104 VEGETABLE PHYSIOLOGY 



those unicellular or filamentous plants which live on the 

 surfaces of rocks or tree-trunks, the process is only slightly 

 modified, for the gases of the atmosphere readily dissolve 

 in the water which the cell-walls contain and diffuse thence 

 into the interior of the cell. 



In the cases of those more bulky plants which we have 

 especially been considering in the last chapter, a further 

 mechanism is' necessary, as the external air cannot gain 

 access into the interior of a large mass of cells without 

 special arrangements for its admission. This is especially 

 the case with such plants as are possessed of protective 

 mechanisms like the corky layers of the bark, or the 

 strongly developed cuticle of the leaves. The arrange- 

 ments of the structural elements in these plants we have 

 seen to include a very complete system of intercellular 

 spaces, passages, or canals, by means of which almost all 

 the constituent cells are placed in nearly or quite complete 

 communication with the external air. The intercellular 

 space system has consequently a very important function 

 to discharge in this particular, as well as to serve as the 

 means of carrying off from the interior the aqueous vapour 

 exhaled from the cells. 



The intercellular space system begins to appear at a 

 very early period in the development of the young plant. 

 While all its cells are merismatic, as is the case when it 

 begins to emerge from the seed, they are united together 

 entirely, a condition which persists at all the growing 

 points of the plant as its age increases. During this 

 condition the aeration of the internal cells is provided for 

 by the slow diffusion of the gases from cell to cell, absorp- 

 tion from the exterior by the external cells being possible 

 so long as their walls are not cuticularised. Some of the 

 cells situated deep in the interior of the adult parts are 

 dependent upon a similar process, but the majority of the 

 protoplasts are provided with access to the air by the early 

 formation of spaces due to the splitting of certain of the 

 cell-walls, and the subsequent partial separation of the 



