5* 



dermal cells. This we may call the period of Epidermal trans- 

 piration. As the leaf grows older this epidermal transpiration 

 becomes interfered with by the gradual development of the 

 cuticle ; hence we get a diminution in the amount of transpir- 

 ation, and the curve, as we saw, falls. But simultaneously with 

 the production of cuticle we find the development of the stomata 

 taking place, and at the period of the history of the leaf when 

 the cuticle has reached the highest point in its development, 

 the stomata have already begun to play an important part in 

 the function of transpiration which, in some measure, compen- 

 sates for the diminution of its amount occasioned by the cuticle, 

 and so we find the curve rising again. This period may be 

 called that of Stomatal transpiration. The amount at the 

 maximum of this period is never so great as at the maximum 

 of the period of epidermal transpiration. As the leaf grows 

 older the amount of transpiration becomes less and less, and the 

 curve gradually and permanently falls. These experiments 

 show us the importance of considering the age of the leaf as 

 modifying transpiration, and they also teach us what we 

 shall have to consider more fully hereafter, the great im- 

 portance of the stomata in relation to the function of trans- 

 piration. 



Next let us enquire — Do we find any definite sequence of rise 

 and fall in the amount of transpiration taking place within the 

 twenty-four hours, or in other words, is there any daily periodi- 

 city of transpiration, such as we shall subsequently find occurs in 

 the case of the root-pressure ? I can only say, in passing, that all 

 we do know is that in the daytime the amount of transpiration 

 is much greater than it is at night, but we have not at present 

 materials for constructing a definite sequence to express this 

 difference with points of maxima and minima. Both light and 

 external circumstances affect the transpiration directly, and 

 hence whatever periodicity is present may perhaps be due to 

 these forces, and not to any internal inherent periodicity in the 

 plant itself, such as we will find in the case of the root- 

 pressure. Yet, while we have no direct proof that an inherent 

 periodicity does exist, there is some reason for thinking that it 



