GROWTH OF ROOT AND STEM 



earth is positive geotropism, or progeotropism. The turning of 

 the stem away from the earth is negative geotropism, or apogeo- 

 tropism. The primary root is 

 therefore progeotropic, the pri- 

 mary stem is apogeotropic. 

 Lateral roots show transverse 

 geotropism, or dia geotropism, 

 since their direction of growth 

 is sideways or lateral. 



38. 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 lat- 

 eral direction to an upright 



Fig. 29. 



Pea seedling ; lateral roots turning downward 

 after primary root has been cut off. 



direction. This is very strik- 

 ing 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 remain- 

 ing spread out for some distance. 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 sur- 

 face of several of these lateral branches grow directly upward 

 making a small forest or grove of trees in the top of a single 

 tree. 



39. 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 



