THE STEM OR CAULOME. 73 
their contents they communicate important properties. (See 
also Laticiferous Vessels, page 57). 
d. Intercellular Substance.—A peculiar substance which was 
termed, from its position, intercellular substance, was formerly sup- 
posed to be universally distributed between the walls of the cells, 
glueing them together as it were ; and in some plants occurring 
in great abundance, as in many Alge, the horny albumen or 
endosperm of some seeds, and in the collenchymatous cells of 
the common Beet, Begonia (fig. 95, cl, cl), &c. But in all these 
cases this appearance is due to alterations and changes which 
have taken place in the cellulose forming the cell-wall and in the 
contents of the cell. Thus, in the Sea Wrack, it is caused by 
the enormous imbibition of water, which makes the outer part 
of the cell-wall swell up, and eventually to be converted into 
mucilage. Hence this special intercellular substance does not 
exist in plants. 
CHAPTER 3. 
ORGANS OF NUTRITION. 
HAvine now considered the elementary structures of plants, we 
proceed to describe in detail the various compound organs 
which they form by their combination. These, as already 
noticed (page 13), are arranged in two divisions, namely: 1. 
Organs of Nutrition; and 2. Organs of Reproduction. The root, 
stem, and leaves form those of nutrition, and the flower and 
its parts those of reproduction. Upon the whole, it is most 
convenient to commence our notice of the organs of nutrition 
with the stem. 
Section 1. THe Srem orn CAULOME. 
The stem may be defined as that part of the axis which at 
its first development in the embryo takes an opposite direction 
to the root, seeking the light and air, and hence termed the 
ascending axis, and bearing on its surface the leaves and other 
leafy appendages (jig. 20, ¢). This definition will, in numerous 
instances, only strictly apply to astem at its earliest development, 
for it frequently happens that, soon after its first appearance, 
instead of continuing to take an upward direction into the air, 
it will grow along the ground, or even bury itself beneath the 
surface, and thus by withdrawing itself from the light and air 
it resembles, in such respects, the root, with which organ such 
stems are, therefore, ordinarily confounded. In these cases, 
however, a stem is at once distinguished from a root by bearing 
