194 
PLANT STUDIES 
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 Oxalis 
(see 14), whose leaves change their position readily in 
reference to light. Motile leaves have been developed most 
extensively among the Leguminosce, the family to which 
FIG. 171. 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 STRASBURGER. 
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. 171). 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 
