THE GRAVEL-SLIDE COMMUNITY. 



85 



run off in all directions to a distance of 6 or 8 inches or more. However, the 

 most profound branching arises from the horizontal rootstocks. Like the 

 former, these are not large in diameter, but are branched so profusely, extend- 

 ing vertically upward as well as downward and laterally, that they com- 

 pletely occupy the soil from a depth of 2 inches to a maximum depth of 4 feet. 

 Many of these deeper roots originate from strong laterals 1 to 2 mm. in diam- 

 eter, which branch off and run in various directions from the rootstocks. 

 Figure 34 illustrates the profound absorbing surface characteristic of these 

 gravel-slide plants. In color, the larger roots are dark brown, while the finer 

 ones vary from tan to almost white. 



Smilacina stellata. — Families of this plant often occur on steep gravel- 

 slides, or they are the dominants of a gravel-slide colony. Stout stems, from 



3 to 7 mm. in diameter, arise at intervals of an inch to more than a foot from 

 the stout horizontal rhizomes. The latter are about the same diameter as the 

 stem and lie usually at a depth of about 4 to 6 inches, although they are some- 

 times shallower where the gravel has rolled away and often much deeper 

 where the gravel has covered them (plate 24, c) . Some were found at a depth 

 of 30 inches. The rhizomes are much branched, frequently at right angles, a 

 single rhizome system often connecting a whole family of plants through a 

 distance of 7 or 8 feet or more. These rhizomes furnish not only an excellent 

 means for propagation, but also serve as storage organs for these herbaceous 

 perennials. 



The plants frequently grow in such dense clumps that the soil at a depth of 



4 to 6 inches is quite filled with dense masses of these rhizomes. Although the 

 vertical stems do not give rise to rootlets, the rhizomes are uniformly covered 

 on all sides with rootlets about 0.5 to 1.0 mm. in diameter. These run off in all 

 directions, including the vertical to a distance of 3 to 6 inches. Although 

 they are entirely unbranched, they are completely and uniformly covered with 

 a dense coat of root-hairs. Thus the plant is well provided for absorbing the 

 moisture in the shallower soil. In addition to these shorter rootlets, groups 

 of 2 to 5 larger branch roots arise at frequent intervals at the base of the 

 vertical stems or where these stems have been, a place now marked by a seal- 

 like scar. These vary from 2 to 4 mm. in diameter and throughout their 

 course are densely covered with root-hairs. They penetrate the soil to a 

 maximum depth of 44 inches. Through the first 6 to 12 inches of their course 

 they are unbranched or at least poorly branched, but from this point they 

 branch profusely to the third or fourth order, the branches spreading widely 

 and the ultimate rootlets being rather coarse, often 0.2 to 0.5 mm. in diameter. 

 Since these branches run obliquely and even horizontally as well as vertically 

 downward, they furnish with the shorter roots already described an excellent 

 absorbing system. Altogether the finer rootlets are very coarse and poorly 

 branched when compared with Aletes, Thlaspi, and certain other gravel-slide 

 plants. The entire underground parts vary from tan to white in color. 



Pachylophus csespitosus. — This is a common plant on bare gravel-slides 

 where it forms pioneer consocies. The social habit is due to the method of 

 vegetative propagation (fig. 35). The plants are frequently connected by 

 rhizomes from which arise roots 2 to 5 mm. in diameter, none of which were 

 found to reach a depth greater than 45 inches. Frequently at 4 to 8 inches 

 deep, they turn off abruptly up or down the slope and run parallel with the 

 surface for long distances. Large branches arise from these roots and do much 

 of the absorbing for the plant. Of numerous specimens examined, the one 

 illustrated in figure 35 is typical. 



