16 THE OAK 
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 juxta- 
position. 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 
surfaces of extremely delicate strands or cords of 
