150 GENERAL BOTANY 



conservation of water characterize the typical dry-land xero- 

 phytes of desert regions. In regions like the Florida peninsula 

 and the coastal regions of the American continent sandy soil 

 and brackish salt water often bring about a condition approxi- 

 mating that of the desert, since the roots of plants in these 

 regions are unable to secure a large amount of water from the 

 dense soil solution. Similar conditions exist in undrained fresh- 

 water bogs and marshes. 



Tropophytes. The student must not entertain the idea, from 

 the above account of typical xerophytes, that xerophytes and 

 mesophytes are sharply marked off from each other in all regions. 

 Dry conditions obtain in most mesophytic areas at certain peri- 

 ods of the year, and the xerophytic and mesophytic areas often 

 graduate insensibly into each other. Thus, our common broad- 

 leaved trees are typical mesophytes during those parts of the 

 year when they are supplied with abundant moisture and when 

 temperature conditions are suitable for the development of the 

 more delicate leaves, flowers, and fruit. When winter comes on, 

 however, the frozen soil restricts the absorption of water by the 

 roots, and the frost makes it difficult for leaves and flowers to 

 survive. These tree mesophytes then become practically winter 

 xerophytes, in which evaporation is restricted and temperature 

 changes are modified by a thick coating of bark on trunks and 

 branches (Fig. 25). The buds of trees, likewise, in temperate and 

 arctic regions, are adapted to the xerophytic conditions of winter 

 and are protected by the highly modified (indurated and hairy 

 or resinous) bud scales common .in oaks, poplars, and evergreens. 



Herbaceous plants likewise hibernate largely underground in 

 the form of xerophytic storage roots or stems, while the more 

 delicate mesophytic aerial parts die down with the advent of 

 frost and are reproduced each year at the beginning of the 

 warm season. It will thus be seen that some plants have 

 become typical dry-land plants and retain permanently a xero- 

 phytic habit and structure. Others, which are called tropo- 

 phytes, adapt themselves to the changing seasons and take on 

 alternately a typically mesophytic or xerophytic form which is 

 adapted to the water supply at a given seasonal period. 



