ROOTS. 



91 



absorb moisture from them. 



Fig. 85. Apparatus to show the influence 

 of water (hydrotropism) upon the direc- 

 tion of roots. The ends {a) of the box 

 have hooks for hanging, while the box 

 proper is a cylinder or trough of wire 

 netting and is filled with damp sawdust. 

 In the sawdust are planted peas {g), 

 whose roots {h. i, k, m) first descend until 

 they emerge from the damp sawdust, but 

 soon turn back toward it.— After Sachs. 



By these root hairs the ab- 

 sorbing surface, and hence 

 the amount of absorption, 

 is greatly increased. Indi- 

 vidual root hairs do not last 

 very long, but new ones are 

 constantly appearing just 

 behind the advancing root 

 tips, and the old ones are 

 as constantly disappearing. 

 (1) Geotropism and hy- 

 drotropism. — Many outside 

 influences affect roots in 

 the direction of their 

 growth, and as soil roots 

 are especially favorable for 

 observing these influences, 

 two prominent ones may 

 be mentioned. The influ- 

 ence of gravity, or the earth 

 influence, is very strong 

 in directing the soil root. 



Fig. 86. 



A raspberry plant, whose stem has been bent down to the soil and has 

 "struck root."— After Beal. 



