174 THE BIOLOGY OF FLOWERING PLANTS 



and then remained constant. The conditions of the experi- 

 ment were then ahered by placing the apparatus in still 

 air, so that transpiration was much depressed ; absorption 

 now predominated, and the water content of the shoot 

 increased ; the original conditions were restored, and tran- 

 spiration at once increased, not to the previous constant 

 value, but to a much higher one. In this case, therefore, 

 an increase in the water content of the shoot was directly 

 responsible for an increase in transpiration, and vice versa, 

 Knight (1922) has shown that the stomatal opening which 

 occurs at the beginning of wilting is accompanied by an 

 increase in transpiration. The maximum transpiration is 

 however reached, and a decline sets in, before the stomata 

 reach their maximum opening. This decline in transpira- 

 tion must therefore be caused by the decreasing water 

 content of the leaf. As we have seen, the decrease in 

 water content which is effective may be very small. The 

 slight loss of water which may lead to diminution in 

 transpiration has been called by Livingston " incipient 

 wilting." 



It may be noted that in all this work no attention seems 

 to have been paid to the cuticular fraction of transpiration. 

 Yet we have seen that this may be an appreciable part of the 

 whole in mesophytic plants, and the work of F. Shreve 

 (i9i4rt) has shown that in some rain-forest plants it may be 

 greater than the transpiration from the stomata. It is quite 

 possible that at the critical points in the march of tran- 

 spiration the cuticular fraction may be an important factor, 

 and its study should not be neglected. 



Another possible factor in the regulation of transpiration 

 is change in the concentration of the cell sap. In the plants 

 studied by Livingston the daily fluctuation in water content 

 was, on an average, about 4 per cent. This means an increase 

 in the concentration of the cell sap, which might be further 

 increased by an accumulation of sugars. The consequent 

 lowering of vapour pressure would, however, hardly produce 

 an appreciable retardation of evaporation. It is possible 

 that in the plants with very high osmotic pressure discovered 



