98 



THE ECOLOGICAL RELATIONS OF ROOTS. 



a short distance on the branches, was strongly marked by wrinkles, indicating a 

 considerable degree of contraction. Indeed, throughout the entire root system, 

 not excluding the smaller branches, characteristic wrinkles and constrictions 

 occurred. At the point of origin the branches were about 7 or 8 mm. in diam- 

 eter. They traversed the soil in diverse directions, for the most part running 

 more or less parallel with the surface, but giving off some vertically or 

 obliquely descending branches. The former sometimes ran off laterally to a 

 distance of 2 or 3 feet and at a depth of from 6 to 18 inches below the surface 

 of the soil. As the ultimate laterals approached their extremities they were 

 characterized by a marked, abrupt branching habit, which resulted in a tuft 

 of from 6 to 10 or more rather coarse, poorly branched termini varying from 

 2 to 5 cm. in length (fig. 41). The diameter of the laterals at the point where 

 they develop these tufts is often 2 mm. or more. Arising from these more or 

 less horizontal laterals, vertically descending roots were found to penetrate a 

 maximum depth of 24 to 30 inches. Like the tap, the branches of all orders 

 were more or less fleshy and very brittle, affording a decided capacity for food 

 storage. Several plants of different ages were examined. In general the above 

 description answers for all. 



Aster porteri. — This plant, which grows abundantly on the half-gravel- 

 shde, has the stem clusters connected by short rhizomes and possesses the root 

 system characteristic for plants of this habitat. From the base of the stems 

 arise a multitude of fibrous roots, the largest seldom being over 2 mm. in 

 diameter. Great numbers descend vertically or somewhat obliquely, some to 

 a maximum depth of 30 inches. Many others run off rather horizontally or 

 obliquely to a distance of 6 to 12 inches on either side of the plant before 

 turning downward. All are profusely branched and rebranched into a fine 

 absorbmg network, so that every cubic inch of the soil for nearly a foot on 

 each side of the plant and to a depth of over 2 feet is forced to yield up its 

 available water and solutes to the excellent absorbing system. 



HALF-GRAVEL-SLIDE ROOT SYSTEMS AND THE HALF-GRAVEL-SLIDE 



ENVIRONMENT. 



While the plants of the half-gravel-slide habitat still retain the 

 superficial and wide-spreading root habit of their predecessors of the 

 gravel-slide, this is supplemented by a much deeper portion which 

 extracts water and nutrients below the 18-inch level, as well as very 

 largely from the second and third foot of soil. The surface-spreading 

 roots may be explained by the frequent mountain showers which 

 supply water rather continuously to the shallow soils, while the 

 deeper-seated ones reflect the better deep-soil conditions, but especially 

 the more intense competition for water of this denser population. 

 The thick surface layer of loose rock fragments of the gravel-slide has 

 here become more disintegrated and closely packed, and soon gives 

 way to soil. Hence water evaporates more freely. This, with the 

 competition of the taller half-gravel-slide plants, accounts for the 

 disappearance of gravel-slide species. The former shade those of 

 lesser height, but especially compete with them for water, undoubtedly 

 relying upon their deeper penetrating roots during periods of drought. 



