TRANSPIRATION 



203 



transpiration consists in the facilitation of the gaseous exchange 

 between the transpiring cells and the external air. 



The leaves are the principal organs of transpiration, though 

 this action is carried on by all tissues furnished with stomata, or 

 lenticels, and to a slight degree by all surfaces of the plant as 

 noted above. 



Transpiration is affected by a number of external conditions, 

 the most important of which are humidity of the air, temperature 

 of the plant and the air, light, electric potential, air currents and 



Fig. 102. Showing terminations of elements conducting water in leaves. A, in 

 Euphorbia splendens, with a laticiferous tube at m. B, end of fibrovascular bundle 

 and contiguous parenchyma cells in Ficus elastica. After Haberlandt. 



mechanical vibration, amount of water in soil, and composition of 

 the salts in the water absorbed. 



On account of the above factors, the amount of water transpired 

 by any given type of leaf, is much more in some localities than in 

 others, which has resulted in the development of a large number 

 of forms of transpiring organs with many adaptive structures. 

 Many species are so plastic that the transpiratory conditions under 

 which individuals are found are met by responses in the way of 

 formation of leaves of a size and structure suitable to the environ- 

 ment. 



Transpiration is also affected by a number of internal condi- 



