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 
