320 



Introduction to Botany. 



Fig. i63. 



Food bodies of Cecropia 

 adenopus, somewhat 

 magnified. After 



SCHIMPER. 



Plants which grow under ordinary 

 conditions of moderately moist soil 

 and average humidity of atmosphere, 

 and are not subjected to prolonged 

 drought during the growing season, 

 are termed mesophytes. These have 

 less pronounced characteristics of the 

 hydrophytes. Most of the mesophytes 

 cast off their leaves at the close of the 

 growing season, and many of them 

 die back to the ground and survive 

 as bulbs, tubers, etc. Such meso- 

 phytes are also termed tropophytes. 



204. Xerophytic Conditions. — The following physical 

 conditions give rise to xerophytes : A dry soil, such as is 

 found in desert regions ; a low temperature of the soil, the 

 roots being able to abstract but little water, even when 

 plenty is present, from soils whose temperature approaches 

 zero; soils, such as those of peat bogs, abounding in humic 

 acids, these acids in some way diminishing the absorptive 

 power of roots. 



205. Halophytic Conditions. — ■ The halophytes have been 

 evolved in soils whose water contains more than 0.5^ of 

 salts in solution, as is the case along seacoasts, in salt 

 marshes, and in the dry beds of ancient salt waters. A low 

 relative humidity,- a high temperature, and a low pressure 

 of the atmosphere all hasten transpiration, and assist in 

 the evolution of xerophytic and halophytic characters. 



206. Hydrophytic Conditions. — The hydrophytes have 

 been produced under conditions opposite to those above 

 enumerated ; namely, in soils abundantly provided with 

 water, even to saturation and submergence, of sufficiently 

 high temperature to enable the roots to absorb the water 



