XEEOPHYTE SOCIETIES. 



199 



leaves are in a fixed position, there are certain ones whose 

 leaves are able to perform movements according to the need. 

 Mention has been made already of such forms as O.ralfs 

 (see §1-4), whose leaves change their position readily in 

 reference to light. Motile leaves have been developed most 

 extensively among the LeguminoscB, the family to which 



Fig. 166. Two twigs of a sensitive plant. The one to the left shows the numerous 

 small leaflets in their expanded position ; the one to the right shows the greatly 

 reduced surface, the leaflets folded together, the main leaf branches having 

 approached one another, and the main leaf-stalk having bent sharply downwards. 

 — After Steasbtjeger. 



belong peas, etc. In this family are the so-called "sen- 

 sitive plants," which have received their popular name 

 from their sensitive response to light as well as to other 

 influences (see Fig. 166). The acacia and mimosa forms 

 are the most notable sensitive plants, and are especially 

 developed in arid regions. The leaves are usually very 

 large, but are so much branched that each leaf is com- 

 posed of very numerous small leaflets. Each leaflet has 

 14 



