32 



ROOTS 



be noticed that the growing tips of the roots of the sprouting 

 peas point almost directly outward from the center of the disk 



on which the seedlings are fastened. 

 In this case the so-called "centrif- 

 ugal force" due to the rotation of 

 the wheel is sufficient wholly to 

 overcome geotropism. 



42. Direction taken by secondary 

 roots. As the student has already 

 noticed in the seedlings which he 

 has studied, the branches of the 

 primary root usually make a con- 

 siderable angle with it (Fig. 2). 

 Often they run out for long dis- 



FIG. 25. Sprouting peas on a 

 rapidly whirling disk 



The youngest portions of the roots 



all point directly away from the tances almost horizontally. This is 

 axis about which they were re- especially common in the roots of 



volved. After Detmer 



forest trees, above all m cone-bearing 



trees, such as pines and hemlocks (Fig. 26). This horizontal, 

 or nearly horizontal, position of large secondary roots is the 



FIG. 26. Roots of a white pine 



most advantageous arrangement to make them useful in stay- 

 ing or guying the stem above to prevent it from being blown 

 over by the wind. 



