84 



THE ECOLOGICAL RELATIONS OF ROOTS. 



to a point several feet on each side of the plant. A single plant thus lays hold 

 of the surface soil to a depth of 18 inches or more within a radius of 4 or 5 

 feet from its base. 



Apocynum androssemifolium. — Large areas, especially of the steeper gravel 

 slides, are frequently covered by extensive communities in which this species 

 is dominant, often forming families (fig. 34). The vertical portions arising 

 from the horizontal rootstock are 2 to 5 mm. in diameter. From these origi- 



FiG. 34. — Apocynum androscemifolium, showing rhizomes and dense network of roots. 



nate groups of 2 to 5 erect stems. The rootstocks lie at a depth varying from 

 6 inches to 2 feet, but usually at about 8 to 16 inches. These run for distances 

 of 6 to 10 feet or more, giving rise at irregular intervals to erect portions 

 which bear new plants. Sometimes these connected plants are only a few 

 inches apart, while at other times the interval between them may be 3 or 4 feet. 

 The vertically ascending parts seldom have large branches, but are well 

 clothed with abundant laterals 1 mm. or less in diameter. These|branches 

 divide and subdivide into many branch orders, forming brushlike mats which 



