110 VEGETABLE PHYSIOLOGY 



can diffuse through the cell- wall, which, as we have already 

 seen, is saturated with water. If we turn to 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 arrangements 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 con- 

 stituent cells are placed in nearly or quite complete com- 

 munication with the external air. The intercellular space 

 system has consequently a very important function to dis- 

 charge 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 completely 

 together, a condition which persists at all the growing 

 points of the plant as its age increases. During the young 

 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. As age advances 

 and the adult condition is gradually attained, while some 

 of the cells situated deep in the interior are dependent 

 upon a similar process, the majority of the protoplasts are 



