THM WORK OF LEAVJSS 



203 



carried away, and it is consequently stored up as 

 starch. 



Every one has noticed that leaves removed from 

 the plant quickly dry up unless placed in water. 

 It would seem that they must lose water 

 rapidly by evaporation. Is this normally 

 the case when they are growing on the 

 plant? Obtain two watch-crystals of 

 about the same size (pieces of mica 

 or glass may be used if the watch 

 crystals are not obtainable) , and fasten 

 them to opposite sides of a leaf by a 

 piece of wire bent to form a clip, as 

 shown in Fig. 117. Seal the joints 

 between the leaf and the glass air-tight with vaseline 

 (the leaf should not be removed from the plant and 

 care should be taken not to injure it in any way) . If 

 the leaf gives off water-vapor, it will condense in the 

 form of drops inside the watch-crystal, especially when 

 cooled. Examine the leaf from time to time and re- 

 move to a cool place if necessary. Do you find that 

 the leaf gives off water ? ^ From which side is more 

 given off ? Is this the side on which the greater num- 



I A more quick and accurate method is to cover the leaf with paper im- 

 pregnated with cobalt chloride (4 to 5 per cent dissolved in water); this is 

 dried over a lamp until it appears intensely blue. It is then placed on the leaf 

 and covered with mica or glass held in place by a clip, as shown in Fig. 117. 

 If water-vapor is escaping from the leaf the paper quickly assumes a reddish 

 tint. 



117. Arraneement for 

 determining whether 

 a leaf gives off water 

 vapor. 



