74 VEGETABLE PHYSIOLOGY 
part in the original absorption of water. The water is 
drawn from the woody elements into the parenchyma of 
the leaf, and as it passes from cell to cell the leaf tissue is 
made turgescent. The turgescence is very largely due to 
the ascending stream, whose progress we have traced; at 
the same time we must remember that the turgid cortex of 
the root is continuous through that of the stem with the 
soft tissues of the leaves, and hence the slow movement of 
diffusion assists in its maintenance. In plants which have 
but little woody tissue, such as the greater number of 
herbaceous annuals, this slow movement plays relatively a 
more important part than in those trees which have a 
conspicuously woody trunk. 
As we have seen, the turgid mesophyll tissue has a 
great part of the surface of its cells abutting on the inter- 
cellular spaces of the leaf. The cortical cells of the axis are 
also similarly placed, though the spaces are much smaller 
in that region. The intercellular spaces of the plant are 
in communication throughout, and the cells which abut 
upon them are in most places, and particularly in the 
leaves, furnished with very delicate cell-walls, which readily 
allow a process of evaporation to take place, watery vapour 
passing into the passages. The whole intercellular space 
system thus becomes charged with vapour, the process of 
evaporation from the cells being, however, much more 
marked in the leaves, owing to the greater development of 
the spaces there. At particular spots in the leaves and 
other green portions of the plant, these intercellular spaces 
or canals communicate with the external air by means of 
small openings or crevices in the outer layer of cells, which 
are known as stomata (fig. 61). Each stoma is surrounded 
by two cells of peculiar shape, known as guard-cells, which 
by being approximated to each other to a greater or less 
degree, enable the extent of the communication to be 
varied from time to time according to the conditions of 
the plant. The ultimate escape of the watery vapour from 
the interior of the plant is subject by means of these 
