494 RELATION TO ENVIRONMENT. 



plant population show how sensitive the plant habit and struc- 

 ture are to environment. So in temperate regions there are all 

 gradations from vegetation resembling that of the humid tropics 

 in luxuriance to the vegetation of arid regions. 



956. Resting season. During the winter season, or in some 

 milder climates (California) during the dry summer season, the 

 whole aspect of the mesophytic vegetation is changed. The 

 forests become leafless, except in some for the sprinkling of the 

 evergreen, and xerophytic types found in conifers, heath plants, 

 etc., or where the forest is dominated by these types. The 

 deciduous trees and shrubs having discarded their leaves, which 

 provided them with an immense working surface during the 

 growing season, now enter upon their long rest, with only bulky 

 structures exposed to the air, the surface of which is adapted to 

 greatly resist the loss of water. The twigs, branches, and trunks 

 are protected by their bulk and by the covering of bark, or in 

 some cases the twigs are covered also with hairs. The buds are 

 bulky, and while there are small and delicate leaves and tender 

 growing parts, these are protected from loss of water and conse- 

 quent death from drying out during freezing weather by the cov- 

 ering of thick dry scales, and in some cases by the additional 

 protection of woolly or hairy scales underneath these. 



957. Provision against injury and loss by the fall of the 

 leaf. Before the fall of the leaf much of the nitrogenous food sub- 

 stances in the leaf slowly passes back into the stem, where it is 

 stored for future use. While this is going on in deciduous trees, the 

 petiole of the leaf near its point of attachment to the stem is pre- 

 paring to cut loose from the latter by forming what is called a sepa- 

 rative layer of tissue. At this point the cells in a ring around the 

 central vascular bundle grow rapidly, so as to unduly strain the 

 central tissue and epidermis, making them brittle. In this con- 

 dition a light puff of wind breaks them off, and the separative 

 layer of tissue forms a covering which retards loss of water at the 

 wound. 



958. Perennial herbaceous tropophytes. With the approach 

 of winter, perennial herbaceous tropophytes prepare for exces- 



