THE MATERIAL INCOME OF PLANTS 315 



of a root and may enter it if the conditions permit. But inasmuch as 

 the mineral solutes in the soil waters are very similar, no matter what 

 the character of the soil may be, this is probably of less importance to the 

 plant than it would seem to be at first sight. 



Available water. By no means all of the water in the soil is free to 

 migrate into the roots. There comes a time, as the films about the soil 

 particles become thinner and thinner, when the adhesion of the water 

 to the soil grains is equal to its diffusion tension. Leading up to that 

 equilibrium, it grows increasingly difficult for the plant to balance its 

 loss of water by that entering ; its cell sap has become more and more 

 concentrated; and when the outgo surpasses permanently the income, 

 permanent wilting usually ensues and often more or less extensive 

 death of the foliage. 



The water content of a soil from which no more water can enter a 

 plant manifestly depends upon the plants concerned, the nature of the 

 soil, and other physical factors. It is no fixed quantity in any case, and 

 at best can be determined only roughly. To say that it is in sand less 

 than 0.5 per cent, in clay about 10 per cent, in loam about 12 per cent, 

 in humus about 14 per cent, and in muck about 20 per cent, is merely 

 to indicate the order of magnitude, not to state a fixed amount. These 

 figures become more instructive when compared with the total capacity of 

 such soils for water, which runs about as follows: sand, 15 per cent; clay, 

 50 per cent; loam, 65 per cent; humus, 70 per cent; muck, 120 per cent. 



Effect of roots on soil. A considerable amount of carbon dioxid 

 (CO 2 ) and less quantities of other substances diffuse from the root into 

 the soil-water films. Solution of carbonates is increased by the pres- 

 ence of CO 2 in water, as is shown by the readiness with which a polished 

 marble plate may be etched by roots traversing its surface and giving 

 off CO 2 . Reactions due to other solutes which diffuse from the root, or 

 to excretions from it, may determine the solution of other sorts of soil 

 particles, and the substances so dissolved may then enter the root. It 

 is not known that these changes so produced in the soil are of any con- 

 siderable importance in plant life. Whether by diffusion from the roots 

 of live plants or by the decomposition of dead roots, or by both, it is 

 certain that various complex organic compounds, not yet fully known, 

 exist in soils, which may interfere seriously with the growing of plants 

 thereon. In certain soils the character and quantity of these little 

 known substances are so injurious that the soils are almost sterile. 

 Even a watery extract from them proves harmful. In such cases the 



