Fig. 81. Formation of polar bodies in a lichen, Basidiobolus ranarum. A, the two conjugating cells with the bill-like processes in which the nuclei lie. B, the nuclei dividing. ksp, the nuclear spindle. C, after the division into a polar body (rk) and a sex-nucleus (♂ k and ♀ k). D, after the union of the nuclei to form a conjugation nucleus (copk); the fertilized ovum is surrounded by envelopes and modified into a lasting spore. After Fairchild.
We have come to know the processes of fertilization among phanerogams chiefly through Strasburger, Guignard, and more recently through the Japanese botanist Hirase. The agreement with the animal process is surprisingly great, notwithstanding the notable differences in the external conditions of fertilization.
As is well known, the male cells in the highest flowering plants are not zoosperms but roundish cells, each of which, enclosed, together with a sister-cell—the so-called 'vegetative' cell—in a thick cellulose capsule constitutes a pollen-grain. The pollen-grains reach the stigma, under which, buried deep within the 'ovule,' the female sex-cell rests, enclosed in a long, sac-like structure called the 'embryo-sac' (Fig. 82, A). Beside it (eiz) there lie several other cells, usually seven in number, two of which, the so-called 'synergidæ' (sy), have their place at one end of the embryo-sac, just in front of the ovum (eiz). Probably these give off a secretion which exercises an attractive (chemotactic) influence on the male fertilizing body ('the pollen-tube'), and thus, so to speak, show it the way to the ovum.
Fig. 82. Fertilization in the Lily, Lilium martagon, after Guignard. A, the embryo-sac before fertilization; sy, synergidæ; eiz, ovum; op and up, upper and lower 'polar nuclei'; ap, antipodal cells. B, the upper part of the embryo-sac, into which the pollen-tube (pschl) has penetrated with the male sex-nucleus (♂k) and its centrosphere; below that is the ovum with its (also doubled) centrosphere (csph). C, remains of the pollen-tube (pschl); the two sex-nuclei are closely apposed. Highly magnified.
When a pollen-grain has reached the stigma it sends out a tube, usually after a few hours, which penetrates into the soft tissue of the style, and grows deep down into the interior of the ovule, ultimately penetrating as far as the embryo-sac through a special little opening in the covering of the ovule, the so-called 'micropyle' (Fig. 82B, pschl). Its blunt end is now closely apposed to this, so that the true sperm-nucleus (B, ♂k), surrounded by some protoplasm, can leave the pollen-tube and wander in among the cells of the embryo-sac. Later on we shall see that two generative nuclei migrate from the pollen-tube, but in the meantime we shall devote our attention only to one of them, the fertilizing nucleus, which immediately moves towards the ovum-nucleus and apposes itself closely to it. Then follows the fusion or conjugation of the two nuclei, which are alike in size and appearance, just as in the fertilization of the animal ovum (C, ♂ k and ♀ k). Whether in this case, too, the sperm-nucleus brings with it a central corpuscle, or whether, as Guignard believed he observed, the ovum retains its central corpuscle (C, csph), or finally, whether both modes occur, is not yet known with certainty. The fact that, as a rule, seeds capable of reproduction only form in an ovule when the stigma has been previously dusted with pollen, leads us to suppose that, in this case, as among animals, the ovum lacks something that is necessary to induce embryonic development, only retaining this power in very exceptional cases, namely, when adapted for parthenogenesis. And this something may very well be the dividing apparatus of the cell, the centrosome with the centrosphere. But whether this supposition prove correct or not, a nuclear spindle always forms simultaneously with the fusion of the two sex-nuclei into a segmentation nucleus, and this spindle is the starting-point of the young plant, thus exactly corresponding to the first segmentation of the animal ovum. It agrees with it also in the important respect that it again contains the full number of chromosomes—twenty-four in the lily—while the two nuclei, male and female, only exhibit half the number each, that is, twelve.
Thus a reduction in the number of chromosomes to half takes place in plants also, but it is not yet known with certainty whether this is brought about in the same way as among animals, namely, by reducing divisions. Without entering more fully into this still unsolved and very complex problem, I should like to state that I consider this very probable; indeed, I agree with the view of V. Häcker[13], that the reducing divisions of plants are only more difficult to recognize as such, and, furthermore, are often disguised by the fact that they often occur alongside of, or between divisions which are not reducing. If it were possible to reduce the number of chromosomes in a cell to half without the aid of cell-division, if, for instance, only half were to integrate again from the chromatin-network, this must have been quite as possible in the case of animal cells, and then, moreover, the single chromosome would not have had the significance of an individuality, and no special form of nuclear division would have been introduced to reduce their number. That it has been introduced seems to me to prove that it was necessary, and since it was so among animals, it could not have been dispensed with among plants either.
[13] See V. Häcker, Praxis und Theorie der Zellen- und Befruchtungslehre, Jena, 1899, pp. 144-5.