88 THE BIOLOGY OF FLOWERING PLANTS 



two ; but we may draw the definite conclusion that stomatal 

 transpiration is predominant, and that this predominance 

 is much accentuated in moving air — the condition in which 

 the plant most often transpires. A different result, obtained 

 by F. Shreve (i9i4rt) for Pilea nigrescens, a herb of the 

 Jamaican rain forest, must be noted. He found that the 

 cuticular transpiration was 30 per cent, greater than the 

 stomatal. This may be characteristic of rain-forest plants, 

 living in a very moist atmosphere, and having leaves with 

 an extremely thin cuticle. 



The path of diffusion of carbon dioxide in respiration 

 and assimilation has been investigated by Blackman (1895), 

 and by Browne and Escombe (1900). Table XIX gives 

 some of Blackman's results for the intake of carbon dioxide 

 in assimilation and escape of carbon dioxide in respiration. 



TABLE XIX 

 Path of Gaseous Exchange in Respiration and Assimilation 



There is a close relation here between the passage of 

 carbon dioxide and the number of stomata. It is clearest 

 in the plants which have no stomata on the upper surface ; 

 in these there is practically no diffusion of gas through 



