TRANSPIRATION 211 



The rate of transpiration depends upon the moistness of 

 the surrounding air. — A plant placed under a glass bell-jar 

 soon begins to transpire very slowly, because the air becomes 

 saturated with moisture ; whereas if pieces of calcium chloride 

 be suspended near the plant it transpires rapidly, because the 

 calcium chloride sucks the moisture from the air and renders 

 it dry. Thus we find the drier the air is the more rapidly a 

 plant loses water by transpiration. 



Movements of the Air (Wind) often increase the rate of 

 transpiration. — This is not as easily proved in a simple 

 manner, but a familiar fact illustrates its truth. Plants placed 

 in draughty passages in a house are liable to wither and fade 

 sooner than those placed in ordinary rooms, because they 

 transpire more rapidly without being able to make up by 

 a corresponding increase in the rate of absorption of water. 



These facts explain various more or less familiar phenomena. 

 Plants transpire more rapidly at daytime than by night, because 

 the air is warmer and often ■ drier, and because the leaves are 

 exposed to light. Again, if cut flowers or shoots be placed at 

 once in a dark box they remain fresh for a longer time than 

 if they are exposed and carried for the same length of time in 

 the hand. The fading is due to transpiration without corre- 

 sponding absorption. In the dark box, transpiration is slower 

 because of the absence of light and the stillness of the air 

 inside the box, also because the air in the box soon becomes 

 damp. 



Function of Transpiration. — We have seen that the root 

 absorbs water and dissolved salts, and that this solution is then 

 carried up the stem to the leaves, and finally a large part of 

 the water is returned to the air. Why should the plant take 

 the trouble to drink in so much water, carry it up to the leaves, 

 and then throw most of it away? Transpiration confers at 

 least two important benefits on the plant. In the first place, 

 the water transpired brings with it salts in solution : the salts 

 are required as food. But the available salts are only present 

 in the soil in the form of very weak solutions ; and even if 

 they were present in the form of strong solutions the roots 

 could not absorb them. The result is that the plant is com- 

 pelled to absorb a great quantity of salt-solution in order to 

 obtain the salts it requires. The water is taken in largely in 

 order that the plant may have the salts. When the solution 



