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 
