Pollen tube

“ To secure the production of seed, the first necessary step is pollination, or the transfer of pollen from the stamen to the stigma. When this is effected—the means will be considered immediately—and a pollen grain alights on the surface of the stigma, which is usually sticky or hairy to aid its retention there, the pollen grain commences growth, and sends out a slender tube (the pollen tube) , which pursues its way through the substance of the stigma, down the style, into the ovary, and from its tip a male cell passes out and fuses with the ovum. ”

“ 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 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. ”

“ When the pollen is wanted, the branches should be placed in a container of water and set near a window where sunlight will reach them. Usually, after one day of exposure to bright sunlight, the staminate blooms will expand and begin to shed their pollen. The pollen may easily be collected by allowing an extended catkin to droop inside a vial or test tube and then, as the catkin rests against its inner wall, tapping the outside of the tube sharply with a pencil to jar the pollen grains loose. A separate test tube must be used for each variety of pollen to be experimented with. ”

“ If we take into account Darwin's remarkable experiments showing that in "illegitimate unions" the pollen exerts a sort of poisonous action on the stigmas or ovules, it is possible to arrange a series of cases starting with perfectly legitimate pollinations where the pollen tube feeds as it descends the style on materials provided by the cells, and proceeding to cases where the pollen is more and more merely just able to penetrate the ovary and reach the ovules, to the extreme cases where no union at all is possible. ”

“ The full-grown embryo sac shows the same structure as that already described in Monotropa (page 276) , but as the walls of the full-grown ovule are thicker here, its structure is rather difficult to make out. The ripe stigma is covered with little papillæ (Fig. 95, F) that hold the pollen spores which may be found here sending out the pollen tube. By carefully opening the ovary and slightly crushing it in a drop of water, the pollen tube may sometimes be seen growing along the stalk of the ovule until it reaches and enters the micropyle. ”

“ It is down these style-branches that the pollen-tube passes on its way to the ovules or embryo seeds. The top of the style is expanded into a cupped stigma on which are many glutinous points. One can observe the browning and ripening of the stigma after pollen has been deposited by wind, bees or other agencies. When the ovules are fertilized, the forming fruit enlarges regularly unless it meets with misfortune or is crowded out for lack of room and nourishment. ”

“ Gazette, xlvi., 1908, regards this tissue as belonging to the nucellus.) At the time of pollination the long tubular integument secretes a drop of fluid at its apex, which holds the pollen-grains, brought by the wind, or possibly to some extent by insect agency, and by evaporation these are drawn on to the top of the nucellus, where partial disorganization of the cells has given rise to an irregular pollen-chamber (fig. 17, A, pc) . The pollen-tube, containing two generative and one vegetative nucleus, pierces the wall of the megaspore and then becomes swollen (fig. 17, B and C, pt) ”

“ The central part of the style is found to be either hollow or composed of a soft tissue through which the pollen tube can easily grow. Upon germination, the pollen tube grows downward through the spongy center of the style, follows the path of least resistance to the space within the ovary, and there enters the ovule. ”

“ It cannot move of itself, for one thing, and if it could it is too large to pass between the tissues of the style. So it simply sends down the long tube, which grows fast, pushing along through the style, whose tissues are rather loose, and carrying with it the only valuable part of the pollen grain, its living protoplasm. No ovule can possibly grow into a grain without this tiny bit of pollen. ”

“ Protandry is universal. In the bud the tips of the styles, covered by the sweeping-hairs, lie closely enveloped by the anther-tube; in the next stage the style grows through the tube and sweeps out the pollen as it proceeds; ultimately the stylar branches expand and the stigma is then prepared to receive the pollen. In many, the sensitiveness of the filaments assists in sweeping out the pollen at the exact moment of the insect visit. Regular self-pollination is found e.g. ”

“ The pollen spore bursts its outer coat, and sends out a tube which penetrates for some distance into the tissue of the ovule, acting very much as a parasitic fungus would do, and growing at the expense of the tissue through which it grows. ”

“ But the stamens, being shorter than even the germen, cannot discharge the pollen so as to throw it upon the stigma, as the flower stands always upright till after impregnation. And hence, without some additional and peculiar aid, the pollen must necessarily fall down to the bottom of the flower. Now, the aid that nature has furnished in this case, is that of the Tiputa Pennicornis, a small insect, which, entering the tube of the corolla in quest of honey, descends to the bottom, and rummages about till it becomes quite covered with pollen ”

“ Crüger within the stigma of a female plant they emitted "here and there a rudimentary tube." The petals then faded and the ovarium enlarged, but after a week it turned yellow and finally dropped off without bringing any seeds to perfection. This appears to me a very curious instance of the slow and gradual manner in which structures are modified; for the female pollen-masses, which can never be naturally removed or applied to the stigma, still partially retain their former powers and function. ”

“ Even before leaving the anther cells the grains in these cases have protruded their pollen tubes; these seek the pistil and penetrate to the ovules.
It might perhaps be supposed that, as the seed can be produced so easily, all plants would have cleistogamous flowers. But here
comes into play the fact that continual close fertilization is a great detriment and not a benefit, and that it is better in the end that flowers produce an apparently wasteful amount of pollen and take the chances of a cross, than to be more economical and be perpetually self-fertilized. ”

“ 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. ”

“ See in my future paper on Linum pollen and stigma recognising each other. If you will tell me that pollen smells the stigma I will try and believe you; but I will not believe the Frenchman (I forget who) who says that stigma of Vanilla actually attracts mechanically, by some unknown force, the solid pollen-masses to it! Read Asa Gray in 2nd Review of my Orchis book on pollen of Gymnadenia penetrating rostellum. I can, if you like, lend you these Reviews; but they must be returned. R. Brown, I remember, says pollen-tubes separate from grains before the lower ends of tubes reach ovules. ”

“ Why I am curious to know is because I put pollen into the ovarium of monstrous primroses, and now, after sixteen days, and not before (the length of time agrees with slowness of natural impregnation) , I find abundance of pollen-tubes emitted, which cling firmly to the ovules, and, I think I may confidently state, penetrate the ovule. But here is an odd thing: they never once enter at (what I suppose to be) the "orifice," but generally at the chalaza...Do you know how pollen-tubes go naturally in Primula? ”

“ The stigma should receive an abundant supply of pollen. No harm can come from supplying too much while if too little is applied, some of the seeds will not mature or even the fruit may not set. It is well known that in many plants, at least, the « 295 » pollen stimulates the development of the fruit-walls, as well as fertilizes the ovules, and a greater amount of pollen than is sufficient to produce the seeds themselves may therefore exert an important influence. Sometimes it is not necessary to use an instrument to transfer the pollen. ”

“ As we know that the long-styled form, when standing near plants of the other two forms and fertilised by insects, produces on an average 93 seeds per capsule, we see that this form, fertilised by its own two pollens, yields only between one-fourth and one-fifth of the full number of seed. I have spoken as if the plant had received both its own kinds of pollen, and this is, of course, possible; but, from the enclosed position of the shortest stamens, it is much more probable that the stigma received exclusively pollen from the mid-length stamens ”

“ On each side a membrane (cl, B) extends from the edges of the stigma to the filament of the anther, thus forming a membranous cup or clinandrum, in which the lower ends of the pollen-masses lie safely protected.
Each pollinium consists of two leaves of pollen, quite disconnected at their lower and upper ends, but united for about half their length in the middle by elastic threads. A very slight modification would convert the two pollinia into four distinct masses, as occurs in the genus Malaxis and in many foreign Orchids. ”

“ So long as the masses of pollen cells remain erect on the bee’s head, they cannot reach the stigma of any other orchis that it visits. As the sticky glands on the head contract, the pollen masses incline forward and become horizontal, so that they touch the next sticky stigma that is visited. The greater stickiness of the stigma detaches the pollen masses from the bee’s head, so that the flower is fertilized. It takes so long for the pollen masses on the bee’s head to assume the horizontal position that the insect has visited all the flowers on one orchis and then visits another plant. ”

“ 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. ”