Pollen, Pollination
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Read about Pollen, Pollination in the Standard Cyclopedia of Horticulture
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Pollen, Pollination. Pollen is the fecundating material contained in the anther, usually in the form of many very small grains. In many orchids it is in the form of masses of cohering parts or grains, termed pollinia. Pollen represents the male or fertilizing phase of reproduction in seed plants. Forms of pollen are shown in Figs. 3094-3097. All gymnosperms (conifers, and the like) and angio- sperms (true flowering or ovary-bearing plants) normally reproduce by means of seeds. For the fertilization of the ovule, in order that seed may result, the intervention of the pollen is necessary. The "dust of the flower" is therefore of far more interest to the horticulturist than this old popular name would imply. Studies in hybridization and self-sterility have long made evident the practical importance of a knowledge of pollen. Every plant provides for the production of this material, and usually in definite pollen-bearing parts termed stamens. The stamens are organs of the flower, and as essential as the carpels. The pollen is produced in definite sacs or compartments of the anther, comprising the tip of the stamens; and when the pollen is ripe, or mature, the fine grains are set free in quantity by the rupture of the inclosing sacs. The abundance of pollen produced may suggest wasteful management of the plant's resources, but a liberal supply of this substance is necessary. Although it requires but a single one of the small grains to fertilize a single ovule and produce a seed, pollen-grains are produced often a thousandfold more abundantly than ovules. The best offspring are usually produced when cross-fertilization occurs, and in the transfer of pollen from plant to plant it is only a small part which can reach its destination. There are many chances and such great losses that abundance of pollen is a necessary provision. In general, flowers are pollinated by the wind and by insects; that is, pollen is transported by these two agencies. Flowers principally dependent upon the wind for pollination are termed anemophilous, while those visited by insects are designated entomophilous. These distinguishing terms may also be applied to the pollen itself. Anemophilous pollen is of a more or less spherical form; readily yielding to the wind, and correlated with this is a dry and inadherent outer surface. Such is the case, for example, in the various families to which the oak, willow, grasses, and pine belong, all of which plants are devoid of any stock of brilliant color or rich odors that might attract bug, moth, butterfly, or bee. The pollen of the pine has even developed bladders, so as to be borne more lightly upon the wind. On the other hand, those plants largely dependent upon the visits of insects for pollination may have the pollen grains provided with some kind of spines, ridges, furrows, or viscid coatings that they may the more readily adhere to hairy limbs or other surfaces of the insect which may come in contact with them. Here, then, is to be found a reason for the beauty and specialization of external wall. In entomophilous pollen the elliptical form of grain predominates, but the general shape is extremely various; and the plants producing such pollen are usually provided with beauty of flower, fragrance, or other insect attraction. In order that the pollen which has been transported to the stigma may be effective, it must be healthy. Experiments have shown that weak and poorly nourished orchard trees often produce ineffective pollen. The nature of the season may also have much influence upon its character, continued rains causing great losses by preventing the maturity of this product as well as by mechanical injury and by precluding the winged carriers. Most plants have some special provision for the protection of the pollen against rain; that is, either by the closing of the flower under moist conditions, or by the location of the anthers in a sheltered tube, under projecting hairs, lobes, or other corolla appendages. The individual particles of pollen are in the form of delicate grains only readily visible in some quantity, as in powdery masses. At the time when they are set free, the grams are generally entirely distinct from one another, to be blown about by an accidental wind or carried by visiting insects. In some cases, however, the grams are bound together loosely or by means of delicate glutinous threads (Rhododendron); they may be closely united in fours (heath family); or the whole tissue of an anther or its divisions may remain intact as pollinia (some orchids, milkweed, and others). A particular species of plant will produce pollen quite constant in form and attire; but an aggregation of cultivated varieties originated from a single species may show considerable variation in this regard. Nevertheless, form, size, color, surface markings, texture of wall, and translucency of contents are not fixed qualities even for related genera or species. See Figs. 3094- 3097 for different forms of pollen. When the healthy pollen of one plant falls upon the ripe stigma of a plant of the same species, the grains germinate in the sugary excretion of the stigma by the protrusion of a tube which penetrates the style and effects fertilization as described under Fertilization (Vol. Ill, page 1221). Furthermore, it is well known that while the flowers of many plants may be readily fertilized by their own pollen, the offspring are stronger when pollen from another plant or another variety has had access to the flower. Sometimes pollen from a foreign variety is absolutely essential to the best fruit formation. This is particularly true of certain varieties of the pear. A poor quality of fruit can be prevented only by growing together different varieties. Again, although a plant may readily pollinate itself, yet the pollen from another plant or variety may be prepotent over its own. That is to say, if the plant be pollinated by its own pollen along with that of a foreign variety, that of the foreign variety will usually effect fertilization. This can be explained only on physiological grounds, and at present merely from a theoretical point of view. Any pollen penetrates and effects fertilization because it is attracted, first by substances in the style, and later by the egg-cell itself. When a foreign variety is prepotent it is so because it is more readily attracted, due, we may say, to a greater difference of potential between the two elements, the two elements from the same plant being more in equilibrium and less markedly attractive. As regards pollen from a foreign species, it seems to be the rule that hybridization does not occur so readily, and we must then assume that the differences have become so great as to cause repulsion. The detailed development of pollen is highly interesting and instructive on morphological grounds, but in this place a very brief account of the formation of the grains will suffice. The developmental phases in Big- nonia (Pyrostegia) venusta will serve as an example. A cross-section of the young flower-bud will show that in the anther-sac regions, semicircular layers of large well- nourished cells (called archesporial cells) are differentiated. These cells divide and the layer increases in extent, yet in this case it is always only one cell in thickness. When these cells have finally attained considerable size and provided themselves with a thick wall, they divide more or less simultaneously; and then each of these daughter-cells divides again by a division following quickly upon the first. Each cell has then formed four new cells within its original walls. The new cells remain thus united in fours until each is provided with a stout wall of its own, and then they separate. Each cell is then an immature pollen-grain, and technically a spore, that is, exactly homologous with the microspores of the vascular cryptogams. As a rule, before these pollen-grains are set free, another change occurs denoting maturity. This consists in the division of the nucleus of the spore in such a way that two cells of unequal size result (in some conifers several small cells are formed). On germination the large cell, which now incloses the smaller, protrudes the tube which penetrates the style; whereas the nucleus of the small cell divides into two. and one of these fuses with the egg-cell in the ovule, thus fertilizing it. B. M. Duggar. Pollination. In botanical usage, pollination is the transfer of pollen from the anther to the stigma. In horticultural usage, particularly with reference to orchard fruits, the term is often applied in a general way to designate all the influences concerned in the setting of fruit. For the benefit of those who are uninformed in botany it may be said that pollination is concerned primarily with the "essential organs" of the flower, the stamens and pistils. The stamens bear the pollen in their anthers, and they die after the pollen is shed. The pistils bear the ovary or seed-case, the style, and the stigma. The pollen falls upon the stigma. In some plants these organs are separated in different flowers or even on different plants. (Fig. 3098.) Aside from those cases in which the stamens and pistils are so intimately associated that the pollen falls directly upon the stigma, flowers are pollinated mainly in two ways: by wind and by insects. The grasses, sedges, and pines are examples of wind-pollinated plants. The flowers of wind-pollinated plants are usually inconspicuous and without nectar or fragrance. They produce a great abundance of light dry pollen, which is wafted away by the slightest breeze and is often carried many miles by a strong wind. The pistils of these plants are long and feathery, and thus are well adapted to catch flying pollen. The flowers of insect-pollinated plants, on the other hand, are usually showy, and have nectar or fragrance, or both. The pollen is more or less moist or sticky, so that it is not easily blown away. Insects are probably attracted by the showy colors and by the perfume, both of which bespeak the presence of nectar. As the insect reaches down for the nectar, which is near the bottom of the flower, some parts of its body are almost sure to become dusted with pollen. When the insect visits another flower some of this pollen may be brushed upon the stigma and a fresh supply received. This pollen likewise may be carried to another flower, and so on. Thus cross-pollination, or the transfer of pollen from the anthers of one flower to the pistil of another, is accomplished. Many flowers, notably the orchids, have special modifications of structure apparently developed for the purpose of securing cross-pollination by insects and preventing self-pollination. The bodies of some insects, also, have corresponding adaptations which insure the cross-pollination of certain flowers which they are in the habit of visiting most frequently. This correlation between flowers and their insect visitors has been the subject of extended observation. "Fertilization of Flowers," by Herman Muller, contains a bibliography of the subject up to 1886. For the distinction between fertilization and pollination, see the article Fertilization, page 1221. The value of crossing to plants was first clearly proved by Charles Darwin in 1859. From the observations of Kolreuter, Sprengel, Knight, and his own exhaustive experiments, Darwin showed that continued self-fertilization is likely to result in inferior offspring; while cross-fertilization, within certain limits, gives greater vigor to the offspring. Cross-fertilization between different flowers on the same plant usually has no appreciable advantage. The probable reason for this is that the plant resulting from the union of unlike parents, as in cross-fertilization between flowers on different plants, is more variable than one resulting from self-fertilization or crossing between different flowers on the same plant, and hence has a better chance of fitting into new conditions. Plants are endlessly modified to secure cross-fertilization and avoid self-fertilization. The principal means by which this end is gained are: (1) Special contrivances in the structure of the flower which favor cross-pollination. (2) A difference in the time at which the pollen matures and the stigmas become receptive in the same flower (dichogamy). This condition is very noticeable m some varieties of orchard fruits. The prematurity of the pistil is more common than the prematurity of the stamens. (3) Self- sterility, which is the inability of a flower to set fruit with its own pollen. Self-sterility is not usually due to a deficiency of pollen or to defective pistils. The pollen-grains often germinate on the stigma, but fertilization does not take place. The embryological reasons for this are not clearly understood. The ultimate cause of self-sterility in the grape has been studied by Dorsey. Cytological studies of the pollen of self- sterile varieties showed distinct degenerative processes in the generative nucleus, or arrested development previous to mitosis in the microspore nucleus. Dorsey concludes that self- sterility in the grape is not due to hybridity alone, as suggested by Beach, since there are both fertile and sterile hybrid varieties; but is due also to deep-seated influences operating to produce declinism and dioeciousness, the native species of grapes being mostly dioecious. Dorsey finds the nuclei of the pollen of many self-sterile varieties of native plums to be degenerated and disorganized. Degeneration of the pollen cannot be the main cause of self- sterility, however, since two self-sterile varieties may be mutually fruitful when planted together. About sixty species of plants are known to be more or less self-sterile. (4) The separation of the sexes in different flowers or on different individuals. It is thought by some that there is a gradual evolution among some kinds of plants toward unisexuality, and that adaptations for insect-pollination, dichogamy, and self-sterility are steps in this process. Self-sterility has an important economic aspect in the culture of certain fruits. It is common in varieties of pears, apples, plums, and grapes; it is uncommon or unknown in cherries, peaches, raspberries, currants, gooseberries, and strawberries. Whenever isolated trees or large blocks of a variety blossom full year after year, but drop most of the fruit before it is half- grown, the variety may be self-sterile, provided the failure cannot be attributed to excessive vegetative vigor, marked lack of vigor, disease (especially scab, brown- rot, and fire blight), insect attack, unfavorable weather during the blossoming season, or other untoward circumstance. Self- sterile varieties are detected experimentally by inclosing the unopened blossoms in thin paper sacks, and dusting the pistils, when receptive, with the pollen produced by these blossoms ; or by emasculating them and hand-crossing with pollen of the same variety. If very few fruits are produced from a large number of these selfed blossoms, but the variety fruits abundantly when crossed with other sorts, it is self-sterile. A few varieties of fruits are more or less self-fruitful, as distinct from self-sterile; they bear good fruit with their own pollen, but the fruits are seedless, as in the banana. Ewert found that many apples in Germany have this parthenocarpic development; that is, they grow without fertilization. It is not common in North American varieties of fruits. Self-sterility is not a constant factor in any variety. It appears to be almost as easily influenced by the conditions under which the plant is grown as is the shape or color of the fruit. A variety is frequently self-sterile in one locality and self-fertile in another. Waite found several varieties of Japanese plums self- sterile, but concluded, "With plums, as with other fruits, self-sterility is purely relative; under favorable conditions these varieties are able to set fruit without cross-pollination." Powell proved that in different parts of the Delaware-Maryland peninsula the Kieffer pear is self-sterile, partially self-fertile, or completely self-fertile. The Ben Davis apple is self-sterile in Vermont, according to Waugh, but self- fertile in Kansas, in the experiments of Greene. Bartlett pear is self-sterile in most of the Atlantic States, but usually self-fertile on the Pacific Coast. Beach found that varieties of grapes which are weakly self-fertile vary in this respect in different localities, and even in different parts of the same vineyard, being entirely self-fertile in one place and completely self-sterile in another. It is quite evident that the degree of adaptation of a variety to its environment of soil and climate has much to do with its ability to fruit abundantly with its own pollen. It is not possible, therefore, to give a list of varieties that are self-sterile, and another list of those that are self-fertile, that would have more than local application. There are certain sorts, however, that are less dependable in this respect than others. Out of eighty- seven varieties of apples tested in Oregon by Lewis, fifty-nine were self-sterile, fifteen self-fertile, and thirteen partially self-fertile. Powell found practically all the commencal varieties of apples in Delaware self- sterile, except several summer sorts. Some of the prominent commercial varieties that are usually more or less uncertain are: Arkansas (Mammoth Black Twig), Gravenstein, Grimes, Jonathan, King (of Tompkins), Limbertwig, Paragon, Northern Spy, Ortley, Rome, Spitzenburg (Esopus), Twenty Ounce, Winesap. Among those generally quite dependable are Ben Davis, Baldwin, Oldenburg, Rhode Island Greening, Yellow Transparent, Yellow Newtown. Anjou, Bartlett, Clairgeau, Clapp, Howell, Kieffer, Lawrence, Nelis, and Sheldon pears are frequently uncertain, while Angouleme (Duchess), Bosc, Flemish, and Seckel are usually self-fertile. Practically all the varieties of Japanese and native plums are self-sterile, the single exception, according to Waugh, being Robinson. Wild Goose and Miner are notoriously infertile. Hooper and Backhouse report that the European varieties are largely self-sterile in England, but in America the defection is confined chiefly to Coe, French Prune, and Italian Prune. The experiments of Close, Whitten, and Howard, indicate that all the leading varieties of peaches are self-fertile, and are not benefited by cross- pollination. In Germany, however, Ewart finds peaches "sparingly self-sterile." No cherries are known to be self-sterile, although Napoleon, Belle de Choisy, and Reine Hortense have that reputation among commercial growers. Of one hundred and forty-five varieties of grapes tested by Beach, thirty-one were self-fertile, forty-one self-sterile, and seventy-three uncertain. Brighton, Herbert, Lindley, Merrimac, Salem, Wilder, and other hybrid varieties are decidedly unfruitful with their own pollen; while Concord, Delaware, Diamond, Niagara, Winchell, and Worden are among those strongly self- fertile. Reimer found the Scuppernong and other varieties of the Muscadine grape so defective in pollen that they are fruitful only when planted near male vines of the Muscadine. No varieties of the quince, raspberry, currant, gooseberry, or strawberry have been found self-sterile, but many varieties of strawberries lack well-developed stamens and so must be planted near perfect-flowered sorts. A self-sterile variety often may be made fruitful by planting near it another variety to supply pollen; or by top-grafting part of the tree with cions of another sort. No benefit is derived from other trees of the same variety, even if brought from a distance, since all are but divisions of the same original seedling. In the selection of a pollinizer, several points must be considered: (1) The two sorts must blossom approximately at the same time in order that cross-pollination may be possible. The transfer of pollen from one variety to another is performed mainly by insects. Waugh and Backhouse have shown that practically none of the pollen of the plum and other stone-fruits is carried by wind, it being moist and sticky. The same is true of pears, but apple pollen is somewhat drier and is windblown to a slight extent. The honey-bee is the most important pollen-carrier. Hooper estimates that in England 80 per cent of the cross-pollination is done by the hive bee, 15 per cent by various wild bees, especially the bumblebee, and 5 per cent by miscellaneous insects. In tree-fruits it is necessary to select varieties that come into bearing at about the same age, otherwise one might be without cross-pollination for the first two or three years. Several state experiment stations have published lists of varieties blossoming at the same time, for the guidance of the planter. See New York (Geneva) Bulletin No. 407. (2) There should be an affinity between the two varieties, so that the self-sterile sort may find the pollen of the other acceptable. This can be determined only by hand-crossing. Beach found that the pollen of sefr-sterile varieties of grapes is practicably incapable of fertilizing other varieties; but this does not hold for tree-fruits since two self-sterile varieties planted together usually are mutually fruitful. Powell found no affinity between Paragon and Stayman apples; Kerr none between Wild Goose and Whitaker plums, and there are a number of other instances. Undoubtedly some varieties are more acceptable as pollinizers of a self-sterile variety than others. Spitzenburg apples produced by Lewis from Jonathan pollen averaged 144 grams in weight; from Baldwin pollen, 157 grams. In general, however, varieties of the same species that blossom simultaneously cross-fertilize readily, and there is no appreciable and constant difference in the fruit. (3) In commercial orchards the pollinizer should be a standard variety, valuable for market. (4) It should produce a large amount of pollen. Winesap produces little pollen; it would be unsatisfactory as a pollinizer for other sorts; Grimes, Ben Davis, and Rome are abundant pollen-bearers. Cross-pollinated fruits may be larger and heavier than self-pollinated fruits, but there is rarely any other influence. The shape, color, flavor, and keeping quality remain the same, regardless of the variety selected as a pollinizer. Kieffer pears pollinated with Seckel look and taste no different from Kieffer pears pollinated with Le Conte. Many supposed instances of the immediate influence of pollen have been recorded, but in most cases proof is lacking that the changes were not due to bud-variation. It cannot be doubted that this influence is exerted occasionally, but certainly much less frequently than is commonly supposed. In small orchards there is no need of mixing the varieties with special reference to cross-pollination. In orchards covering more than 10 acres, it is desirable to intersperse the varieties at regular intervals. It is more convenient in spraying, harvesting, and other orchard operations to plant the pollinizer in a solid row instead of mixing it in the rows with the self-sterile sort. If the pollinizer is not very valuable, one row in ten may be sufficient; but usually one in four to six is safer. If the pollinizer is a valuable variety, the two should be alternated in blocks of four to six rows each. It is not necessary to plant more than one variety as a pollinizer. Orchard pollination, however is a broader problem than the mere detection of varieties that are inclined to be unfruitful when planted alone, and discovering which are the best pollinizers for each of them. Experiments in crossing and observations in orchards indicate that nearly all varieties, whether self-sterile or self- fertile, will produce more or better fruit with foreign pollen than with their own. Powell found that some self-fertile trees of Kieffer in Delaware bore a light crop with their own pollen, 4 per cent of the self-pollinated blossoms producing fruit; but bore a much heavier crop when pollinated with Duchess, Lawrence, and other varieties, 76 per cent of the crossed blossoms producing fruit. Yellow Newtown is distinctly self-fertile in Oregon, yet Lewis noted a decided improvement in the fruit when Jonathan and Grimes pollen was used upon it. He concluded, "All varieties of pome fruits, at least of apples and pears, even though they may be termed self-fertile, are benefited by having other varieties planted with them as pollenizers." The benefit will usually more than offset the slight inconvenience in orchard management occasioned by this mixed planting. The chief economic problem for the experimenter, therefore, is to determine what commercial varieties may be planted together for best results; and the rational course for the fruit-grower is to practise mixed planting on the basis of these experiments. Those who wish to study the subject of fruit-pollination in greater detail should consult the following publications: Vermont Experiment Station Reports, 1896- 1900; Delaware Experiment Station Reports, 1900- 1902; Oregon Experiment Station Bulletin No. 104, Circular No. 20, Research Bulletin No. 1; New York (Geneva) Experiment Station Reports, 1892-1895; Bulletins Nos. 153,157,169,223; Wisconsin Experiment Station Reports, 1894-1896; New York (Cornell) Experiment Station Bulletin No. 181; North Carolina State Experiment Station Bulletins Nos. 201, 209: United States Department of Agriculture, Division of Vegetable Pathology, Bulletin No. 5; Minnesota Experiment Station Bulletin No. 144; Missouri Experiment Station Bulletin No. 117; Virginia Experiment Station Report 1909-1910. S.W Fletcher. CH
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References
- Standard Cyclopedia of Horticulture, by L. H. Bailey, MacMillan Co., 1963
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