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 most remark- 
able of halophytes. It grows in shallow water along the 
seashore and sends out many aérial 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, I). 
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 
