260 The Physiology of Plants BOOK in 



in 1879 and 1880 by Von Hohnel. The results showed 

 considerable variations, which might well have been expected 

 as plants of very different habits of life were employed for 

 the determination, and the difficulties of computing even 

 the leaf area were almost insuperable. Haberlandt gave 

 80 grammes per diem as the amount exhaled by a plant 

 of maize, while a plant of hemp transpired 191 grammes 

 in the same time; a sunflower plant gave off about as 

 much as the hemp. Von Hohnel made more careful 

 observations than Haberlandt, as he made allowance for 

 the varying conditions of different parts of the plant, 

 and for differences of situation. He estimated that a birch- 

 tree with 200,000 leaves transpired 30-40 kilogrammes of 

 water during a single hot day. 



The relation of stomata to transpiration was treated by 

 the older writers as purely a mechanical one, and was 

 even then only partially understood. Their mechanism 

 was investigated with considerable care and accuracy by 

 Von Mohl in 1856, and was shown to depend in the first 

 instance on the turgidity of the guard cells. He pointed 

 out that internal hydrostatic pressure made them curve 

 outwards in consequence of the way in which their ends 

 are attached, and tentatively ascribed the variations in 

 their turgor to varying quantities of osmotic substances 

 in their interior. 



No material advancement of our knowledge of them was 

 secured until about 1880, when the idea of irritability or 

 response to changes in environment dawned upon botanists 

 in consequence of the writings of Pfeffer, Sachs, and Darwin, 

 to which reference has already been made. We have seen 

 that a conception of the vital character of transpiration 

 had been slowly making its way in scientific opinion since 

 1860, but it had so far been confined to the process of 

 the formation of the vapour. The exhalation of the latter 

 through the openings of the air passages having been shown 



