CLOVERS AND ALLIED PLANTS. 



51 



own little stalk. If there are only three leaflets the 

 leaf is said to be trifoUate, and this character is so 

 general among the clovers that their botanical name is 

 TrifoUum. In other plants of the order there may be five 

 or a dozen leaflets. At the base of the leaf stalk, where it 

 joins the main stem are a pair of structures hiterto un- 

 mentioned in this work. They are membranous or leaf-hke 

 expansions, always paired, and a.Te called stipules. Differences 

 among the stipules are the easiest way of distinguishing 

 between certain clovers when only their leaves are present. 

 One of the chief characters of plants of the order 

 Leguminosae is the possession of nodules on their roots. 

 These nodules are usually about 

 the size of Turnip seeds, and 

 may occur singly or in groups 

 up to the size of a hazel nut or 

 larger. The nodules are the 

 result of the action of certain 

 bacteria Uving in the roots of 

 the plants. These bacteria have 

 the power of causing the free 

 nitrogen gas of the air to com- 

 bine with other substances to 

 form proteids, which are stored 

 up during the hfe of the bac- 

 terium and remain in the 

 nodules when the bacterium 

 goes into its resting stage. 

 These compounds now form 

 the very kind of food that the 

 clover needs in large quantities, 

 and so they are seized on by the clover, which thus indirectly 

 uses the nitrogen of the air. As an accompaniment of the 

 plentiful and direct supply of available nitrogen, the 



Fig. 24. — ^Nodules on root of bean, 

 (after Percival) 



