672 ECOLOGY 



root excretions have accumulated, but the persistent occupation of the same spot 

 makes this view rather untenable, except, possibly, for those rhizomes or rhizome 

 colonies (e.g. in Polygonatum) which migrate as a whole into new areas; this con- 

 sideration weighs equally against the soil exhaustion theory. Maintenance of soil 

 level probably is advantageous, since great depth interferes with the development 

 of aerial organs, while extreme shallowness would mean lessened soil protection and 

 greater difficulty in the invasion of new areas, because of the increasing occupation 

 of space towards the surface. Rhizomes are advantageous further as organs of 

 persistence through unfavorable seasons (p. 716) and as organs of food accumula- 

 tion ; in Psilolum and in Corallorhiza they replace roots, and even may bear " root 

 hairs." 



Runners. General features. Runners are horizontal stems at or 

 above the ground level, taking root in the soil, and differing from 



FIG. 984. A plant of the creeping juniper (Juniperus horizontalis), illustrating radial 

 migration; horizontal branches advance in all directions, rooting in the sand; in the 

 background are numerous plants of Artemisia; Waukegan, 111. Photograph by 



McCALLUM. 



rhizomes chiefly in not being subterranean. Stolons are essentially 

 identical with runners, though the term sometimes is applied to certain 

 rhizomes. Runners usually are much slenderer than rhizomes, and often 

 their internodes are greatly elongated; chlorophyll commonly is present. 

 If runners come into contact with moist soil, roots and buds develop 

 at the nodes, thus giving rise to potential or actual new plants (fig. 985). 

 A representative runner is that of the strawberry in which there is a con- 

 tinuation of horizontal elongation accompanied by repeated rooting, 

 thus giving rise to a number of potential plants. Sometimes (as in 

 Sempervivum and Saxifraga} the runner ceases to elongate after a bud 

 has developed; in such cases the term offset may be used (fig. 1165). 

 Creeping stems (as in white clover) are runners which lie close to the 



