THE HALF-GRA\^L-SLIDE COMMUNITY. 



95 



transitional form between the gravel-slide type and that of ordinary soil. 

 The roots are dark tan in color and are fairly tough and resistant. Five 

 plants were examined. 



Geraniuni csespitosum. — This plant is often rather abundant on the half- 

 gravel-sHde, where its size and duration give it considerable importance. 

 Several plants were examined, which in general were similar (fig. 40). The 

 largest had a tap-root an inch in diameter, which at a depth of 3 inches gave 

 off a strong lateral more than 1 cm. in diameter. This ran off horizontally, 

 following a curved course to a distance of 42 inches, and ended only 18 inches 

 below the surface. At a depth of 6 inches the root broke up into 3 parts, all 

 of which grew more or less horizontally, none of the branches reaching depths 

 of more than 37 inches. Most of these laterals approached the surface, the 

 profuse branches often ending within the second to the fourth inch of gravelly 

 soil and forming a fine system for surface absorption. Another lateral ran off 

 at a depth of about 15 inches to a distance of 52 inches from the base of the 

 plant. Geranium also has the same root habit of forming large mats of fine 

 rootlets, not only at the ends of the larger branches but also at the extremities 

 of the numerous smaller ones. The roots are reddish-brown in color, rather 

 brittle, and are uncovered with considerable difficulty. The older roots and 

 the crown are frequently more or less decayed. 



Calamagrostis purpurascens. — This plant holds an important place in the 

 composition of the half-gravel-slide community. Like Elymus triticoides, 

 its root system is so well developed that it can compete successfully with most 

 other species (plate 26, b). Two clumps, each about 8 inches in diameter, 

 were examined. These were old plants with the flower-stalks 15 inches high 

 and in full bloom. From the base of the clmnps almost countless numbers of 

 rather tough fibrous roots arose, the largest scarcely more than a millimeter 

 in diameter, while many were much smaller. Among them were many new 

 roots only a few centimeters long. These ran out in all directions hke the 

 radii in a half sphere. The soil was well filled with those that extended verti- 

 cally and sHghtly obHquely to a depth of 18 or 20 inches, few of the longer 

 roots reaching depths of 32 inches, while the lateral extent of those running 

 vertically just beneath the surface was at least 2 feet on either side of the 

 plant. Many of the deepest and widest-spreading laterals maintain about 

 haff their original diameter for a distance of 12 to 14 inches. From the very 

 base of the plant to the extreme tips the supply of fine rootlets is remarkable. 

 They branch and rebranch again and again into conspicuous mats of almost 

 microscopic rootlets which penetrate every crevice of the gravelly soil. Few 

 if any of the grasses examined had such great masses of delicate clusters of 

 absorbing rootlets. Such a grass is excellently adapted to live in the half- 

 gravel-slide, as more thorough occupancy of the soil can scarcely be imagined. 

 In color the roots are bro-v\Ti to light tan. They were removed from the 

 gravelly soil with no great difficulty. 



Kceleria cristata. — This grass, which is a characteristic dominant of the 

 half-gravel-slide, is distinguished by an extremely fibrous root system. The 

 main roots at their outset vary from 0.2 to 0.3 mm. in diameter. At a depth 

 of 1 to 4 inches many pf these run off parallel with the surface to a maximum 

 distance of 12 inches. These are branched and rebranched to the third 

 and fourth order, the ultimate termini being almost microscopic in size 

 (plate 26, a). 



Gilia aggregata. — The long pink or red racemes of this abundant biennial 

 are very conspicuous on the half-gravel-sUde, where the plant is quite abun- 



