TRANSPIRATION. IO/ 



with an improved form of apparatus by means of which a 

 more complete and a more equal absorption of the water 

 transpired by the two surfaces of the leaf is ensured, and he 

 inclines to the opinion that a proportion of this kind may 

 really exist. 



It has long been an accepted view that the activity of the 

 transpiration of a leaf varies with its age, but it is to von 

 Hohnel that we are indebted for a definite account of this 

 relation. He finds that the youngest leaves are those .which 

 transpire most freely; the activity of transpiration gradually 

 diminishes-as the leaf grows, but it subsequently rises, so that 

 a second, but lower, maximum is attained when the leaf is 

 fully developed; it then steadily diminishes. The explana- 

 tion which he gives of these facts is that in the very young 

 leaf the cuticle is but feebly developed and offers but little 

 resistance to the passage through it of aqueous vapour; fur- 

 ther, there are no stomata: the loss of water at this stage 

 is entirely due to what he terms "cuticular transpiration." 

 With the growth of the leaf, the cuticle becomes thicker and 

 the stomata begin to be formed, the former process, however, 

 taking place more rapidly than the latter, so that the 

 transpiration is on the whole diminished. As the stomata 

 are developed, the transpiration increases again until the 

 secondary maximum is reached : this secondary maximum 

 is the expression of the "stomatal transpiration" of the leaf. 



We may now proceed to consider the mode in which the 

 stomata affect transpiration. It has long been known that 

 the guard-cellsfare capable of opening and closing the aper- 

 tures of the stomata. Sir Joseph Banks, who was one of the 

 first to notice this fact, was of opinion that the stomata are 

 closed in dry and open in wet weather, and that they thus 

 regulate the transpiration of the plant. Moldenhawer found, 

 however, that the stomata are closed on-rainy days and dewy 

 nights and are open when the sun is shining upon the leaves, 

 and his observations have been confirmed by those of Amici, 

 von Mohl, and linger. The mechanism of the movements of 

 the guard-cells was thoroughly investigated by von Mohl. 

 He shewed that the opening of the stomata depends in the 



