GROWTH AND WORK OF PLANTS 



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 



Fig 30 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 surface 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. 



40. 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 

 by such delicate plants as mushrooms is seen where they burst 



Roots of peas entering soil after anchorage by root 

 hair. 



