THE AEEATION 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 FIG 71 CELLS SPLITTING 

 the tissue in the growing part of an $^^: 

 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 FIG. 72. CORTEX OP ROOT, SHOWING INTER- 



11 i , 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 



