2IO TRANSPIRATION 



(ii.) Cobalt-paper method. — If we soak some white filter- 

 paper in a weak solution of cobalt chloride, and dry it near 

 a fire or in the sunlight, the paper will assume a blue colour. 

 When this blue paper is brought into contact with damp air it 

 gradually assumes a red colour, and the damper the air is, the 

 quicker does ' the red tint appear. Therefore, held near a 

 transpiring leaf, the rate at which the blue paper turns red 

 affords us a means of judging the speed at which the leaf is 

 making the air around it damp, or, in other words, it shows us 

 how fast the leaf is transpiring. 



Leaves are tlie chief transpiring organs of a plant. — If we 

 compare (by weighing) the rate at which two branches of a tree 

 transpire, we find that a branch bearing many leaves transpires 

 much faster than the branch having few leaves. Again, if we 

 cut the leaves off a branch, we ascertain that the branch tran- 

 spires very much more slowly than when the leaves were 

 present. We therefore conclude that the leaves, exposing a 

 large surface to the air, are the parts of a plant which are 

 mainly responsible for transpiration ; a green stem does tran- 

 spire to a certain extent, but a woody stem scarcely transpires 

 at all. 



Usually tlie lower face of a leaf transpires more rapidly 

 than the upper face. — We can easily prove the truth of this 

 statement by experiments by the cobalt method on leaves of a 

 plum, cherry, pear, oak, etc. The leaf is placed between two 

 pieces of glass, with a piece of cobalt paper on part of each 

 face. The cobalt paper in contact with the lower face soon 

 becomes red, whereas the piece touching the other face 

 remains blue for a longer period. This rule generally holds 

 true only for leaves which are extended so as to have one 

 surface pointing upwards and one facing downwards ; leaves 

 like those of the onion, which are nearly erect, transpire 

 equally on all faces. 



The rate of transpiration vai^ies with the temperature of 

 the air. — A plant placed in a warm position in a room tran- 

 spires more rapidly than in a cool position in the same room. 

 A rise of temperature causes a plant to transpire more rapidly, 

 and a fall in temperature retards the process. 



Light favours transpiration. — This may easily be illustrated 

 by comparing the rate of transpiration of a plant or leaf at first 

 placed near a window for a time and then taken into a darker 

 part of the same room. 



