CHAP. XII WATER IN THE SOIL 49 



differences between species of plants depend upon the sum of the leaf- 

 surfaces and on the leaf-structure, the nature of the root-system, and 

 whether this last is shallow or deep ; thus, in the forest various species 

 act as weeds because they consume water before this can reach the tree 

 roots. We may thus explain how it is that some species are less protected 

 than others in the same habitat. In forests the surface of the soil is 

 protected by the tree-trunks, and consequently remains moist ; but the 

 sub-soil, on the contrary, is robbed of its moisture to a greater extent 

 than when under herbaceous vegetation, because of the activity of the 

 roots of the trees. 



Roots can utihze water present in the soil only to a certain degree. 

 The more the water in the soil decreases in amount the more firmly is 

 the remaining water held fast, until a point is reached at which the plant 

 can obtain no more, although a large quantity may still be left behind. 

 Sachs was the first to demonstrate this by investigations on the tobacco 

 plant.i A young plant began to wither when the soil (a dark humus) 

 still contained water equivalent to 12-3 per cent, of its dry weight ; the 

 water-capacity of the soil was determined by drying it at 100 C, as being 

 46 per cent, of its weight ; the plant was able to take up only 337 per 

 cent., and the rest of the water was unavailable to it. Under the same 

 conditions the plants withered on loam and on sand when the percentages 

 of water remaining were 8 and 1-5 per cent, respectively. According 

 to Heinrich's experiments, plants first began to wither in coarse sand 

 when the amount of water had sunk to 1-5 per cent., but in peat when 

 the amount was still 477 per cent. 



A soil from which a species is incapable of extracting water may be 

 described as dry to that species, even though a large quantity of water 

 may be present in it {physiologically dry)? Physiological dryness alone 

 plays a part in distribution of plants.^ 



A dead vegetable covering also influences evaporation.^ 



A soil of considerable humidity may partially replace a moist cUmate. 

 In tropical savannahs the banks of streams are clothed with forest. 

 Furthermore, in steppes and deserts, trees occur where there is running 

 water or where ground-water approaches the surface. Many perennial 

 herbs which in Europe favour a dry sandy soil, occur in the hot dry lowlands 

 of Madeira exclusively on wet soil in the vicinity of springs and water- 

 courses.^ But it is worthy of note that a moist soil cannot always replace 

 atmospheric humidity. Most species of Erica flourish on very dry soil, 

 but are excluded from places with dry air. On the other hand, Tamarix 

 gallica clings to the banks of rivers not only in the Sahara, but also in 

 Central Europe. 



The significance of water in the soil to plant-form. In addition to what 

 has been said in Chapter VII, concerning the significance of water, it may 

 be mentioned that the production of adventitious roots on prostrate 

 shoots is evidently promoted by moisture ; nowhere else is the production 

 of adventitious roots so abundant and common as in moist places.*^ 



' Sachs, J. von, 1865, p. 173. " Schimpcr, 1898; see p. 134. 



* Schimper, 1898 ; also see Kihlman, 1890 ; Hedgcock, 1902 ; Clements, 1904 ; 

 Burgerstein, 1904. 



' See Chapter XVIII. M. Vahl, 19046. 



' Warming, 1884, 1892. 



WARMING E 



