x.] PHYSIOLOGICAL DRYNESS 273 



salt marshes invaded by the sea, develop a xerophytic 

 structure, because they would be injured if they absorbed 

 large amounts of saline soil water. Peaty areas also 

 act in the same way, for it is found that the humic acids 

 in such soils withhold the water from the plant very 

 obstinately. Exposed elevated regions with a low 

 temperature, by reducing the power of the roots to 

 absorb moisture, render it necessary that the plant 

 should lose little by transpiration; hence we see 

 certain conifers flourish both on dry sandy soils and 

 wet elevated moors. 



As regards the chemical side of the question, the 

 most important soil constituent affecting vegetation is 

 calcium carbonate; a large number of plants seem 

 absolutely intolerant of lime in the soil, while others 

 are rarely seen off limestone and chalk areas. Even 

 among the humus-loving plants a different flora is found 

 on the acid peaty areas from that prevalent on the 

 mild humus areas where the soil water contains calcium 

 bicarbonate in solution. 



But, however characteristic the general aspect of 

 the vegetation may be upon the different types of soil, 

 it is rare to find cases of plants entirely intolerant of 

 a different kind of soil from that which they habitually 

 frequent; many plants show a preference for one soil 

 or other without being exclusively confined to it For 

 example, the common primrose is undoubtedly a clay 

 lover, yet it will be found widely distributed over all 

 the English soils; the beech and the yew are typical 

 trees of the chalk, good oak and hornbeam of the clay ; 

 Spanish chestnut, and many conifers like the Scotch 

 fir, are sand lovers ; yet each of these trees will be 

 found commonly enough on other kinds of soils. It will 

 rarely be found that plants will absolutely refuse to grow 

 or even to flourish on soils of which they are naturally 



S 



