ROOTS 59 



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. Explain the differ- 

 ence between this result and that obtained in Exp. XX. 



EXPERIMENT XX 



How do Primary Roots point when uninfluenced by Gravity ? Pin 



some soaked Windsor beans to a large flat cork, cover them with 

 thoroughly moistened chopped peat-moss, and cover this with a thin 

 glass crystallizing dish. Set the cork on edge. Prepare another 

 cork in the same way, attach it to a clinostat, and keep it slowly 

 i-evolving in a vertical position for from three to five days. Com- 

 pare the directions taken by the roots on^ the stationary and on the 

 revolving cork.' 



71. Direction taken by Secondary Roots. — As the stu- 

 dent has already noticed in the seedlings which he has 

 studied, the branches of the primary root usually make a 

 considerable angle with it (Fig. 2). Often they run out 

 for long distances almost horizontally. This is especially 

 common in the roots of forest trees, above, all in cone- 

 bearing trees, such as pines and hemlocks. This horizon- 

 tal or nearly horizontal position of large secondary roots 

 is the most advantageous arrangement to make them use- 

 ful in staying or guying the stem above, to prevent it from 

 being blown over by the wind. 



72. Fitness of the Root for its Position and Work. — The 

 distribution of material in the woody roots of trees and 

 shrubs and their behavior in the soil show many adaptar 

 tions to the conditions by which the roots are surrounded. 



1 See Ganong's Teaching Botanist, pp. 182-186, for complete directions. 

 The brief statement above given is abstracted from that of Professor Ganong. 



