WATER RELATIONS OF PLANTS 



303 



A typical daily trend in transpiration is naturally observed 

 only on absolutely cloudless days, when meteorological condi- 

 tions are normal. It will show sharp fluctuations in changeable 

 weather, largely according to alterations in temperature and light. 



70. Leaf Structure as a Factor in Transpiration. Stomatal 

 and Cuticular Transpiration.— The rate of transpiration is deter- 



I 7S 



\ 



5 eo} 



ss 

 eo 



Fig. 95. — Graphs showing the daily march of transpiration and the most 

 important factors determining it (according to Briggs and Shantz). 



4 



mined not only by external factors, but to a high degree likewise 

 by the structure of the leaf, as well as the state of its cells and 

 tissues during transpiration. These factors complicate con- 

 siderably the process of water loss. 



Evaporation proper, i.e., the transition of water from a liquid 

 to a vaporized condition, takes place at the surface of the paren- 

 chymatous cells lining the intercellular spaces of the leaf. These 

 open spaces represent a special aeration system, the outlets of 

 which are the stomata. The rest of the leaf is covered by a solid 



