THE DIFFEKENTIATION OF THE PLANT-BODY 37 



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- FIG 

 tem extending 

 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 



35. CORTEX OF HOOT, SHOWING INTER- 

 CELLULAR PASSAGES BETWEEN THE CELLS. 



FIG. 36. SECTION OF LEAF SHOWING THE LARGE INTERCELLULAR SPACES 

 OF THE MESOPHYLL. 



all the ramifications of the stem, and are seen 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 



