124 EXPERIMENTS WITH PLANTS 



due to the absorption of water; when we draw this 

 water out, the root-hairs collapse. 



Just as in the ease of the seed (see page 18), the 

 presence of water- attracting substances (e. g., salts, 

 etc.) in the soil- water hinders absorption by the root; 

 this is the case along the seashore where the water 

 is brackish and in alkali soils. 



In order that the roots may be able to explore the 

 soil freely and absorb water from it, it must be in the 

 proper physical condition or, to use a more common 

 term, it should be mellow, i.e., of a loose, friable 

 texture. The physical condition of the soil is" known 

 as tilth. 



How may the soil be kept in good tilth! This in- 

 volves two things: (1) keeping the soil-crumbs of the 

 proper size, (2) keeping up the circulation of air in 

 the soil. 



1. The size of the soil-crumbs is of the greatest im- 

 portance. If they are too large, the surface exposed 

 to the action of the roots is relatively small. Suppose 

 chat by tillage we break them up so that they are only 

 one -tenth of their original diameter, we thereby in- 

 crease the surface available to the roots ten times and 

 multiply the possibilities of plant-growth accordingly. 

 Soil composed of too large crumbs not only allows the 

 rain-water to leach through too rapidly, but is unable 

 to lift it up again to any great extent from the water- 

 table. 



