Pollen grain

“ From clump to clump he flies, and through his help each one of the pale blue blooms is sure to get its food, each flower fertilized by the pollen of another.
Another beautiful provision is seen in the difference in size of the pollen grain of the two flowers, those of the high anthers being much larger than those from the lower anthers. These larger grains are intended for the high stigma, which they are sure of reaching, while those of smaller size, on the top of the tongue, which should happen to be wiped off on the high stigma, are too small to be effective for fertilization. ”

“ Chemists have determined that nectar is the heat and force-producer in the food of the bee, while pollen supplies its nitrogenous tissue-building qualities. It is evident that bees select certain pollens for their superior nutritive powers, just as in bread-making we prefer wheat to any other species of grain. In the kinds of pollen most in favour with bees a good microscope will reveal the fact that the pollen-grains are often accompanied by a certain amount of true farina, as well as essential oils, which must greatly enhance their food-value. ”

“ Before leaving the pollen-sac a division takes place in the pollen-grain into a vegetative cell or cells, from which the tube is developed, and a generative cell, which ultimately divides to form the male cells (see Angiosperms and Gymnosperms) .
When the pollen-grains are ripe, the anther dehisces and the pollen is shed. In order that fertilization may be effected the pollen must be conveyed to the stigma of the pistil. This process, termed pollination (see Pollination) , Pollination. is promoted in various ways,—the whole form and structure of the flower having relation to the process. ”

“ For example, in some strawberries the stamens are absent or useless; that is, they bear no good pollen. In such cases the grower must see to it that near by are strawberry plants that bear stamens, in order that those plants which do not bear pollen may become pollinated; that is, may have pollen carried to them. After the stigma has been supplied with pollen, a single pollen grain sends a threadlike sprout down through the stigma into the ovary. This process, if successfully completed, is called fertilization. ”

“ If the experiment with the geraniums is not practicable, the child can be told that the pollen is necessary to the development of the seed, that it falls upon the stigma and nourishes the little ovules down in the ovary, and that no seed can form without the aid of the pollen. All the seeds we plant in the flower gardens or in the vegetable gardens, and all the grain we sow in the fields, are produced by the help of pollen. All the peas and beans and other seeds we eat owe their existence in part to the pollen, and without it they could not develop. ”

“ The Pollen Tube Passing through the Style to the Ovary
To understand how the pollen substance finds its way to the ovule substance let us examine the pollen grain a little more carefully. Pollen grains are of many shapes, though usually they are globe-shaped, or football-shaped. Tiny as they are, the outer skin is often marked with grooves and ridges in a very ornamental manner. They have two skins, an outer hard one, a softer inner one. The outer skin is not equally thick and hard all over. ”

“ When a grain of pollen is placed by any agency whatsoever—wind, an insect, or a camel's-hair brush, as the case may be—on a neighboring stigma, it sends out a pollen-tube which penetrates the ovary and at last enters into and coalesces with the embryo seed itself. The fresh material thus added to the embryo seems to upset the dormant condition—allows the failing growth to continue. The seed swells, the fruit ripens, and a new plant is shed forth upon the earth, the product of two distinct prior individuals. ”

“ The tassel produces a great deal of pollen. You have, on passing through a patch of corn, noticed the pollen falling everywhere and covering everything. Ears of corn sometimes have as many as 1000 grains of corn to the cob. Each grain sends out one or more little silks beyond the shuck to catch some of the pollen. Should these little silks fail to catch the grains of pollen, no grains of corn would form on the cob. The father and mother natures must unite if little seed are formed. ”

“ The pollen-grains have remarkably thin transparent coats; when exposed to the air they shrivel up quickly; when placed in water they swell, and are then 8-10/7000 of an inch in diameter, and therefore of smaller size than the ordinary pollen-grains similarly treated, which have a diameter of 13-14/7000 of an inch. In the cleistogamic flowers, the pollen-grains, as far as I could see, never naturally fall out of the anther-cells, but emit their tubes through a pore at the upper end. I was able to trace the tubes from the grains some way down the stigma. ”

“ Thus the sterility appears to depend on other things besides the pollen, the climate, perhaps, having some effect. Sometimes, too, it happens that, if the home pollen grows, and then foreign pollen be applied, this last grows faster and crowds out the first sort.
The immense number of pollen grains produced by a single flower apparently militates against the saying that Nature allows nothing to be formed but what is needful. ”

“ In these the flowers are much larger owing to the increased size of the petals, which are of brilliant colour and of various shape. Pollen is mostly much reduced in quantity, since insects flying direct from flower to flower afford a far more economical mode of distribution than is offered by the wind. The pollen grains, moreover, are sticky and covered with tiny spines or knobs, to render them more liable to adhere to the body or head of an insect; the pollen grains of wind-fertilized flowers being, on the other hand, smooth, dry, and dust-like. ”

“ As soon as the flower is emasculated it must be securely tied up with a bag, to exclude pollen, as seen in Fig. 100. Netting of any kind is not safe in ordinary practice, for the pollen grains are small enough to pass through it. It often happens that the flower-stem is not strong enough to hold the bag, nor large enough to allow the bag to be puckered tightly about it. ”

“ Both present flowers not merely conspicuous in themselves from their size, form, and colour, but conspicuous also by reason of their grouping. Here, if anywhere, we should expect that beauty should justify itself by its utility. But the facts appear to be just the other way. The pollen is collected together into waxy masses, which are arranged in a very peculiar manner on the pistil; and the pollen tubes pass from the pollen grains whilst still enclosed within the anthers, and so bring about fertilization without the intervention of insect agency. ”

“ In these, and hundreds of other plants, the wind will blow great clouds of pollen through the air. When we stop to consider that a single, or at most a very few pollen grains are all that are necessary—in fact, are all that can be of real service—the enormous wastage of the male fertilizing substance, in order that mating be secured, gives us some idea of how prodigal is nature in this supreme function. ”

“ As each ovule requires at least one, probably several, pollen-grains, [18] and as the seeds produced by Orchids are so inordinately numerous, we can see that it is necessary that large masses of pollen should be left on the stigma of each flower. Even in the Neotteæ, which have granular pollen, with the grains tied together by weak threads, I have observed that considerable masses of pollen are generally left on the stigmas. ”

“ In this class the stamens and style unite into a column, and at the top of the column the pollen masses are situated; these are covered by the anther-cap, and in a cavity is the stigma with its viscid surface to receive the pollen grains.
So diverse and intricate are the forms of the flowers, and especially labellums, that there is little doubt that insect aid is necessary in their natural habitats to bring about pollination. It has been proved by the operations carried out in cross-fertilisation in gardens that no class of plants can be so readily crossed under artificial conditions. ”

“ The whole flower is much reduced in size; but what is much more important, an extremely small quantity of pollen has to be formed, as none is lost through the action of insects or the weather; and pollen contains much nitrogen and phosphorus. Von Mohl estimated that a single cleistogamic anther-cell of Oxalis acetosella contained from one to two dozen pollen-grains; we will say 20, and if so the whole flower can have produced at most 400 grains; with Impatiens the whole number may be estimated in the same manner at 250; with Leersia at 210; and with Viola nana at only 100. ”

“ Other pollen grains worthy of examination, are those of various lilies, of Eschscholtzia and of Scotch Fir; the last named have curious little air-bladders, for the purpose of rendering them more buoyant.
Many lowly plants thrive in weak sugar solutions, after the manner of pollen grains. The yeast plant is one of them. A very small portion of yeast, in a drop of sugar solution, will show us one of the simplest methods of vegetable reproduction. Yeast is a fungus and it is also a plant composed of only one cell. Under the microscope, it appears as a [108] colourless oval body. ”

“ The failure of pollen to germinate in the catkins at 100% humidity suggested the possibility that the catkin tissue might contain some substance which prevented germination of the mature pollen grains until after it was shed. ”

“ Cephus in Wheat and Rye. Barley occasionally withers after flowering, the grain atrophying from no known cause, terms like consumption given to the disease conveying no information.
Atrophy of young fruits is commonly due to the flowers not setting—i.e. some agent has interfered with the normal transference of the pollen to the stigma. This may be due to excessive rain washing out the pollen (e.g. Vine) , to a lack of the necessary insects which effect pollination, often seen in greenhouse plants; to the stamens being barren—e.g. ”

“ The pollen-tube grows from the stigma, through the “conducting cellular tissue” (style) , to the ovules. The pollen-grains contain two cells, a vegetative and a free generative cell. The latter passes into the pollen-tube and there divides into two, one of which is the sperm-nucleus. The female prothallium, which is intended to serve as nutritive-tissue, is formed after fertilisation. Archegonia are wanting. ”

“ In a cross-pollinated plant like maize this is not the case. The pollen is carried by the wind through long distances and varieties planted close together are continually intercrossed. The isolation of a particular type is not simple as in the case of wheat, but may be prolonged through many generations. Each prize ear selected for future planting will have had at least a few and possibly many of its seeds fertilized by pollen from less desirable strains. When these seeds are grown they of course again fertilize the seeds of the desirable plants with a frequency proportionate to their number. ”

“ The sticky fluid which causes pollen grains to cohere is found upon all of the legs, in the region of their brushes, although the pollen combs and auricles of the hind legs are likely to show it in greatest abundance, since nearly all of the pollen within each basket has [26] passed over the auricle, has been pressed upward and squeezed between the auricle and the end of the tibia and the pollen mass above, and by this compression has lost some of its fluid, which runs down over the auricle and onto the combs of the planta. ”

“ It might with the sweet-pea, but even if it did, the pollen from the same flower is not as good as the pollen from another flower from a different plant, and the seed would be poor and the plants would grow weaker every year. There are many insects that act as servants to the flowers, and the wind is one of the servants, too. It shakes the corn-tassel so that the pollen falls on the silk and makes the ear, and it carries the pollen of one stalk to the silk of another—sometimes from one field to another. ”

“ There are many current statements to the effect that all varieties, whether self-sterile [101] or self-fertile, are more fruitful and produce better fruit with foreign pollen than with their own. To old pear-growers, this seems to be putting it rather strong, but the statements come from accurate experimenters and observers and should have consideration. Cross-pollination, be it remembered, is not a cure-all for failures to set fruit. Unseasonable weather, lack of vitality in trees, various fungi, and no doubt other agencies, may be the cause of unfruitfulness. ”

“ In the mean while plenty of barren flowers are produced on a distinct root, on short straight stalks, from which they rise like little separate white bubbles, suddenly expanding when they reach the surface, and floating about in such abundance as to cover it entirely. Thus their pollen is scattered over the stigmas of the first-mentioned blossoms, whose stalks soon afterwards resume their spiral figure, and the fruit comes to maturity at the bottom of the water. ”

“ Not all the flowers produce seeds. Note that an apple tree may bloom very full, but that only relatively few apples may result (Fig. 207) . More pollen is produced than is needed to fertilize the flowers; this increases the chances that sufficient [153] stigmas will receive acceptable pollen to enable the plant to perpetuate its kind. At any time in summer, or even in fall, examine the apple trees carefully to determine whether any dead flowers or flower stalks still remain about the apple; or, examine any full-blooming plant to see whether any of the flowers fail. ”