CHAP, xii WATER IN THE SOIL 45 



i 



Water in the soil is (i) chemically combined water, which for the 

 most part plays no considerable part in plant-economy ; (2) water 

 absorbed from aqueous vapour of the atmosphere ; (3) water received 

 from atmospheric precipitations and retained by capillary action ; 

 (4) ground-water, or water sucked up from it. 



Ground-water is that collected above the impermeable stratum of soil, 

 and moving according to the laws of gravity or remaining in the soil 

 in sheets, just as does water exposed above the surface of soil. A layer 

 of clay mostly serves as the substratum of ground-water ; sand and 

 gravel permit the passage of water. Ground-water may contain many 

 soluble substances, especially calcic salts ; but when it lies deep it is, as 

 a rule, poor in substances nutritive to plants (it is pure), because these 

 have been retained by the over-lying layers ; it is also devoid of bacteria, 

 because the upper layers of soil have acted as a filter. 



The level of the ground-water and fluctuations in this according to 

 the seasons of the year depend upon the amounts of atmospheric precipita- 

 tion and evaporation, and are of very considerable oecological signifi- 

 cance, and play a most important part especially in the desert. In 

 many cases ground-water lies too high for certain plants ; in other cases 

 it is so far below that the roots cannot utilize it directly or indirectly ; in 

 still other cases it is at such a depth as to be reached by the roots at 

 certain seasons, but not at others. In these cases the height to which 

 the water can be raised by capillary action is an important item. 



The level of ground-water obviously influences the temperature of 

 soil. 1 



The ability of plants to utilize water is very diverse, because the 

 roots penetrate to different depths. Dry summers acquire great signifi- 

 cance in relation to different species, some of which suffer or die sooner 

 than others. 2 Trees with deep roots can thrive even in a dry climate 

 when they are able to reach ground-water. It may be noted that, accord- 

 ing to Ototzky, the level of ground-water invariably sinks in the vicinity 

 of forest, and always lies higher in an adjoining steppe than in a forest ; 

 forest consumes water. 



The significance of the level of ground-water is very clearly demon- 

 strated in Denmark. Here chemical differences in the soil, which has 

 been pulverized and deposited by glaciers, are scarcely so great as in 

 mountainous countries where the rock lies near the surface and possibly 

 reacts on the vegetation by reason of its chemical nature. A case in 

 point, according to Feilberg, 3 is provided by the sandy plains near Skagen 

 in Jutland. When the ground- water in summer is at a depth of three 

 inches, Juncus-vegetation and meadow - moor prevail ; at six inches 

 mosses (Hypnaceae) and Cyperaceae still play a part, but grasses begin 

 to occur ; at nine inches these latter*become dominant ; at twelve inches, 

 normal grass-growth occurs in ordinary summers ; at fifteen inches, 

 cereals thrive in somewhat warm summers ; at from eighteen to twenty- 

 four inches, cereals thrive in cold or moist summers ; at from thirty to 

 forty inches, the soil is unsuitable to cereals, and xerophytes reign. Other 

 examples are given by Feilberg, who lays greater stress, and rightly so, 

 than perhaps the majority of other investigators do, on the importance of 



1 See Chapter XIII. * See Deherain, 1892, and others. 



3 Feilberg, 1890. 



