106 



THE ECOLOGICAL RELATIONS OF ROOTS. 



usually not more than 1 to 2 mm. wide (fig. 49). Of the 10 plants examined 

 only 3 had roots penetrating deeper than 6 inches. In these cases the tap 

 penetrated rather vertically downward, having no large branches but being 

 covered below a depth of 4 inches with multitudes of fine laterals only a 

 centimeter or two in length. Near the tips they branch profusely and reach 

 depths of 15, 18, and 20 inches, respectively. Even in these plants the 2 or 3 



Fig. 48. — Senecio cernuus. 



strong laterals, 4 to 7 mm. in diameter, originated just below the crown and, 

 tapering rapidly like the tap-root, ran off horizontally at a depth of about 2 

 inches to a distance of 18 to 24 inches from the base of the plant. Unlike the 

 tap, these large laterals were well branched with rather widely spreading 

 rootlets, which gave off multitudes of tertiary branches, all of which ended in 

 a great network of absorbing rootlets. Many fine, much-branched rootlets, 

 only a few inches long, also originated from the crown of the plant where it 

 was buried in the moist duff. The root systems of those described were the 

 same as the others examined, except that the tap-roots of the latter group 

 took a course up the slope and assumed the role of laterals already described. 



Saxifraga bronchialis. — This plant forms large mats, often several feet in 

 extent, which cover the moist soil on the forest floor or even occur on moist 

 rock surfaces. It is especially well developed in the Douglas fir community. 

 From the main root prostrate stems from 1 inch to over 1 foot in length extend 



