34 



DICOTYLEDONS 



and hang down vertically (fig. 31, ii.). We say, the leaf "sleeps" 

 now. On the following morning it resumes its original position. 



These move- 

 ments, which 

 are very regu- 

 lar, are effect- 

 e d by the 

 swollen joint 

 called pulvi- 

 nus, which can 

 be observed at 

 the base of the common 

 leaf-stalk, as also on 

 each of the stalks of 

 the three leaflets. 



What does this cu- 

 rious behaviour of the 

 leaf mean for tlfe plant? 

 We know that the plants 

 take from the soil 

 n u r i sh m e n t w h i ch , d i s- 

 solved in w^ater, ascends 

 to their leaves, where 

 the water is evaporated 

 leaving the salts of the 

 soil behind it in the 

 leaves. The work of 

 tlius pumidng up new 

 food to the leaves must 

 stop when the evapora- 

 tion l)y the leaves is 

 obstructed; and this is 

 exactly wliat takes phice when dew settles upon the leaves. 

 Now,' it is a known fact that articles laid horizontally on the 

 ground have a greater deposit of dew than such as hang verti- 

 cally. It is therefore advantageous for the leaves to assume a 

 position which prevents the dew from covering them and thus 



