486 RELATION TO ENVIRONMENT. 



from a mesophytic to a xerophytic habit. If the usual broad- 

 leaved trees and shrubs retained their green living leaves through 

 the winter, the loss of water would be so great as to kill the 

 trees. The soil being so cold, root absorption is at a very 

 low ebb, or often ceases for considerable periods. By discarding 

 their leaves, transpiration is greatly lessened, and the life of the 

 tree or shrub is preserved. 



2. Perennial herbaceous plants. In these plants the aerial 

 shoots as well as the leaves usually die on the approach of winter, 

 while the underground shoot, in the form of a bulb, tuber, rhi- 

 zome, or simple shoot-stem, is protected from drying out by its 

 covering of soil or leaf-mold. These plants are sometimes termed 

 glophilous, because they are dependent on the subterranean parts 

 for preservation through the winter. 



3. The annuals and biennials might be treated of as a third 

 type of tropophytes. In the annuals the plant lives only through 

 the growing season, and on the approach of winter, or of the 

 period when extreme xerophytic conditions prevail, turns to 

 seed. The seed is the xerophytic stage of the plant. Biennials 

 might even be made a fourth type, the underground stem being 

 developed the first year, exists through the winter, and the sec- 

 ond year the plant forms seed. 



III. Plant Structures Adapted to Conditions of 

 Environment. 



949. The normal plant condition. Theoretically the normal 

 plant condition is one in which the plant lives under uniform 

 conditions of environment throughout the year, with slight 

 fluctuations of temperature, humidity of the air, etc. These 

 conditions are more nearly approximated in humid tropical 

 countries. Temperature and moisture conditions, both of soil 

 and air, are relatively high and uniform. These conditions 

 favor luxuriant vegetation, which is characteristic of the humid 

 tropics. Many tropical plants, like the palms, are evergreen, 

 because they hold their leaves for more than one year. But 



