26 EXPERIMENTAL PLANT PHYSIOLOGY 



4. When the apparatus is moved to a warmer 

 place. Transpiration is increased. 



Inferences. — The rate at which a branch transpires is 

 affected by external conditions. The rate of transpira- 

 tion increases when the temperature rises, and decreases 

 with fall of temperature. 



The comparison between transpiration and the drying 

 of clothes can now be carried further. In both cases 

 water is being given off. In both cases the amount of 

 water given off is increased by the movement, warmth, 

 and dryness of the surrounding air. It seems then, at 

 first, that the transpiration of a plant is exactly on a 

 par with the drying of clothes, and that the loss of 

 water from the plant, like that from the clothes, is 

 altogether regulated by the surrounding atmosphere. 

 This idea is strengthened to a certain extent by the 

 drooping of plants at the end of a hot day in summer ; 

 evidently they have lost more water than they can 

 really atlbrd. And yet, on the other hand, careful 

 watching shows that the plants have much more 

 control over the amount of moisture given off than the 

 islothes have. 



The way in which this control is brought about must 

 now be investigated. 



Experiment 14 



Aim. — To determine whether a plant transpires 

 equally from both surfaces of the leaf. 



Method. — The problem in this experiment is to find 

 out from which surface of the leaf most water-vapour 

 is given off, so a delicate test for the presence of 

 moisture is needed. 



If a piece of paper be dipped into cobalt chloride 

 solution and then djried the colour changes from pink 

 to blue. On again moistening the paper the pink 

 colour reappears. Here then is a substance that will 

 serve our purpose. 



