THE HALF-GRAVEL-SLIDE COMMUNITY. 



97 



dant. Of the 6 plants examined all had a large tap-root from 4 to 8 mm. m 

 diameter. The tap sends off numerous branches near the surface and tapers 

 so rapidly that at a depth of 10 inches it seldom exceeds 1 or 2 mm. in diameter. 

 The laterals are mostly wide-spreading, frequently nearly horizontal ui posi- 

 tion, and much forked and rebranched into very abundant fine termini. They 

 reach distances of over a foot from the base of the plant. Like most plants of 

 this habitat the root system is not deep. No Gilia roots were found below the 

 28-inch level, but the moist surface soil is well filled with great quantities of 

 fine absorbing rootlets, especially to the depth of 18 inches. 



Potentllla arguta glandulosa.— This species frequently forms families on the 

 half-gravel-sHde. A large clump of these plants was examined. The com- 

 plexity of the rootstocks is such that they are hard to describe. The individual 

 clumps are connected throughout long distances by much-branched under- 

 ground parts, which often run horizontally or sometimes obliquely at depths 

 varying from 3 to more than 18 inches. These underground connecting parts 

 may reach a diameter of a centimeter, but are usually much smaller. They 

 are very much branched and give rise to the clusters of stems at mtervals of 

 3 inches to more than a foot. At a depth of 2 to 5 inches the roots often throw 

 off a large number of small, short, but exceedingly well-branched surface 

 absorbing laterals. The larger roots are usually only 2 to 6 mm. in diameter 

 and run off in all directions, some to a maximum depth of 4.5 feet, while the 

 surface of the soil is filled with the extremely well-branched brush-like termini 

 of the laterals. These clusters are from 6 to 10 inches in length. The deeper 

 soils are also completely occupied by the irregularly branched and rebranched 

 root network, often to a depth of 4 feet (plate 26, c). 



Frasera speciosa.— This striking plant starts with a strong, rather fleshy, 

 glistening white tap-root, which is 1.5 to over 2 cm. in diameter. It penetrates 



Fig. 41. — Root system of Frasera speciosa. 



vertically downward to a depth of 9 inches, where it divides into 4 branches 

 of ahnost equal diameter. This main tap, throughout its length and even to 



