THE HALF-GRAVEL-SLIDE COMMUNITY. 



97 



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

 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 in 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. 



Potentilla 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 intervals 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 



c 



} 















G 











Fig. 41. — Root system of Frasera speciosa. 



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

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



