156 PLANT PHYSIOLOGY AND ECOLOGY 



hydrophytes at a percentage which in a finer soil suffices only for 

 mesophytes. This is largely due to differences in the amount 

 of available water. Such differences serve also to explain the 

 apparent lack of correspondence in alkaline soils, etc. 



175. General features of xerophytes. .Most xerophytes possess 

 a deep-seated root system, which is able to draw water from the 

 lower moist soil, and to conserve it from loss in the dry upper 

 layers. Reservoirs for water storage are rarely developed in the 

 root. The stem usually shows modifications more or less similar 

 to those of the leaf. The stem is often reduced and sometimes 

 disappears, though not all stemless plants are xerophytes. The 

 stem is most modified as a rule when the leaves are greatly reduced 

 or absent. 



The organ which is most strikingly modified in xerophytes is 

 the leaf. This exhibits a large number of variations in size, form, 

 texture, and structure. Several of these are often combined in the 

 same leaf, though as a rule one alone is characteristic. The most 

 satisfactory grouping of xerophytes is upon the basis of the leaf, 

 since it is the organ most directly affected. Hence, those plants 

 on which the leaves are present and properly modified may be 

 termed leaf xerophytes, and those in which the stem has been 

 modified in the absence of the leaves, stem xerophytes. Various 

 groups of xerophytes have also been distinguished with respect 

 to certain factors which reduce the water supply. Thus, species 

 of saline and alkaline soils have been termed halophytes or salt 

 plants, those of arctic habitats, polar xerophytes, and those of 

 many bogs and swamps, bog xerophytes. The latter are probably 

 not xerophytes at all, while the other two show no essential 

 differences from the ordinary types. They are due to the lack of 

 available water, and exhibit most of the common xerophytic 

 modifications. 



176. Types of leaf xerophytes. In these, adaptation has acted 

 primarily upon the leaf, while the stem has remained normal, or 

 has changed but slightly in most instances. In some cases the 

 leaves have been reduced to scales, but even then they persist 

 throughout the growing season, and continue to take the primary 

 part in photosynthesis. Leaf xerophytes may be arranged in 

 groups based upon the form of the leaf or its structure. Since the 

 same leaf sometimes shows two or more structural modifications, 

 a grouping with respect to form is the most satisfactory. The 



