116 



THE ECOLOGICAL RELATIONS OF ROOTS. 



Three plants were examined. The description for one will answer in general 

 for all. The two largest had tap-roots 6 and 7 mm. in diameter respectively; 

 the smaller was only 3 mm. in diameter. Both of the larger plants had 3 

 stems which reached a height of 14 to 16 inches and were either in blossom or 

 in fruit; hence these may be considered typical mature plants. The tap-roots, 

 except where they twist and curve irregularly back and forth for an inch or 

 more in crevices of the cloddy soil, pursue a vertically downward course. The 

 smallest reached a depth of 34 inches, the largest 40 to 43 inches respectively. 

 As a whole the root is very poorly branched, no large laterals being given off. 

 Beginning near the surface, however, and extending quite to the tip, short 

 threadlike laterals, seldom exceeding 0.2 to 0.3 mm. in diameter and often 



Fig. 55. — Abronia fragrans from the sandhills. 



arising in groups of 2 or 3, run out laterally for a distance of less than 1 cm. 

 to more than 4 inches. The longer ones occur within the surface foot of soil. 

 These branches are very scattering, often 3 or 4 cm. of root having no branches 

 whatsoever. The tip is only poorly branched. The older roots are black in 

 color, this being confined to the outer layer of cortex, the interior being white. 

 The younger roots vary in color from brown to yellowish. They are removed 

 only with extreme difficulty from the hard soil. The tiny lateral rootlets are 

 seldom branched beyond the second order and do not form a network (plate 

 29, c). 



Four more rather large plants, all in flower or fruit, were examined. They 

 were growing on a captured sand-dune and had tap-roots varying from 6 to 

 10 mm, in diameter. One of the largest of these is shown in figure 55, The 

 tap is prominent throughout and tapers slowly to the tip, which reaches a 

 depth of from 40 to 48 inches. The plant is characterized by 2 types of 

 branches; the shorter ones vary from 2 to 20 mm, in length and occur in tufts 



