16 THE OAK. 



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 pro- 

 duced 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 par- 

 tition 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 sepa- 

 rating 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 mem- 

 brane 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 



