NATURE OF PLANTS 55 



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. This sensitiveness to touch is very well shown by 

 fastening with mucilage a bit of sand paper to the tip of the root 

 when the irritation will cause such an excessive growth in the 

 cells of the curving zone as to sometimes cause the tip to move 

 through a complete loop. 



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

 and moisture and food. Roots avoid light and cold and they 

 are thus brought into soils at the proper temperatures. In arctic 

 regions where often only a few inches of the surface of the soil 

 thaws, the roots will be directed straight down to within a short 

 distance of the frozen earth when they will curve at a sharp 

 angle. Every one is familiar with the fact that roots are caused 

 to curve towards and follow the course of decaying tree trunks, 

 being stimulated by the moisture and foods contained in them. 

 Numerous instances might be cited where roots are attracted to 

 wells, drains, etc., even surmounting considerable obstructions 

 in order to reach the water. This localization of the sensitive- 

 ness to all external conditions that affect the root at the very 

 tip is altogether admirable, because it enables it at the very start 

 to come into the most helpful relations to its surroundings. It 

 was a very clever fancy of Darwin to compare this localization 

 to a brain center as found in certain low orders of animals. 



27. Other Root Relations. — While 70 per cent, of our plants 



