126 EXPERIMENTS WITH PLANTS 



excludes air. Paved streets and sidewalks often cause 

 injury to trees by preventing access of air; for this 

 reason it is better to leave an open space about them 

 and aerate the soil by frequent cultivation. We fre- 

 quently see Willows and other trees on the banks of 

 streams living with their roots completely submerged in 

 water or in saturated soil ; this seems at first glance to 

 be in contradiction to what we have just learned about 



92. Diagram to illustrate the effect of ideal plowing. The compactness of the 

 soil is indicated by the density of the shading. Before plowing, there is a 

 compact surface crust (*), below which the soil grows less compact as we go 

 deeper; after plowing, this compact mass is broken up into a loose, friable 

 mass of soil-crumbs, or floccules, with a consequent increase in the bulk of 

 the furrow-slice (fs); compacted plow sole at (pi). (After Hilgard.) 



the necessity for a supply of air for the roots. In this 

 case, however, the roots are in running water and are 

 able to make use of the small but continually renewed 

 supply of air which it contains. Where the soil is satu- 

 rated with water, without free circulation, the air-supply 

 is quickly exhausted. Not only are the roots unable to 

 breathe but chemical processes injurious to the plant 

 are set up in the soil. 



Plowing and surface tillage are the principal means 

 used to secure good tilth. The result of ideal plowing 

 is shown in Fig. 92, in which the density or compact- 

 ness of the soil is indicated by the density of the 



