76 



THE ECOLOGICAL RELATIONS OF ROOTS. 



varying in diameter from 1 to 3 mm., not including numerous smaller ones. 

 These larger laterals usually pursue a course almost parallel with the surface 

 for a distance of 5 to 22 inches, and then almost invariably turn abruptly 

 downward, reaching depths of 10 to 35 inches. Throughout their course they 

 branch freely into both large and small rootlets, the larger branches spreading 

 widely and then turning downward after the fashion of the main laterals. 

 Throughout the course of the laterals and their larger branches, a striking 



Fig. 28. — Gilia longiflora. 



system of fine absorbing rootlets, branched to the second and third order, 

 is found. Below the point where the tap throws off the larger laterals, it is 

 much more poorly branched, the branches being relatively much smaller and 

 shorter, but well supplied with fine rootlets, as are the laterals above. From 

 the figure and description it will be seen that although this is an annual, the 

 absorptive system is characteristic of that of most of the species found in the 

 sandhills. The roots are glistening white and quite tough for herbaceous plants. 



Euphorbia petaloidea. — Euphorbia is a sandhill pioneer which forms both 

 families and colonies. The root system begins with a strong, smooth tap- 

 root, varying in diameter according to the age and size of the plant, the 



