THE DIFFERENTIATION OF THE PLANT-BODY 387 
A somewhat similar mechanism is provided in the case 
of terrestrial plants. At the time of their first formation 
all the cells are in close approximation to each other at 
all points of their surface. This condition is, however, 
only temporary ; dur- 
ing the early stages of 
growth the cell-walls 
split apart at particu- 
lar places, usually the 
angles of the cells. A 
.. system of intercellular 
spaces is thus formed 
which, as growth pro- 
ceeds, become con- 
tinuous with each 
other and form a sys- 
: Fie. 35.—Cortex or Root, sHowtna InrErR- 
tem extendin g CELLULAR PASSAGES BETWEEN THE CELLS. 
throughout the plant. 
They can be detected in the root, in the cortex of which 
they are conspicuous (fig. 35); they may be traced through 
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all the ramifications of the stem, and are geen to form a 
very prominent feature of the mesophyll of the leaves (fig. 
36). They communicate with the exterior in all the green 
parts of the plant, especially the leaves. In the epidermis 
of all such parts are small openings known as stomata 
