How Roots Advance in Soil 63 



in the upper four to six feet are never everywhere in close con- 

 tact with one another. There are great numbers of empty spaces 

 all through the surface layers of earth, and we get a very forcible 

 illustration of this fact in setting fence posts. Here we dig a 

 moderate sized post hole, 2 or 2% feet deep, place a 6-inch post in 

 the hole, and then scrape and ram into the same hole all of the 

 dirt which was removed from it, and if the job is well done we 

 have a scant supply to fill it. It is the existence of these unoccu- 

 pied cavities in the soil which enables roots to make their way 

 through it by wedging it aside. In a thoroughly puddled soil it 

 is impossible for roots to develop, not simply for lack of air, but 

 because there is no room into which it is possible to set the soil 

 aside to make place for the root. When a fine-grained soil is 

 thoroughly puddled, all of the small clusters of grains which in a 

 soil in good tilth hold together, are completely broken down, and 

 the smallest particles are packed in between the larger ones until 

 its cavities are so completely obliterated that even water will 

 not penetrate it ; and when this is true there is not even room for 

 the root -hairs to make their way between the angles formed by 

 the soil -grains, for the finest silt and clay particles have been 

 forced into these to fill them up. 



The second fact needed to understand how the root advances 

 itself in the soil is, that it makes use of osmotic pressure to set 

 the soil grains aside. Most of us know with what force dry wood 

 will expand when it becomes wet and is allowed to swell. Iron 

 hoops are burst by the pressure developed. A primitive method 

 of blasting rock was to drive dry blocks of wood into the holes 

 and then wet them. Another method of blasting is to fill the 

 drill holes with unslaked lime and then add water to slake it. In 

 all of these cases, the work is done by osmotic pressure, and 

 the results illustrate how very great this force is when it is 

 restrained, and how thoroughly adequate it would be for the pur- 

 poses of the root in setting aside the soil particles if it could make 

 use of it. 



The method by which the root uses osmotic pressure in mak- 

 ing its way through the soil may be explained with the aid of 

 Fig. 12, which represents diagrammatically the tip of an advancing 



