STOMATA AND GASEOUS EXCHANGE 87 



From this it is clear that the stomata are most important 

 as regards transpiration, but that transpiration does also 

 take place through the cuticle. In Fuchsia and Aucuba 

 water vapour is given off by the upper surface on which 

 are no stomata. The cuticular transpiration from the upper 

 surface is one-eighth of the combined cuticular and stomatal 

 transpiration of the lower surface in Fuchsia, while in Aucuba 

 it is only one-fortieth ; this is evidence of the effect of the 

 cuticle, which is thick in the latter, and thin in the former. 

 In the sunflower the transpiration ratio is very close to the 

 stomatal ; in the tobacco the two are different. The latter 

 is the common case and indicates that factors such as the 

 relative size and state of the two sets of stomata, the relative 

 thickness of the cuticle above and below, and the different 

 conditions of light, etc., at the two surfaces, must enter into 

 the result. Renner (19 10) has estimated the ratio between 

 the total stomatal and total cuticular transpiration for a 

 number of leaves. Some of his results are given in 

 Table XVIII. 



TABLE XVIII 

 Relation of Cuticular and Stomatal Transpiration 



These figures must not be taken as constants for the various 

 leaves, since in different experiments they vary considerably, 

 and there is a marked difference between the ratios found in 

 still air and in wind. The ratios in wind for three plants 

 are given in brackets. 



There is, therefore, no constant relation between the 



