88 THE ECOLOGICAL RELATIONS OF ROOTS. 



the plant for a radius of more than 18 inches. These were extraordinarily 

 well-branched and rebranched and were very dense. At 5 inches in depth 

 another large lateral occurred, and the root broke into 2 nearly equal parts at 

 6 inches in depth. One of these ran off into the bank and slightly upward to 

 a distance of 2 feet from the base of the crown, but it was really much longer. 

 It ended 3 inches below the soil surface. This lateral, scarcely more than 1 

 mm. in average diameter, gave off both short and long branches in a profuse 

 manner, all being repeatedly branched into minute termini and densely covered 

 with root-hairs. They also dipped upward to within 2 inches of the surface 

 and downward to 8 or more inches, but were most abundant at 4 or 5 inches in 

 depth. The other root at the fork ran out into the slope for over 5 feet at an 

 average depth of 16 mches. The roots often run long distances without much 

 branching, but the last 2 or 3.5 feet form a great network of branches. The 

 roots are reddish in color and relatively tough. None of those examined were 

 deeper than 2 feet (fig. 36). 



In another specimen the root system began to divide almost from the 

 crown, some of the main parts, however, being much larger than the others, 

 varying from 2 to 13 mm. All the rootlets, whether large or small, were 

 marked by a diffuse dichotomous branching. Most of the branches were 

 superficial, extending to a depth of 1 to 12 inches. These branches were 

 themselves branched many times, and the termini of the branches of all sorts 

 were persistently marked by capillary roots, ramifying and forming a con- 

 spicuous network in the soil. The course of most of these laterals was par- 

 allel with the surface. The maximum spread of the main absorbing branches 

 was found to be 4.5 feet. A very few of the larger branches take a more 

 marked downward course, branching very much the same as the horizontal 

 laterals, but the branches are not so numerous nor so long. The maximum 

 depth recorded was 39 inches and the tip of the root had wandered 43 inches 

 from the vertical. 



GRAVEL-SLIDE ROOT SYSTEMS AND GRAVEL-SLIDE ENVIRONMENT. 



All of the plants of this community are similar in possessing roots 

 adapted to secure moisture and nutrients from the surface soil. They 

 are characterized by a shallow, widely spreading root system confined 

 largely to the first 18 inches of the soil and in fact usually best developed 

 in the surface foot. The lack of depth is compensated by a remarkably 

 wide lateral extent combined with a profound system of branching. 

 An explanation of these adaptations is readily found in a study of 

 the soil and the distribution of the rainfall. 



The gravel-slide soils consist of a superficial layer of coarse angular 

 rock fragments varying in size from over an inch to a few miUimeters 

 in diameter. Except during rains this surface layer is very dry. The 

 slope is so steep thkt there is often a constant movement of the rock 

 particles down the mountain-side, the course of these moving pebbles 

 being marked by depressions looking not unlike the tracks of harrow 

 teeth. Most of the plant tops have slipped down the slope from 2 to 

 8 or more inches. This surface layer is very efficient in preventing 

 run-off, as well as in forming a dry mulch and thus protecting the 

 underlying soil from high evaporation. A concrete illustration of the 



