GROWTH OF ROOT AND STEM 



29 



growing. Even where the soil is quite hard and compact we 

 often find it permeated with a perfect maze of delicate roots. 

 The action of the root in penetrating the soil is much like that of 

 a wedge, except that the driving force is different, and the for- 

 ward movement of the 

 root lies in a short section 

 just back of the tip which 

 is constantly shifting 

 from old cells to new 

 ones. The driving force 

 for forward movement in 

 the root comes from the 

 growth of the cells in the 

 zone of elongation, while 

 the widening force comes 

 mainly from growth in 

 thickness. Since the 

 driving force comes from 

 elongation of cells near 

 the tip of the root, the older part of the root must be held in 

 place, otherwise the root would simply be pushed backward 

 out of the ground. If the seed germinated when on the sur- 

 face of the ground the root could not well enter. In case of 

 the seedling the root hairs serve to anchor the young root until 

 the lateral roots are formed, when the young root system as a 

 whole furnishes the anchorage. The tip of the root is pushed 

 with considerable force against the soil particles in advance, 

 and being conical in form turns them to one side. The rigidity 

 of the older parts of the young root, as well as the wall of earth 

 around it, prevents the root from bending. 



47. Force exerted by growth. The force exerted by roots 

 and stems as a result of growth is remarkable. Even in the case 

 of young seedlings the hard crust of the soil is often broken as 

 the stem rises. In rocky places it. is a common thing to see small 

 crevices in rocks, where the slender roots of trees enter, broaden, 

 and the rock is split apart as the root enlarges. The force exerted 



Fig. 33- 



Roots of peas entering soil after anchorage 

 by root hair. 



