320 FOUNDATIONS OF BOTANY 



contents of root-hairs and young roots than between salt 

 water and the liquids inside the root. Halophytes, there- 

 fore, are put on short rations as regards water,, even 

 though they may be growing in a watery marsh. Con- 

 sequently halophytes often have much the appearance of 

 fleshy xerophytes and the structure of xerophytes. 



The mangrove tree (Fig. 226) is one of the ifiost remark- 

 able of halophytes. It grows in shallow water along the 

 seashore and sends out many aerial roots which at length 

 find their way down into the salt mud. In this way it 

 collects drift material and gradually extends the shore line 

 farther out to sea. 



392. Other Kinds of Ecological Classes It is easy to 



class plants according to their habits in many other regards 

 than according to their relative power of transpiration (see 

 Chapter XXVI). Only one other kind of classification 

 need, however, be mentioned in this chapter, that is, the 

 division into sun-loving and shade-loving plants. Even in 

 very dense forests some plants will be found growing on 

 the soil in the twilight formed by the shade of the trees. 

 Some of this undergrowth is of seed-plants, and there are 

 many ferns and mosses which flourish in such situations. 

 Shade-plants commonly have large pale leaves, and gener- 

 ally (except in ferns) the leaves are not much cut or 

 lobed (Fig. 227, 1). Sun-loving plants, on the other hand, 

 usually have comparatively little leaf-surface, and the 

 leaves are often cut into narrow divisions (Fig. 227, II). 

 Apparently the broad leaf-surfaces in the one class are to 

 expose many green cells to the light for starch-making, 

 while in the other class the slender leaf-divisions expose 

 enough assimilating cells, and at the same time the 



