28 



GROWTH AND WORK OF PLANTS 



the gravitation influence. The turning of the root towards the 

 earth is positive geotropism. The turning of the stem away from 

 the earth is negative geotropism. The primary root shows posi- 

 tive geotropism, the primary stem negative geotropism. Lateral 



roots show transverse geo- 

 tropism, since their direction 

 of growth is sideways or 

 lateral. 



45. Change in the direc- 

 tion of growth of lateral 

 branches. This takes place 

 when the main shoot or 

 "leader" is destroyed or re- 

 moved. One or more 

 branches change from growth 

 in a lateral direction to an 

 upright direction. This is 

 very striking in certain trees 

 like some conifers (pines, 

 spruces, etc.). When the top 

 of a young pine is cut off one 

 or two of the lateral branches 

 gradually curve upward and take the place of the leader. This 

 is sometimes very common in the white pine where the larva of 

 a beetle kills the top of the main shoot or leader. Pine or spruce 

 trees struck by lightning in such a way that a large part of the top 

 is broken off, present a few years later a very singular appearance, 

 the topmost lateral branches remaining spread out for some dis- 

 tance. They are too old and firm for one or two of them to 

 turn upward and take the place of the " leader." But a number 

 of small branchlets on the upper surface of several of these lateral 

 branches grow directly upward making a small forest or grove of 

 trees in the top of a single tree. 



46. Work performed by roots in penetrating the soil. 

 One of the important kinds of work which the roots perform is 

 the penetration of the soil or substratum where the plant is 



Fig. 32. 



Pea seedling; lateral roots turning downward 

 after primary root has been cut off. 



