LECTURE VII. 



TRANSPIRATION. 



UNDER ordinary circumstances, the exhalation of watery 

 vapour is always going on from the surface of all parts of 

 plants which are exposed to the air. Its activity is not the 

 same in all, for the structure of the cell-walls of the superficial 

 cells is not everywhere the same. The walls of the epider- 

 mal cells of terrestrial plants for instance, are more or less 

 cuticularised, and the amount of watery vapour which can 

 pass through them varies with the degree of their cuticularisa- 

 tion : it is however in all cases smaller, as Ad. Brongniart 

 pointed out, than that which passes through the uncuticularised 

 cell-walls of aquatic plants when they are exposed to the air. 

 As a consequence, the latter wither much more rapidly than 

 the former. The property which the cuticle is thus seen to 

 possess of offering a resistance to the passage through it of 

 aqueous vapour is due, according to Garreau, to the resinous 

 and waxy substances which it contains. 



The structure of all terrestrial plants, from the Mosses 

 upwards, is such that direct communication is established 

 between the external air and the interior of the plant by 

 means of stomata and lenticels. We have already seen that 

 these openings communicate with intercellular spaces which 

 run between cells with uncuticularised walls, cells, therefore, 

 which can readily exhale aqueous vapour. It is, as a rule, in 

 the leaves that the greatest area of uncuticularised cell-wall 



