496 ECOLOGY 



sole bush {Larrea tridentata) has ample tops and few root hairs, while the reverse 

 is the case with a plant in dry soil. 



Variations in the occurrence of root hairs. — While air roots and water 

 roots commonly are hairless, most soil roots have hairs. Wet soil, as in 

 swamps and beneath ponds, presents a condition transitional between 

 water and mesophytic soil; in such habitats most roots have hairs, but 

 some are hairless. Hairs are more abundant, even in the same species, 

 in soil beneath flowing water than in soil beneath standing water. The 

 root hairs of some xerophytes have rigid lignified walls; for example, 

 in Pinus edulis they are stiff and brown. In some succulent xero- 

 phytes (as Opuntia) hairs occur to the extreme root tip. In most 

 plants whose roots are invested with fungi, such as the conifers, oaks, 

 and many tuberous and bulbous plants, root hairs are either almost 

 or entirely wanting (fig. 1106). The roots of parasites commonly are 

 hairless, and in the green partial parasites all gradations are found 

 between species with abundant hairs and those with none (p. 772). 



The structure and role of soil roots. — General features. — Roots are 

 more uniform in their appearance than are stems and leaves, and there 

 probably is some connection between such uniformity and the com- 

 paratively uniform habitats in which roots grow. The root of the seed- 

 ling, known as the primary root, is a tap root, being the main descending 

 axis of the plant (fig. 701). At first this root with its hairs represents 

 the entire absorptive and anchorage system. Very soon branches 

 appear, known as secondary roots, which differ from stem branches in 

 their irregular position and in then: endogenous origin; shortly the root 

 system becomes a most complex affair, owing to repeated branching. In 

 many plants the primary root persists throughout life, continuing to 

 elongate and to grow in diameter; persistent primary roots of this char- 

 acter, which often are tap roots as well, are found in the carrot, dande- 

 lion, dock, and in many trees (fig. 708). In many other plants, 

 especially in those that reproduce by means of underground stems, the 

 primary root soon dies, its place being taken by adventitious roots 

 (p. 503). Roots, particularly those of trees and shrubs, are made up 

 largely of woody tissue, which serves as an avenue of conduction to and 

 from the root hairs; also, through the stiff cell walls, these tissues 

 give mechanical strength to the roots. Some of these wood cells have 

 relatively capacious cavities and are dominantly conductive, whereas 

 others consist almost wholly of thickened walls and are chiefly mechan- 

 ical; see detailed discussion of conductive and mechanical tissues, 



