THE TKANSPOKT OF WATEK 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 



