ROOTS. 



91 



absorb moisture from them. By these root hairs the absorb- 

 ing surface, and hence the amount of absorption, is greatly 



increased. Individual 

 root hairs do not last 

 very long, but new 

 ones are constantly ap- 

 pearing just behind the 

 advancing root tips, 

 and the old ones are 

 as constantly disap- 

 pearing. 



(1) Geotropism and 

 hydrotropism. Many 

 outside influences affect 

 roots in the direction 

 of their growth, and 

 as soil roots are espe- 

 cially favorable for ob- 

 serving these influ- 

 ences, two prominent 

 ones may be mentioned. The influence of gravity, or the- 

 earth influence, is very strong in directing the soil root. 



FIG. 85. Apparatus to show the influence of 

 water (hydrotropism) upon the direction of 

 roots. The ends () of the box have hooks 

 for hanging, while the box proper is a qylinder 

 or trough of wire netting and is filled with 

 damp sawdust. In the sawdust are planted 

 peas (g), whose roots (A, i, k, m) first descend 

 until they emerge from the damp sawdust, 

 but soon turn back toward it. After SACHS. 



FIG. 86. A raspberry plant, whose stem has been bent down to the soil and has " struck 

 root.' 1 After BEAL. 



