THE ACORN AND ITS GERMINATION 15 
away from the structures just described, he will find that 
each is attached by a minute stalk to a sort of ridge just 
beneath the tiny plumule ; this ridge is sometimes termed 
the collar. He willalso see that the plumule and radicle 
fit closely into a cavity formed by the two cotyledons, 
and so do not interfere with the very close fitting of 
their two flat faces. 
Summing up these essential features of the structure 
of the ripe acorn and its contents, we find that the 
fruit contains within its pericarp (which is a more or less 
complex series of layers of which the outermost is hard) 
the seed; that this seed comprises a membranous testa 
enclosing an embryo; and that the embryo is composed 
of two huge cotyledons, a minute radicle, and a still 
more minute plumule; and that the tip of the radicle 
is turned towards the pointed end of the acorn, lying 
just inside the membranes. 
| Leaving the details of structure of the membranes 
until a later period, when we trace their development 
from the flower, I must devote some paragraphs to a 
description of the minute anatomy and the contents of 
the embryo as found in the ripe acorn, so that the 
process of germination may be more intelligible. 
Thin sections of any portion of the embryo placed 
under the microscope show that it consists almost 
entirely of polygonal chambers or cells, with very thin 
membranous walls, and densely filled with certain granule- 
like contents. These polygonal cells have not their own 
independent walls, but the wall which divides any two of 
