48 PLANT GEOGRAPHY 



(3) Hygroscopic water is that which clings to the 

 particles in thinner films so as not to travel by surface- 

 tension, or to be available to the root-hairs, or to be 

 removed until driven off by steam at temperatures 

 above the boiling-point. 



Permeability, or the readiness with which free water 

 passes through soils, especially coarse-grained soils, is 

 inversely proportional to capillarity. The retentiveness 

 of soil, i.e. its power of retaining liquid, depends mainly 

 on its capillarity; and this again, as we have seen, upon 

 the size of the soil-particles; which practically means 

 the proportion of clay or humus present. 



Few, if any, factors are more effective in altering the 

 character of the vegetation of any area than changes 

 in the level to which capillary water rises in the soil 

 " the water-table," as it is termed. 



It is as dilute solutions in the soil-water that plants 

 receive the nitrates, especially of potash, phosphate of 

 lime, sulphates, chiefly of lime, sodium-chloride (common 

 salt) and iron compounds, probably largely carbonate, 

 which they require, together with other less useful 

 mineral substances, such as carbonate and silicate of 

 lime. There is, however, a limit of concentration beyond 

 which roots will not absorb. This seldom exceeds 

 5 per cent., i.e. 'soil containing water with upwards of 

 5 per cent, of saline matter is physiologically dry. 



Soil temperature is a geographical factor of paramount 

 importance. It depends on the duration and angle of 

 incidence of the sun's heat-rays, the specific heat, colour, 

 porosity, density, and water-content of the soil, the 

 presence of a covering of vegetation, and other causes. 

 The heating power of the sun's rays is proportional to 

 the cosine of the angle of their incidence; so that it 

 diminishes with latitude, and is affected by slope and 

 exposure, or aspect. The specific heat of sand is .2, 

 water being i ; that of peat about .5, i.e. it is much 

 easier to heat sand than to heat peat ; but dark-coloured 

 soils absorb heat more readily by day, and also radiate 

 more rapidly at night. Porous soils are rapidly heated, 

 but lose heat equally rapidly by radiation, while the 

 conductivity of soils and rocks for heat seems to vary 

 with their density. The great specific heat of water, 

 i.e. the large amount of heat required to raise its tem- 

 perature, makes its abundance in the soil render the soil 



