Page:The Oak.djvu/34

 own independent walls, but the wall which divides any two of them belongs as much to one as to the other, and only here and there do we find a minute opening between three or more cells at the corners, and produced by the partial splitting of the thin wall. We may, if we like, regard the whole embryo as a single mass of material cut up into chambers by means of partition walls, which have a tendency to split a little here and there, much as one could split a piece of pasteboard by inserting a paper-knife between the layers composing it; what we must not do, is to suppose that these cells are so many separate chambers which have been brought into juxtaposition. In other words, the cell-wall separating any two of the chambers is in its origin a whole, common to both chambers, and the plane which may be supposed to divide the limits of each is imaginary only.

I have said that the embryo consists almost entirely of this mass of polygonal, thin-walled cells, and such is called fundamental tissue; but here and there, in very much smaller proportion, we shall find other structures. Surrounding the whole of the embryo, and following every dip and projection of its contours, will be found a single layer of cells of a flattened, tabular shape, and fitting close together so as to constitute a delicate membrane or skin over the whole embryo; this outer layer of the young plant is called the epidermis.

Whenever the cotyledons, or the radicle, or plumule are cut across transversely to their length, there are visible certain very minute specks, which are the cut