38 THE NATURE AND WORK OF PLANTS 



tains these acids, and slowly eats away the outer 

 layers, and wherever the roots or their hairs touch, 

 the corrosion is increased so that each small particle 

 in time is entirely dissolved. The rock particles 

 would thus soon disappear from the soil, but frag- 

 ments are constantly being split from the larger 

 rocks by the action of frost, and heat, and the rend- 

 ing action of large roots, so that the supply is kept up. 



42. Food material in the soil and how the plant 

 finds it. The substances which are formed by the 

 corrosion of the rocks and the decay of humus are 

 not found everywhere in the same quantity, but are 

 scattered more or less unevenly through the soil. 

 To be of the greatest service to the plant, therefore, 

 the roots must not only absorb substances, but 

 should be able to find the places where they are 

 most abundant. The roots of the higher plants are 

 capable of doing this, and direct their tips into the 

 places which contain the food and water necessary 

 for the plant. In order to be able to accomplish this, 

 these organs have become sensitive to gravity, light, 

 heat, moisture, and chemical substances. Some of 

 the forms of sensitiveness of the root may be easily 

 observed. 



