— 26 — 



The roots of cultivated plants cannot nnake use of this water, the- 

 refore in considering the percentage of water containd in the soil, this 

 factor must always be kept in view, for the figures „12"/()" may really 

 mean that in a given soil the useful water amounts to only 10, 8, 

 4 and even 2" 'o. 



The soil on the Odessa experiment field holds about 10", useless 

 water and in the following statements the serviceable water will be prin- 

 cipally taken into account; that is, the quantity of water over and above 10"/o. 



In studying the laws of water circulation and the role played therein 

 by the roots the following consideration must not be lost sight of. 

 Uoung soots hold so much water (80" o and more) that their growth is 

 only possible where there is liquid water, and the more frequently the 

 tips of the rootlets meet with water drops in the soil, the more normal 

 will be the growth of the root. Therefore the ideal surroundings for roots 

 are those where all the interstices between the hard particles whether 

 of soil, chaircoal or other matter, are replete wixh water. Then the root- 

 covers will meet with no difficulty at any point in obtaining water and 

 the grov/th of the root will be continuous. 



Now suppose that the interstices of the soil are not all filled with 

 water, but that some contain air, as stated above. The more frequently 

 such vacuities are met with the more difficult will be the growth of the 

 root; and it may happen that the d'stance separating the drops of water 

 is too great for the growing tips of rootlets to traverse, without the aid 

 of the requisite water on the way. Developement in the growth of the root 

 will cease; and if many roots of the same plant happen to be in such a 

 dry vicinity their developement as well as that of the whole plant will 

 cease or the growth may proceed more slowly if the rest of the roots 

 find water. And the oftener any part of the roots of a plant finds water, 

 the better will be the developement. For that reason the whole root- system 

 of a separate plant may show a far from equal developement in all its 

 parts. The hydrotropism of roots (fig. 6) is met with on our dry steppe 

 soils in a very severe form. 



All the stages of this process may be found in the same soil: part 

 of the roots may find themselves in an horizon saturated with water; 

 further on, air gaps may be met v/ith at intervals; then, more frequently, 

 but small in extent and mterrupted, spaces filled with water drops; then 

 only a few drops of water dispersed simply or several together, in a 

 fairly large area of soil; and lastly we find no drops of water at all in 

 the soil but only that which has adhered to the upper surface of the soil 

 particles in the shape of an extremely thin layer. In the last instance 

 we have useless water; for when the tip of a rootlet comes into contact 



