THE AERATION OF PLANTS 105 
cells. Air makes its way into these spaces by a process of 
diffusion outwards from the cells abutting upon them, and 
very soon external orifices in the shape of stomata make 
their appearance. The various constituents of the air 
make their way into and out of the cell by a process of 
diffusion, being dissolved in the water of the cell-wall or 
escaping from such a moist mem- 
brane 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 py, 71,—Cexus spiirtina 
the tissue in the growing part of an Pe eee graces. 
organ begins to take place, the forma- 
tion of the intercellular spaces can be observed. In these 
regions they begin by a splitting of the wall between two 
contiguous cells or at the angles where three cells join 
(fig. 71). The crevice soon extends 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 Fic. 72.—CorTEx oF Root, sHowine Inter. 
. CELLULAR PASSAGES BETWEEN THE CELLS. 
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 occupy more space than the cells themselves (fig. 73). 
Light appears to influence their development somewhat, 
though no definite relation can be shown to exist between 
