NATURE OF PLANTS 63 



structed cells for conveying this impulse comparable to the nerves, 

 but it has been shown that chemical changes are set up that ex- 

 tend from the tip to the region of curvature where a more ex- 

 tended growth of the cells on the upper side of the root is induced 

 than on the lower side. This results in bending the root down 

 into the soil. These reactions apply to the main or tap root. 

 The lateral branches, or secondary roots, tend to grow more or 

 less at right angles to the stimulus of gravity, while the short 

 tertiary branches radiate out in all directions. Why the stimulus 

 of gravity causes a more extended growth, on one side of the tap 

 root than on the other or why the lateral roots are so differ- 

 ently affected can not yet be answered, but the nicety of these 

 reactions in enabling the roots to reach in all directions and 

 thoroughly explore the soil in their quest for food, is very ap- 

 parent. The cells of the root tip are also sensitive to touch or 

 contact. This enables the root to avoid obstacles and grow 

 around rocks and stones. In this case the irritation of the root 

 tip produces a more considerable growth in the zone of curva- 

 ture on the side where the root is irritated with the result that 

 the tip of the root is bent from the object. In this way roots 

 work their way through the soil and avoid obstacles and grow 

 around rocks, for as soon as the irritation of the obstruction is 

 removed by the curvature, gravity will cause the root to grow 

 down again. 



The cells of the root tip also react to light intensities and to 

 definite percentages of moisture and mineral foods. Roots avoid 

 light and as a consequence they grow down into the earth where 

 they are directed in their growth, owing to the peculiar sensitive- 

 ness of their cells, so that they come to lie in soils containing 

 suitable moisture and crude food materials. Every one is fa- 

 miliar with the fact that roots are caused to curve towards and 

 follow the course of decaying tree trunks, being stimulated by 

 the moisture and food contained in them. Numerous instances 

 might be ciited where roots are attracted to wells, drains, etc., 

 even surmounting considerable obstructions in order to reach the 

 water. This localization of the sensitiveness to all external 

 conditions that affect the root at the very tip is altogether ad- 



