WATER-ABSORPTION AND TRANSPIRATION. 



115 



cuticle, for it allows transpiration to be regulated by the open- 

 ing and closing of the stornates. 



The guird cells collapse and close the stomates in darkness, 

 and also where there is a deficient supply of water, or when 

 transpiration is too active. The leaf is unable to assimilate 

 carbon dioxide in the absence of light, so that the stomates 

 may close in darkness without detriment to the plant, and in 

 this way unnecessary evaporation and its accompanying cool- 

 ing effect are largely avoided. 



153. Transpiration Experiments, Does the water- 

 vapour escape from both sides of a flat (bifacial) leaf to an 

 equal extent? Through what channels does it pass in the 

 leaf ? Does the rate of transpiration vary, and what condi- 

 tions cause it to vary? What other questions regarding 

 transpiration occur to you, and how would you attempt to 

 get the plant to answer your questions ? 



Pig. 41. 



(a) Get three similar bottles full of water (Fig. 41 , B, C, D). Into one 

 (T)) place a leafy shoot, into the second (C) a shoot deprived of leaves, 

 and leave the third (B) as a control. See that the level of the water is 



