HALOPHYTES 469 



444. Form and structure of halophytes. Most halophytes 

 present certain peculiarities of form and structure, such as suc- 

 culence of stem or leaves, or both, a highly developed palisade 

 layer, small intercellular spaces, diminution of evaporating sur- 

 face, and often specially developed tissue for storage of water. 

 These are evidently xerophytic characteristics, and their pres- 

 ence may be due to two causes : 



First, the occurrence of salt in the soil renders absorption of 

 the soil water comparatively difficult, since osmosis takes place 

 more readily between nearly pure water and the liquid con- 

 tents of the young roots and root hairs than between salt water 

 and the liquids within the root. Halophytes may therefore be 

 on a short allowance of water even when their roots are con- 

 stantly wet. 



Second, the absorption of much salt would poison the plant, 

 and therefore it is an advantage to keep down transpiration 

 and with it the rate at which salt water is allowed to enter 

 the roots. 



445. Halophytes not dependent on salts. It is worth while 

 to note the fact that halophytes are not usually dependent on 

 a highly saline soil. They are salt tolerators rather than salt 

 lovers. 1 But they nourish in saline localities because they are 

 capable of enduring much more salt than ordinary plants, and 

 so can grow in salt marshes and such localities comparatively 

 unhindered by the competition of non-halophytic species. 



446. Other kinds of ecological classes. One may class plants 

 with reference to their habits in many other regards than 

 according to their relative economy of water or their tolerance 

 of salts. 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 

 are found growing on the soil in the twilight formed by the 

 shade of the trees. Some of this undergrowth is of seed plants, 



1 Or, in technical terms, the plants which grow in saline soils are facul- 

 tative halophytes but not obligate halophytes. 



