212 TRANSPIRATION. 



colour, but this can be restored by drying them again 

 over a flanie. 



274. Transpiration influenced by Opening and 

 Closing of Stomata. As may be proved by simple 

 experiments, light accelerates transpiration. No doubt 

 this is chiefly due to the heating effect of the light rays 

 entering the leaf, but in many cases at any rate the 

 acceleration is increased by the widening of the stomata 

 under the influence of light. The stomata of different 

 plants do not, however, always react in the same way to 

 light ; in Lime and various other plants they open widely 

 only in direct sunlight, while they close in diffuse light ; 

 on the other hand, in Willow they remain open in 

 diffuse light. Another important factor in the move- 

 ments of stomata is the vapour tension of the air 

 abundance of water vapour in the air retards tran- 

 spiration, for purely physical reasons, but at the same 

 time it usually causes the stomata to open widely; 

 on the other hand, in dry air evaporation is in- 

 creased, but in many plants transpiration is reduced 

 because the stomata close when wilting begins. Here 

 again there are exceptions in many plants (especially 

 those growing in marshes and shady places) the wilting 

 leaf has its stomata wide open and therefore continues to 

 give off water and soon becomes shrivelled, when the air 

 is dry and the water-supply is curtailed or stopped. 



(a) Cut leafy twigs of Lime and of Willow, and set each in a 

 bottle of water, covering the water with a layer of olive oil ; weigh 

 the whole apparatus in each case, expose to bright light for two 

 hours, and weigh again. From a leaf of each plant tear or shave 

 off a piece of the lower epidermis, mount in water, and note that 

 the stomata are widely open ; also test a leaf of each plant with 

 cobalt paper. Now set both plants in not too bright diffuse light, 

 and after two hours weigh again ; note that the Lime has lost very 

 little water, while the Willow has transpired vigorously ; examine 

 a piece of epidermis in each case, and note that in Lime the stomata 

 are closed while in Willow they are still wide open also apply 

 the cobalt paper test, comparing the rate of change of colour in the 

 two cases. In winter, use the common indoor plants Cyperus alter- 

 nifolius (which behaves like Willow), and Aspidistra or Ficus (which 

 behave like Lime) for these experiments. 



