50 PLANT GEOGRAPHY 



reaches its maximum where a high rainfall and an 

 impermeable substratum cause the soil to become water- 

 logged and acid, especially in the absence of lime, the 

 result being peat. Humus absorbs heat, gives soil an 

 open texture, and absorbs and retains moisture. 



One ingredient of certain soils must be mentioned here, 

 on account of its marked influence on vegetation. This 

 is sea-salt. Few plants can absorb salt-water, and those 

 that do so are essentially xerophytic in character. Salt 

 in the soil-water, in fact, renders it physiologically dry, 

 so that halophytes, as plants of saline situations are 

 termed, are a class of xerophytes. The unfavourable 

 saline condition generally makes a halophytic flora poor 

 in species and in individuals. Among the principal 

 families of halophytes are the RhizophoracecB, the Man- 

 grove-trees of the tropics (Fig. 2), and the Chenopodiacece, 

 such as Atriplex and the Beets. The most striking 

 external characters of halophytes are reduced, thickened, 

 succulent, glabrous, and often glaucous leaves as in the 

 Yellow Horned-poppy, the Sea-kale, and the Sea-holly. 

 Few others are woody like the Mangroves, and internally 

 we find they have small intercellular spaces, little 

 chlorophyll, and abundant cell-sap. Many species not 

 ordinarily halophytic undergo modification in these 

 directions when growing in saline soil, as, for example, 

 the maritime variety of Lotus corniculatus L. crassi- 

 folius Pers. Others differ in these particulars from 

 related non-maritime forms, as, for example, Daucus 

 gummifer Lam., from D. Carota L.. Convolvolus Sol- 

 danella L., and Silene amcena Huds. On the other hand, 

 halophytes, if transplanted, can grow in ordinary soil. 

 In Asparagus, for instance, almost all the soda-compounds 

 in the wild plant are replaced by potash-salts when it is 

 cultivated in garden ground. Halophytic species fre- 

 quently occur in the neighbourhood of inland salt-springs. 



THE BIOLOGY OF THE SOIL. The formation of humus 

 from dead plant-tissues is an important instance of the 

 action of living organisms in the soil, which is not the 

 inert, merely chemical substance it was once thought to 

 be. Aerobic bacteria, micro-organisms, that is, which 

 require free oxygen for their vital action, break down 

 the carbohydrates of plant-tissues, such as cellulose and 

 wood, and their nitrogenous constituents, into "mould" 

 or " mild humus," such as the leaf-mould in woods or 



