THE GRAVEL-SLIDE COMMUNITY. 



83 



supplied with minute, absorbing rootlets. Others run ofif obliquely, while 

 still others parallel more or less the attenuated tap-root, and like the tap are 

 abundantly supplied with both long and short rootlets, the whole forming a 

 wonderfully efficient absorbing system. Compared with the transpiring sur- 

 face, the root system is very well developed. The surface view in the figure 

 shows the roots at a depth of 2 to 2.5 inches. The dotted lines show where 

 they turn downward. No roots were found to penetrate to a greater depth 

 than 30 inches. 



Aletes acaulis. — ^This low umbellif- 

 erous plant, which is only 6 to 8 inches 

 high in fruit, is the dominant of the 

 gravel-slide, of ten being more abundant 

 and conspicuous than all other plants 

 combined (plate 23, a, b). From the 

 large tap-root arises a large number of 

 stems, each multicipital in character; 

 47 individual stems were counted on 

 a single medium-sized plant. Such a 

 group forms a formidable obstacle to 

 the rock fragments moved by gravity, 

 the latter pushing the top of the plant 



6 to 10 inches downward from the top 

 of the well-anchored tap-root. Both 

 the main stems and the branches show 

 marked wrinkles from profound con- 

 tractions. At the base of the shoots of 

 this perennial, great clusters of dead 

 and decaying gray leaf-bases remain 

 attached to the plant. 



The tap-root is often 4 cm. or more 

 in diameter. It is dark brown in color 

 and quite spongy in texture, as was 

 noted for several species in this habitat, 

 the fleshy roots probably serving for 

 water storage. One plant with a tap 4 

 cm. in diameter, from which originated 



7 distinct stem-clusters, gave rise in the 

 first 3 inches of soil to numerous laterals ranging from 1 cm. to only a few 

 millimeters in diameter (plate 24, a, b) . One of these laterals, 3 mm. in 

 diameter, ran off at a depth of about 4 inches and in a direction parallel with 

 the soil surface to a distance of 4 feet, giving off numerous branches, both 

 large and small, each of which, after branching profusely, ended in a network 

 of tiny, much-branched laterals. Another surface lateral, 8 mm. in diameter, 

 ran up the slope at an average depth of 5 inches to a distance of 4.5 feet. At 

 a depth of 6 inches the tap broke up into 3 parts — 7, 8, and 10 mm. in diameter 

 respectively. These were very much curved and twisted. They followed 

 the crevices of the rocks and none reached a depth greater than 30 inches, 

 but spread laterally to a distance of 3 or 4 feet or more from the base of the 

 plant. All of the laterals branched profusely and terminated in the moist rock 

 crevices in networks of tiny rootlets. The smiace 2 to 5 inches of soil is 

 especially filled with these absorbing laterals, but they are abundant through- 

 out the soil to the tips of the deepest roots. Plate 24, b, shows a fragment of 

 these branches. In fact, the soil is literally filled with these absorbing rootlets 



Fig. 33. — Surface view of a single root of 

 Paronychia jamesii at a depth of 2 to 2.5 

 inches. 



