ROOTS AND RHIZOIDS 



505 



roots grow in diameter, the pressure often becomes sufficient to shatter rocks, so 



that plants contribute materially to rock disintegration. 



Propagation by roots. — As a rule, roots do not give rise to buds, nor do they 



bear stems and leaves. In some plants, notably the silver poplar and the osage 

 orange, stems develop habitually from ordinary soil 

 roots, resulting in a spreading colony of young trees 

 around the parent tree. Many trees and shrubs, 

 especially various poplars and willows, manifest this 

 phenomenon to a marked degree, if the roots are 

 exposed to the air. In the hawthorn and probably 

 in many plants, roots can be induced to develop 

 shoots, if the entire stem system is removed and the 

 roots are exposed to air and light (figs. 721, 722). 

 The exact factors that stimulate stem growth in these 



Figs. 718, 719, 720. — 

 The descent of roots and 

 rhizomes in the soil; 718, a 

 young individual of Erythro- 

 nium mesachoreum, showing 

 a descending rhizome or 

 "dropper" (d, see p. 675); 

 719, an older descending in- 

 dividual, showing remnants 

 of the five preceding years; 

 note the increasing size of 

 the bulb each year; 720, an 

 individual of Zygadenus Fre- 

 monli, illustrating descent by 

 root contraction; note the 

 contractile roots of the cur- 

 rent (r) and the preceding (r') 

 years. — From RiMBACH. 



Figs. 721, 722. — Diagrammatic sections of an eroding 

 clay bluff, illustrating the development of shoots from 

 roots: 721 represents a hawthorn tree {Crataegus) at the 

 edge of the bluff, s being the original trunk; subsequent 

 erosion (722) has caused the destruction of the trunk 

 and the exposure to the air of a root (r), which has 

 given rise to a new shoot {s'). 



rather unusual conditions are not known. Near the 

 growing point within the root of Habenaria Michauxii 

 there is organized a bud that later develops into an 

 imderground stem. 



Water and root form. — The roots of desert 

 plants, as a rule, are extensively developed, 

 at least in comparison with the shoots (figs. 

 723-725). Some plants have long and slender 

 roots that reach to great depths or that have considerable lateral exten- 

 sion. In other cases roots of immense size are formed. In alpine 

 regions there are dwarfed shoots associated with roots of ordinary size 

 (fig. 870), and there are many such plants in dry sandy soil. Per- 



