DIOSMOSIS OF GASES INTO AND FROM LIVING CELLS I33 



travel longitudinally through the length of the plant from its 

 place of intake in floating or aerial leaves, or from its place of 

 liberation by photosynthesis in the green parts. In these plants 

 the intercellular spaces become relatively very large, as in Ne- 

 lumbo, Heteranthera, Juncus, etc. (Fig. 68.) 



It seems that in mesophytic and xerophytic plants (meso- 

 phytes and xerophytes or plants at home under average condi- 

 tions of water supply, and where water is scarce, respectively) 



Fig. 68. — Low-power, camera-lucida drawings of A^ cross section of stem of Juncus; 

 and B, stem of Nymphasa, showing large intercellular spaces at i. In A the black lines 

 traversing the section are really chains of cells and thin-walled cells with relatively large 

 intercellular spaces between them fill up i. 



the intercellular spaces are so small that each part, root, stem, 

 and leaves must take in air more or less independently of the 

 others — the stem cannot supply the root with air, neither can 

 the root or the leaves supply the stem, although seedlings of 

 Pisum and Vicia are known to be exceptions to this rule. If 

 the soil -in which mesophytes or xerophytes are growing be 

 inundated, so that the stems and leaves are not submerged, 

 the roots cannot get air and the plants die. The same thing 

 may happen when earth is filled in around trees to a depth of 

 several feet. 



Diosmosis of Gases into and from Living Cells. — The 

 wall of every living cell is infiltrated with water, and a very 

 thin film of water covers its free surface. Therefore the gases 

 in the intercellular spaces must, before entering the cell, go 

 into solution in this water, and in this condition diosmose through 



