134 ROOTS 



result is that both roots and stems continue to grow in 

 whatever way they began, at least so far as gravity is 

 concerned. 



The tropism induced by water is called hydrotropism. 

 Water seems to produce even more effect upon the direc- 

 tion of root growth than gravity does. If a seedling is so 

 arranged that gravity attracts it in one di- 

 rection while water attracts it in another, 

 the water seems to have a stronger in- 

 fluence. (See Figure 44.} If root tips come 

 through the bottom of a wire cage in which 

 seeds have been sprouted in damp moss, 

 they turn back toward the water against 

 the influence of gravity. 



C. Root-cap. It is evident that the 

 FIG 44- - Seedling tender grow i ng tip of a root would be in- 



of corn grown at 



theedgeof afun- jured if it kept rubbing against the soil 



nei whose surface g rams as jt presses forward. It needs pro- 

 has been kept 

 moist. Note that tection. It is protected by what is called 



the root follows tne rO ot-cap. (See Figure 42.) This cap is 



the moist surface ... . . en A i 



instead of grow- composed of several layers of cells. As the 



ing straight rO ot pushes and turns through the soil, the 



outermost layers of the root-cap are gradually 



worn away. They are replaced by new cells which are 



constantly being formed beneath. That same apical meri- 



stem which forms the cells of the root proper also forms 



cells which become a part of the root-cap. It produces cells 



forward as well as back. 



Root-caps are hardly ever to be found on roots which 

 you pull up from the soil. The tips of the roots and the 

 caps are almost sure to be broken off. The screw-pine, a 



