92 



THE ECOLOGICAL RELATIONS OF ROOTS. 



VII. THE HALF-GRAVEL-SLIDE COMMUNITY. 



The half-gravel-slide community represents a distinct successional 

 advance over that of the gravel-sUde, both in the diversity of species 

 and the density of plant population, as well as in the correspondingly 

 more favorable habitat (plate 25, a). It is an intermediate stage 

 between the gravel-slide and the forest. Here the root systems of the 

 most important grasses, herbs, and shrubs were studied. 



Elymus triticoides. — ^This grass is perhaps the most abundant and important 

 species of the half-gravel-slide community. Because of its excellent root 

 development and consequent ability to compete successfully with other 

 species, it not infrequently controls large areas, sometimes almost to the 

 exclusion of other plants. 



Elymus was excavated at the half -gravel-slide station in coarse, rocky soil 

 (plate 25, a, b). The soil of the upper 18 to 22 inches was light brown to gray 

 in color. The roots were densely matted. The larger were 1.5 to 2 mm. in 

 diameter, being much branched into fine rootlets supplemented by countless 

 finer well-branched and matted ones. In fact, the soil to a depth of 15 or 18 

 inches was completely filled with a dense network of roots. The larger ones 

 penetrated to a maximum depth of 46 inches. The mass of roots at the surface 

 had a lateral spread of about 18 inches on either side of the bunch. Many of 

 the larger roots descended at an angle of about 30 degrees from the vertical 

 to a depth of 15 or 20 inches, when they turned directly downward to a depth 

 of 40 to 45 inches. All of these deeply penetrating roots were, like the others, 

 extremely well branched to the third and fourth order and ramified through- 

 out the crevices of the gravel. Some of these larger roots, after reaching a 

 depth of 13 inches, took a course up the steep slope, following at this depth a 

 line approximately parallel with the surface of the soil. They were well 

 branched, the branches coming up to a distance of only 5 or 6 inches from the 

 surface of the ground. The lower part of the root system was white in color 

 and extremely fragile. 



Solidago oreophila. — ^The autumn landscape is frequently given tone by the 

 masses of yellow flowers of this important half-gravel-slide species. This 

 plant consists of clusters of stems connected by short rhizomes, thus forming 

 a clump. From the base of the rhizomes arise great numbers of fibrous roots 

 about 1 mm. in diameter; as many as 50 to 75 may originate from a single 

 inch of the rhizome. Numerous roots run off parallel with the soil surface, or 

 nearly so, to a distance of 20 to 30 inches from the base of the plant, sending 

 off rather numerous branched and rebranched threadlike laterals from a few 

 centimeters to a few inches long and finally terminating in a much-branched, 

 brush-like ending. Many of the shallower roots also rim off obliquely, so that 

 at a distance of a foot from the plant they may reach to a depth of 8 to 10 

 inches or even more. However, these are not so numerous as the superficial 

 ones. These shallower roots frequently turn down near their tips to a depth 

 of 6 to 15 inches. 



The vertically descending roots send off laterals rather sparingly to a dis- 

 tance of 12 or 18 inches, beyond which depth they become more and more 

 profusely branched, finally terminating in great clusters of hairlike, minutely 

 branched ends. In the more decomposed soils the deepest roots may pene- 

 trate to a distance of 30 to 38 inches, while ia the more rocky substratum 

 they are much shallower. This deeper group of roots usually spreads laterally 



