THE TRANSPORT OF WATER IN THE PLANT 79 
of the veins are not completely empty ; their walls, at any 
rate, are saturated with the water ascending from the roots. 
We have consequently here a resumption of the osmosis 
which we noticed playing so conspicuous a 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 intercellular 
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 
