222 STRUCTUKAL BOTANY 



the day-time, under the influence of light and heat, 

 the stomata are active, and through them water is 

 given off in the form of vapour. That this is so is 

 easily proved. If a plant be placed under a bell- 

 glass, the soil in which it is growing being completely 

 covered with tinfoil, so that no evaporation from the 

 earth is possible, we find that dew is soon deposited 

 on the inside of the glass, if there is a slight fall in 

 the temperature of the surrounding air. This dew is 

 produced by the condensation of the aqueous vapour 

 given off by the plant. Under ordinary conditions 

 the air in the bell-glass soon becomes saturated with 

 vapour, and no further transpiration is possible. If, 

 however, we place under the bell-glass some calcium 

 chloride (CaCl 2 ), or other substance which absorbs 

 watery vapour, we can obtain a measure of the 

 transpiration of the plant, for the increase in weight 

 of the calcium chloride in a given time is due to the 

 water which it has absorbed and which was previously 

 transpired by the plant. The quantity of water 

 given off by transpiration is very large, amounting, 

 as has been calculated, to from 50 to 100 litres a 

 day in the case of a large tree. 



Now, this emission of watery vapour goes on entirely 

 through the stomata. In very young leaves, before 

 the cuticle has been fully formed, the surface of the 

 epidermis itself can give off vapour; but in mature 

 functional leaves the cuticle is quite impermeable to 

 the vapour, and the stomata are the only means for its 

 exit. The stomata, therefore, by their opening and 

 closing, can regulate the transpiration of the plant 



