How Roots Advance in Soil 65 



formed in advance. The root-cap, then, is a sort of point 

 through which the root advances, and which is being continually 

 replaced by a new growth. 



The increase of the root in diameter throughout its length is 

 produced by the addition of new cells wholly within those which 

 lie in contact with the soil, and the same osmotic pressure is the 

 power which is exerted outward on all sides to move the earth 

 away and give room for the increase in size. 



Since this osmotic pressure in the roots of plants may be very 

 great, certainly more than 100 pounds to the square inch, and 

 presumably several times this amount, and since during the 

 growth of the root the pressure is increased slowly, and acts 

 gradually to set the soil aside, it is not difficult to see that the 

 plant has chosen a method of making its way through the soil 

 which is not only effective, but one which utilizes the energy and 

 the materials present in a soil during the growing season with 

 which to accomplish its purpose. The molecules of soil moisture 

 are at once the hammer and the wedge, which are driven by soil 

 temperature into the growing cells to expand them and set the 

 soil aside. 



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