in the afternoon to twilight, and in still others (for example, the saguaro cactus) opening occurs during the night (McGregor et al. 1962). Some (for example, chicory and lettuce) only remain open a few hours; some (for example, cotton), from several hours to most of the day; some (for example, avocado), for 2 days; and some (for example, apple), for several days. The maximum time for a flower to remain open is probably reached in certain orchids which, if not pollinated, remain fresh 70 to 80 days (Kerner 1896*, v. 1, p. 395). There are many more characters that flowers possess, essential for botanists in plant identification, but which do not contribute directly to plant pollination and are not included here. Nectaries and Nectar Secretion Flowers frequently have one or more nectaries, although nectaries are rarely mentioned in botanical descriptions of plants. Nectaries vary in size from microscopic to the 11-inch nectary of the orchid (Angraecum sesquipedale Thou.) (Darwin 1877*). The nectary is most often located within the flower, usually at the base of the sexual column inside the circle of petals. In cotton, however, there is a nectariferous ring just outside the base of the petals on the inner base of the calyx. Nectaries are also found outside the flower, on the stem or leaves. Nectar secretion within the flower usually starts about the time the flower opens and ceases soon after fertilization. Secretion of nectar on the stems and leaves is not influenced directly by flowering and may continue for several weeks. The amount of nectar secreted varies from infinitesimal in numerous species to more than an ounce in the orchid Coryanthes spp. (Kerner 1897*, u. 2, p. 172) and in Protea mellifera Thunb., which natives in Africa reportedly remove and drink (Langstroth 1913 and Holmes 1963). Nichol (1952) reported that the nectar of the Agave parryi Engelm. flower stalk was gathered by Indians in the Southwest and used as a sirup. Numerous bee specialists have calculated the amount of nectar produced in the flowers of various crops. For example, McGregor and Todd (1952*) calculated that the cantaloupe flowers on 1 acre produced 1.7 pounds of nectar in 1 day, whereas alfalfa flowers on 1 acre produced 238 pounds in 1 day. Pollination and Fertilization Certain words associated with pollination are frequently, but sometimes incorrectly, used. For example, a plant may be spoken of as self-fertile or self-compatible if it can produce fruit without the need for the transfer of pollen to it from another cultivar so that no interplanting of cultivars is necessary. Such a plant may not necessarily be self- pollinating. An external agent, such as the wind or insects, may be necessary to transfer the pollen from the anthers to the stigma within the flower or between flowers on the same plant. If the plant is not receptive to its own pollen, it is self-sterile. Even self-pollinating plants are frequently benefited by cross-pollination, the transfer of pollen from one flower to another. They may also benefit from having the pollen more thoroughly transferred and distributed over the stigma at the most receptive period. A plant is cross-compatible if it can normally be pollinated with pollen of another cultivar, but it is cross-incompatible if it is not receptive to pollen of certain cultivars. Horticulturists have sometimes based their decision on the pollination requirements of a cultivar by bagging one or a few branches of the cultivar. If the set of fruit within the bag was somewhat comparable to that of open branches they concluded that the cultivar was self-fertile. In such a test, a 5- to 15-percent difference would most likely not be detected, yet such a difference could be of great economic importance to the grower of the crop. When the stigma is receptive to pollen, it is coated with a colorless, relatively tasteless stigmatic fluid. If viable, compatible pollen comes in contact with this moist stigma, it adheres, germinates, and sends a pollen tube bearing the tube nucleus and the two sperm nuclei down through the style into the ovary and, finally, into one of the ovules. Fertilization follows this pollination process by the sexual union of one of the two sperm nuclei of the pollen grain and the egg nucleus of the ovule to form the fertilized egg or zygote. Through this process of sexual union, a viable seed is formed that is capable of producing another complete plant. In general, the sooner pollination can occur after a flower opens the greater the likelihood that fertilization of the ovule and seed development will occur. As time elapses, the pollen may be lost to insect foragers, wind, gravity, or damage by heat, moisture, or drying out. Also, processes may set in that result in the shedding of the fruit. Unlike asexual reproduction, which produces a plant basically identical to its parent plant, in fertilization following pollination each nucleus bears the genes of the plant from which it was derived; therefore, when they are combined the seed may not produce another plant exactly like that of either parent. For example, if the strawberry breeder is not satisfied with the type of plants he is obtaining asexually, he can transfer pollen from another variety to the stigma of an individual floret of the strawberry blossom of different selections, then save the particular seed that develops from that union to grow and be tested as a mature plant, which he studies for new and improved varieties. There is no way a breeder can forecast which cross will have improved qualities. The manner of sexual reproduction is one of the plant's most interesting characteristics. In some instances, the likelihood of successful reproduction and survival of the plant species through centuries of time seems extremely remote. For example, the yucca plant of the Southwest depends for its survival on a particular species of tiny moth that visits the blossoms at night, collects the pollen from the anthers, and transfers it to a depression in the tip of the stigma. After the pollen is packed into place, the moth lays a single egg on the side of the ovary. The pollen germinates, sends pollen tubes down through the style to the ovary, and fertilizes the ovules. About the time the ovules begin to form seeds, the larva hatches from the egg, burrows into the ovary, and begins to feed on the developing seeds, but it never consumes all of them. Some seeds survive, drop to the ground, and eventually produce new plants. The larva also reaches full size before the seeds mature. It burrows through the side of the seed pod, drops to the ground to pupate in the soil, and emerges as an adult the next year to pollinate new yucca flowers. Each is entirely dependent on the other for survival of the species (Riley 1878). This is an example of sexual reproduction brought about through insect pollination. The elimination of either this insect or this plant could result in the disappearance of the other. In other plants, the insect merely needs to crawl across the anthers and stigma of a flower to transfer pollen and cause fruit to set. In the cantaloupe, the pollen needs to be transferred only 1 or 2 mm to produce a fruit. If this transfer is not made, fruit is not produced. In the saguaro, or giant cactus of the Southwest, pollen must be transferred from the flower of one plant to a flower on another saguaro plant, sometimes several hundred meters away (Alcorn et al. 1961). In the incompatible fruit tree varieties, pollen must be transferred to them from the row or tree producing compatible pollen. If the ovary is divided into segments or locules, the styles and stigmas are also made up of corresponding lobes, carpels, or segments. When a pollen grain falls on one carpel, the pollen tube usually grows down it into its connecting locule of the ovary and fertilizes an ovule to form a seed. If for example, pollen fails to land on one of the three to five lobes of the cotton flower stigma, the corresponding locule or lock of the developing fruit will contain no seed - and consequently no lint that forms on a seed. Because each locule may contain about 10 ovules, at least 10 pollen tubes must safely penetrate them for complete development (Arutionova 1940). The watermelon may have 1,000 ovules in its three locules. This means that at least 1,000 pollen grains must land appropriately distributed on the three lobes, at the proper period of receptivity, if a perfectly formed melon is to develop. Because all pollen grains may not be fertile, or may not land at the appropriate time, many more than 1,000 should be desired by the grower. Mann (1943) observed that a few watermelon pollen tubes crossed from one carpel to another, because the watermelon has no stylar canal within a carpel. However, where the pollen was not well distributed over all the lobes, the fruit was frequently asymmetrical, especially at the blossom end. In most instances, pollen tube growth is limited to the carpel on which it originated. The rate of pollen tube growth depends upon its compatibility with the style. In some cases, the flower is not receptive to its own pollen but is receptive to pollen from other plants of the same cultivar (for example, alfalfa). In other instances, the pollen must come from another compatible cultivar (for example, numerous cultivars of apples). Frequently, when the plant is receptive to its own pollen the tube growth rate is less rapid than that of foreign pollen. In many plant species, as soon as fertilization occurs the stigma and style wither and the petals begin to fade in color and close. As an example, the alfalfa floret wilts within a few hours after pollination but may remain fresh more than a week if not pollinated. Some flowers close at night and reopen the following day, repeating this process for up to several days (McGregor and Alcorn 1959), but usually when the flower closes it never reopens. It either sheds or its fertilization stimulates fruit development. The Fruit Not all fruits develop simply as a result of ovule fertilization. In a few plants, the ovary will enlarge into a "fruit" without the stimulation of pollen. Such fruit development is referred to as parthenocarpic development. Parthenocarpic fruits are usually seedless, although not all seedless fruit arise parthenocarpically. For example, fertilization of the ovule may be necessary to prevent shedding even though the ovule may later disintegrate. Certain hormonal sprays will cause some plants to set seedless parthenocarpic fruit. Some citrus fruits are polyembryonic with one fertilized embryo and sometimes several other non-fertilized embryos that are stimulated to develop adventitiously within the same ovule. This is referred to as apomyctic development or apomyxis. The matured ovary, along with its contents and other structures intimately associated with it, is called the fruit. The fruit may be as varied as a grain of wheat, a walnut, an apple, a strawberry, or a watermelon. Fleshy fruits can be divided into types such as a berry, a drupe or stone fruit, or a pome fruit. A berry is defined as a fruit with a fleshy pericarp or ovary wall, surrounding one or more seeds. The grape, tomato, or watermelon can therefore be classed as berries. A pome fruit has a fleshy part surrounding a papery core. The apple is a common pome fruit. A drupe or stone fruit is one-seeded with a fleshy outer part and a stony inner part. The almond, cherry, olive, and peach are stone fruits. The strawberry is an aggregate fruit type, with each pistil developing into a tiny achene, and the entire mass, including the enlarged fleshy receptacle, developing as a unit. In the raspberry, the pistil develops into a drupelet. The receptacle of the raspberry does not enlarge, and upon harvesting of the ripe fruit it is not removed from the plant. This leaves the well-known hollow space in the raspberry. Development of the Knowledge of Plant Pollination The transfer of the male sex cells to the female portion of the flower, and the fusion of the cells in the ovule is a critical period in the life of a plant. In the manipulation of pollinating agents, man contributes to the efficiency of this fusion and to the insurance that the plant will be productive of fruit or seeds to his benefit. The basic principle of sex differentiation in plants may have been known as early as 1500 B.C. Goor (1967) stated that the Hebrews learned the value and art of date pollination from Egyptian and Babylonian experts. An Assyrian architectural relief of that period shows two divine creatures, each presumably holding a male date inflorescence over a female inflorescence (Faegri and van der Pijl 1966*). Kerner (1897*, v. 5, p. 655) stated, "When we consider that from time immemorial, Chinese and Japanese gardeners have produced asters, camellias, chrysanthemums, peonies, pinks, and roses, of which the majority are the results of crossing, we may assume with certainty that the practice of dusting flowers of one species with pollen of another species first came into use in those countries." Werkenthin (1922) quotes the Arabic writer, Kazwini, who died about 682 A.D., as saying that the date is the only tree that is artificially fertilized. Growers of dates today use this method to assure a set of dates in their groves (see "Dates"). However, if this indicated a recognition of sex in plants, the idea was not carried over to other plants. It was not until 1682 that a botanist, Nehemias Grew, stated that pollen must reach the stigma to insure the development of seeds. Apparently, however, he assumed that the stamens of a flower shed their pollen directly onto the stigma of the same flower (Dowden 1964). In 1694, Rudolph Jacob Camerarius published a letter, "De sexu plantarum epistole" (Werkenthin 1922, Grant 1949), in which he stated that based upon his experiments there are two different parts of the flower, the stamens and the pistil, and that they must work together to produce ripe seed. He concluded that these two parts represented true sexual organs (Faegri and van der Pijl 1966*). Actually, these had been recognized, and even the union of the two sexes was reported on centuries earlier by the Greek philosopher, Theophrastus (300 B.C.), "The Father of Botany" (Dzhaparidze 1 967). In 1750, Arthur Dobbs, communicated to the Royal Society of London that the pollen was the male element which, after falling upon the stigma, was capable of fertilizing the ovary. He further concluded that the pollen must come from its own species (Grant 1949). Watson (1751 ) reported that he transported date pollen 20 miles and pollinated a previously fruitless tree. In 1761, Koelreuter who is usually regarded as the discoverer of sexuality in plants, concluded that bees are agents in the transfer of pollen from the male to the female elements of the flower (Grant 1949). He was the first to cross-pollinate and produce a hybrid between two plant species (Sinnott 1946). In 1763, Arena also wrote rather fully on the subject of cross-pollination in plants and noted that it was carried out by insects (Lutz 1918). Sprengel (1793), however, was the first to really explore sex in plants, the important part played by pollinating insects, and the significance of cross-pollination in plant life. His work stimulated future work on sex in plants and the part played by insects. Thomas Andrew Knight (1799) showed the value of cross-pollination between plants and hybrid vigor: ". . . nature intended that a sexual intercourse should take place between neighboring plants of the same species." He noted that the location of the pollen within the blossom was ". . . generally well adapted to place it on the bodies of insects; and the villous coat of the numerous family of bees, is not less well calculated to carry it." The value of cross-pollination was later supported by Herbert (1837). Not until 1830, however, was the observation made by Amici on the formation of the pollen tube and its passage down the style and into the ovule. This was soon followed by recognition of the fact that there is sexual fusion between gametes in the ovule (Sinnott 1946). It was left for Darwin (1889*) to prove conclusively and to dramatize the importance of pollination in perpetuation and vigor maintenance of the plant species. He studied scores of species, using both hand and insects to pollinate the plants on which he measured the value and significance of cross-pollination. Much of the work on plant pollination since his time is based upon the theories he promulgated. Little has been added to the knowledge of pollination requirements of some plant species since his work was published. The first contribution of great importance on pollination from the United States was the discovery by Waite (1895) of self-sterility in pears and the need for insect-transfer of pollen between varieties. This initiated a new wave of interest particularly in fruit pollination, although many contributions on the value of pollination had already appeared (Crane 1876, Hutchinson 1886, Muller 1883*), and the various apicultural journals were beginning to extoll the virtues of the honey bee as the best pollinating agent. Benton (1896) recommended ". . . 4 or 5 well-populated hives of honey bees for every hundred large apple trees, the hives to be placed in or near the orchard." The renting of colonies for orchard pollination service had its beginnings the first decade of this century (Beuhne 1909, Stricker 1971). The acute need that developed for legume seed during World War II stimulated our Congress to establish the USDA Legume Seed Research Laboratory at Logan, Utah. The combined efforts at this laboratory established the value of honey bees in the pollination of alfalfa for seed production (Utah Agr. Expt. Sta. 1950). As a result, several hundred thousand colonies of honey bees are currently being used to pollinate this crop alone. The latest stage of development in the management of pollinating insects in production of crops is the large-scale use of wild bees, primarily the gregarious ground-nesting alkali bee (Nomia melanderi Cockerell) and the equally gregarious tube-nesting leafcutter bee (Megachile pacifica Panzer) (Bohart 1972, Stephen 1959). (See "Wild Bees.") Some other sources of information on pollination should be mentioned. Clements and Long (1923) spoke in general terms about pollination of numerous plant species. Hooper (1921), Hutson (1926), Kenoyer (1916), and Wellington et al. (1929) discussed the pollination of several specific crops, and Farrar (1931) became concerned about the strength of colonies of honey bees used for pollination. Other smaller but key papers published in the United States include those by Bohart (1960*), Bohart and Todd (1961*), Eckert (1959*), Hambleton (1944), Todd and McGregor (1960), and Vansell and Griggs (1952*). Some broad spectrum publications in other countries include: (Australia) Gale (1897); (England) Butler and Simpson (1953), and Free (1960); (India) Krishnamurthi and Madhava Rao (1963); (Italy) Giordani (1952); (Jamaica) Chapman (1964*), and Purseglove (1968*); and (Russia) Krishchunas and Gubin (1956*), Gubin and Khalifman (1958), and Kasiev (1964). For up-to-date knowledge and completeness, none of these surpasses the recent excellent publication by Free (1970*). He dealt thoroughly with the pollination needs and the management of pollinating insects to supply those needs for each family of plants he considered to be benefited by such pollination. Literature Cited ALCORN, S. M., MCGREGOR, S. E., and 0LIN, G.
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