THE AEEATION OF PLANTS 



111 



FIG. 71. CELLS SPLITTING 

 AT THEIR ANGLES TO 

 FORM INTERCELLULAR 

 SPACES. 



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 

 each cell by a process of diffusion, 

 being dissolved in the water of the 

 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 



FIG. 72. CORTEX OF ROOT, SHOWING INTER- 

 CELLULAR PASSAGES BETWEEN THE CELLS. 



