Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/288

254 CONTRIBUTIONS TO place within the old ones. It is also certain that in this case the formative process accords with that above described. R. Brown and Meyen have enumerated many instances where they observed the cytoblast in very young pollen-cells. In Pinus, Abies, Podostemon, Lupinus and others, I have traced the development of the pollen after Mirbel perfectly; I have distinctly observed the cell-nuclei and their development into new cells within one another in Abies, never having missed the cytoblast in young cells.

Now if the pollen-grains be nothing more than converted leaf-parenchyma, if the anther be merely a metamorphosis of the leaf, we may certainly infer inversely that the process which we have observed in it, and which characterized the formation of the embryo and cotyledons (as prototypes of the leaf) will be again found in all foliaceous organs. For the same reason which was stated with respect to the later stages of the development of the embryo, actual observation is infi- nitely difficult in this case. I have nevertheless examined a great many buds in reference to this point, and have most decidedly convinced myself of the identity of the process both in the constantly elongating apex of the axis, and in the leaves which always originate somewhat beneath it. Succulent plants, the Aloineæ and Crassulacee, are best adapted for this purpose. Crassula Portulaca seemed to me most advantageous, for in it I first succeeded in separating some cells from their connexion, in whose interior young cells were already developed, without, however, entirely filling the parent-cell. But having once be- come familiar with the subject, I was afterwards able to detect these individualities from amongst the apparently semi-organised chaos in all other plants. Another circumstance indeed pre- sents itself here, which renders the subject much more difficult than in the case of the embryo. For, independently of the minuteness of the cells, their walls, in those parts of the plant which are just newly formed, still consist merely of jelly, and are so delicate that it is exceedingly difficult to separate the parts intended for examination without completely destroying the organization. (Compare plate I, figs. 22-4.)

This process is more easily perceptible in articulated hairs, and in such as have a head consisting of several cells, where the same appearances which I have so frequently observed in