THE AERATION OF PLANTS 111 
provided with access to the air by the formation. of spaces 
due to the splitting of certain of the cell-walls, and the 
subsequent partial separation of the cells. Air makes its 
way into these spaces by a process of diffusion outwards 
from the cells abutting upon them, 
while external orifices in the shape of 
stomata very soon make their appear- 
ance. The various constituents of the 
air make their way into and out of 4. 71.—Ceuts sprirrme 
each cell by a process of diffusion, a oe 
being dissolved in the water of the — Spacss. 
cell-wall or escaping from such a moist 
membrane according to the conditions existing, and the 
relation between the internal and external pressure of the 
particular gas in question. 
As soon as the differentiation of the tissue in the growing 
part of an organ begins 
to take place, the for- 
mation of the inter- 
cellular spaces can be 
observed. In these 
regions they begin by 
a splitting of the wall 
between two contigu- 
ous cells or at the 
angles where three 
cells join (fig. 71). 
The crevice soon ex- 
tends and may make 
its way for a considerable distance round any particular 
cell. The cavities so come into communication among the 
cells, each of the latter abutting upon a single one or upon 
several. While the tissue is young these are very narrow 
and slit-like, or are only visible at the angles when the cells 
are polyhedral. They rapidly become larger (fig. 72), and 
in some parts, particularly in the interior of the lower 
strata of the mesophyll of dorsiventral leaves, they may 
INTERCELLULAR 
Fig. 72.—Cortrx oF Root, suowina Inrer- 
CELLULAR PassaAGES BETWEEN THE CELLS. 
