i6o THE BIOLOGY OF FLOWERING PLANTS 



stations. And this means simply that the root, under 

 circumstances of extreme demand, temporarily fails in a 

 sufficient supply. In such cases the plant as a rule makes 

 good the deficit through the night, when transpiration is 

 reduced ; no permanent damage results. But even so, 

 the stomatal closure which follows wilting means a decrease 

 in the assimilating power of the plant. Prolonged periods 

 of drought have more serious effects ; with us they are not 

 common, yet in some years the pastures are " burnt up " — 

 the herbage drying and withering. Under such conditions 

 the mere closure of the stomata on wilting has been insuffi- 

 cient to prevent a ruinous loss of water by the plants affected. 

 It is clear that in more arid conditions, conditions cul- 

 minating in the desert environment, our plants would 

 rapidly perish, and that the vegetation which does survive 

 must have special powers of water conservation and supply. 

 We have seen that as regards the root system this is so, and 

 we now turn to the general question of the manner in which 

 the water content of the plant is conserved in temporary 

 and more prolonged periods of water shortage. 



§ 19. Water Balance, Wilting, and Water Storage 



We must first consider the question of the amount of 

 water the leaf may lose without seriously disturbing its 

 functions. When the leaves of a mesophyte flag, how much 

 water, relative to their full content when turgid, have they 

 lost ? Knight (1922) found that fully turgid leaves of such 

 trees as the ash and apple wilted after they had lost under 

 1 per cent, of their water content. This figure is probably 

 representative for mesophytes in general, and it suggests that 

 these plants work with a very small water balance. Only 

 when absorption and conduction can cover, or very nearly 

 cover, the loss by transpiration is the plant safe from wilting ; 

 it has little reserve to fall back on. This is confirmed by 

 Thoday's experience with the sunflower ; he found that 

 on a fine summer day the leaves could be kept turgid only by 

 forcing water into them under pressure. Livingston and 



