THE RELATION OF THE PLANT TO WATER 157 



tain a much higher percentage of water than a coarse soil. A 

 soil with a high humus content will also retain ,a much higher 

 percentage of water than a soil with a low humus content. 

 Therefore, it will be readily seen that the water-holding power 

 of coarse sand is low as compared with a fine sand, that clay and 

 loam have a higher power of water retention than sand and that 

 humus has a higher power of water retention than clay or loam. 

 The Rooting Habits of Plants. Plants differ very materi- 

 ally in their rooting, habits. Some plants send their roots deep 

 into the soil, while others spread their roots out near the surface. 

 Although the spreading and distribution of the roots in the soil 

 is characteristic of the species of plant under consideration, it 

 is influenced and modified to some extent by the character of the 

 soil and the water supply and by their need for air (page 225). 

 There is always a definite relationship between the top and the 

 root system of the plant. In transplanting it is usually necessary 

 to injure or destroy a part of the root system, and for this reason 

 the growers of many crops, especially trees, prune the tops when 

 transplanting, so that the root system will be proportionately as 

 large or larger than the top. In due process of growth the plant 

 will reestablish the equilibrium between the top and the root 

 system. The extent of the root systems of plants is surprising 

 to those who have not given it some consideration. When we 

 pull a plant from the soil many of the roots, especially the 

 smaller ones, are torn off and we see but a very small part of 

 the system. In the resetting of potted plants, the extent of the 

 root system is revealed to much better advantage. The roots of 

 our cultivated grains, such as corn, wheat and oats, grow to a 

 depth of three to four and a half feet.. A potato plant may, under 

 favorable conditions, spread its roots through the greater part of 



