ROOTS 39 



rate that new growth occurs. Lawn grasses grow In severe 

 competition with each other and also have the leaf surface 

 reduced by frequent cutting, both of which restrict the 

 growth of the root systems, but they doubtless grow new root 

 areas while others die in a similar way. In the same way 

 parts of the roots of trees die as other roots grow. These 

 decaying roots are very beneficial to soils by adding humus 

 and leaving pores for the exchange of air and the entrance of 

 water, preventing run-off except in unusually heavy rainfall. 



Mycorrhiza is a symbiotic fungus growth on the young 

 roots of many plants, especially trees, which prevents the 

 growth of root hairs, but does their work in the absorption 

 of water and salts from the soil for the plant and in return 

 gets mycorrhiza food from the plant on which it grows. It is 

 now considered to be so beneficial to the growth of forest 

 trees that extensive experiments in the inoculation of plant- 

 ings in new areas are in progress. The fungus covers the 

 young roots with a dense compact mantle of fine hyphae like 

 those of the common bread mold but much more extensive 

 than the ordinary growth of root hairs, and may have func- 

 tions in addition to those ordinarily attributed to the root 

 hairs. In the woods, especially in the spring or fall, one may 

 see many toadstools, some of which are the spore-bearing 

 structures of the mycorrhiza on the roots of the nearby trees. 



Chapter 21 deals with the problems that the gardener 

 must consider to improve the conditions that favor root 

 growth. It has been shown that roots grow longer and 

 branch more with a low or medium soil moisture, than with 

 a high soil moisture; this may be associated with aeration. 

 Since roots do not penetrate a hard compact soil so freely 

 as a looser one, deep cultivation, even trenching, before 

 planting will encourage the development of the root system 

 in a larger volume of soil. Roots branch more in a soil with 

 a favorable amount of mineral salts. Organic matter in- 



