28 EXPERIMENTAL PLANT PHYSIOLOGY 



with the glass slides ; fix them in position with india- 

 rubber bands; and support the leaf with a clamp 

 (Figs. 11 and 12). 



Care must be taken that the paper and slides are 



perfectly dry and that the 

 paper is wholly covered by 

 the slides. 



Observation s. — The 



paper on the under side 



of the leaf quickly turned 



pink; that on the upper 



surface turned pink also, 



but much more slowly. 



Inferences. — In the sycamore leaf transpiration takes 



place from both surfaces, but much more rapidly from 



the under than from the upper. 



Fig. 12 



Experiment 15 (alternative method) 



Aim. — As in Experiment 14. 



Method. — Cut four similar leaves from a plant, e.g. 

 lime or beech. Vaseline one on both surfaces; one on 

 the upper surface only ; one on the lower surface only ; 

 leave the fourth leaf without vaseline. 



Observations. — After a few days the unvaselined leaf 

 had completely shrivelled. The leaf that had vaseline 

 on the upper surface only had withered. The leaf with 

 vaseline on the lower surface had withered but little ; 

 and that with vaseline on both surfaces not at all. 



Inference. — The loss of water from the leaf takes 

 place chiefly from the lower surface. 



If the leaves of several plants are tested it will be 

 found that transpiration is generally greater from the 

 lower surface. 



The advantage of this arrangement to the plant is 

 clear : the upper surfaces are exposed to the full rays 

 of the sun, and, if much transpiration were possible 

 from these surfaces, the volume of water given off might 

 soon be in excess of the amount absorbed. 



