ECOLOGY 



85 



202. Subterranean stems and reserve food stores generally 

 are devices for tiding over unfavorable seasons and permitting 

 the plant to make the best of a short growing season. The 



Fig. 42. — Cassia, the wild sensitive pea. 



annual and biennial plant habits are also evidently adaptations 

 to seasonal changes, whether of temperature or moisture. 



203. The usual fall of temperature at night, which is often 

 very considerable, is a condition to which some plants apparently 



