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 



