VEGETATION TYPES. 493 



frequent, the vegetation is mesophytic the year round. In tem- 

 perate regions, where there is a medium rainfall during the grow- 

 ing season, the land vegetation is mesophytic during this season. 

 But during the winter season, or during the dry season in some 

 regions (part of California, for example), the conditions of en- 

 vironment for mesophytic vegetation are extreme. The extreme 

 cold or dryness during this season would be fatal to plants with 

 mesophytic structures were it not for the fact that the plant is 

 enabled to adapt itself to the periodical change in environment 

 by discarding the mesophytic habit and adopting a xerophytic 

 one. Mesophytes of temperate regions therefore differ from 

 mesophytes of the tropics, and have been termed by Schimper 

 tropophytes, because they turn, as it were, periodically from 

 one condition to another. 



954. Tropical mesophytes. Tropical mesophytes in humid 

 districts, being in no danger from extremes of dryness, cold, or 

 excess of salts in the substratum, are free to develop to the high- 

 est extent foliage structures, without interruption, for doing a 

 large amount of work in transpiration, respiration, and photosyn- 

 thesis, which are necessary for rapid growth. We have, there- 

 fore, the luxuriance and permanence of foliage in humid tropical 

 regions. The foliage leaf reaches its highest development. To 

 secure the highest efficiency in work we have already learned 

 that the leaf is broad and thin, so that a great amount of surface 

 in proportion to the bulk is exposed to light and air. 



955. Temperate-region mesophytes, or tropohytes. Grow- 

 ing season. During the growing season the general character 

 of the plant structures is similar to that of the humid tropical 

 region. That is, the conditions are such as to favor a high 

 development of foliage. Where the soil is rich and there is an 

 abundance of moisture and an absence of drying winds, vegeta- 

 tion in temperate regions frequently takes on a tropical aspect, 

 even in the northern United States and southern Canada. In 

 drier situations, or in poorer soil, or where dry winds are preva- 

 lent, the foliage development is less luxuriant. In fact one finds 

 conditions in all regions so varied that great variations in the 



