[Chap. XXIX ROOTS: DEVELOPMENT AND STRUCTURES 



307 



Contrary to common be- 

 lief, roots of our common 

 plants which have been ex- 

 perimentally tested do not 

 grow through a layer of air- 

 dry soil to moist soil beneath 

 (Fig. 127). When a portion 

 of the root system of a plant 

 is exposed to moist soil and 

 the remainder of its system 



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Fig. 126. Roots of hemlock and spruce on 

 a rocky slope in the Smoky Mountains. The 

 layer of mosses and ferns under which they 

 first grew have been washed away. The 

 terminal roots are located deep in the crevices 

 of the rock. Photo by E. N. Doan. 



to dry soil, the portion in 

 moist soil grows extensively, 

 while the portion in dry soil 

 grows but little or not at all, 

 depending upon the dryness 

 of the soil. Similar unequal 

 growth of portions of the root 

 system of a plant occurs in 

 relation to several other fac- 

 tors. 



The horizontal distribution 

 of the root svstem of a plant 

 is not dependent upon either 

 the diameter or the height of 

 the part above ground. The 

 frequent statement in popu- 



