NUTRITION AND SEASONAL LIFE OF PLANTS 133 



by means of an underground stem which is the perennial part 

 of the plant. Each year an aerial annual part, like the stem of 

 the bean, is sent up to construct food and to produce flowers and 

 fruit. During the warm months of summer the green leaves and 

 stem of this aerial part manufacture and distribute foods to the 

 perennial storing 

 underground stem 

 and to the de- 

 veloping flowers, 

 fruits, and seeds. 

 The summer life 

 of these peren- 

 nials, therefore, 

 is like that of 

 the annuals and 

 of the biennials. 

 But when cold 

 weather comes on, 

 the aerial portion 

 of the perennial 

 dies down, and 

 the protected un- 

 derground por- 

 tion stored with 

 food lies dormant 

 until spring, when 

 it again sends up 

 an aerial annual 

 shoot for food-making and reproduction. In the case of grasses 

 some of the old annual foliage often survives the winter, but the 

 really effective aerial and annual leafage is produced each season 

 from an underground stem, or rhizome, which serves the func- 

 tion of storage and hibernation during inclement periods. 



In perennial woody plants (the trees and shrubs of the temperate 

 zones) (Fig. 66) the aerial plant body has become adapted to 

 changing seasons and differences in climate, so that it does not die 

 down, as in herbaceous perennials, with each inclement seasonal 



FIG. 66. The seasonal history of a perennial woody 

 plant, the locust (Eobinia) 



