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 



