44 



PLANT SOCIOLOGY 



otherwise develop deeply are shortened and do not penetrate the basic 

 soil (Fig. 98). 



In his study of the conditions of life in the gravel-slide association 

 Jenny-Lips (1930, p. 154) distinguishes an uppermost soil horizon with- 

 out fine particles and without branching roots; below this a horizon 

 with much fine soil in which the plants of the gravel slide develop their 

 roots into a mat of very considerable density; and, still lower down, a 

 horizon with very little fine earth into which the deep-anchoring roots 

 penetrate (Fig. 24). 



Fig. 24. — Root layering in a thick stand of the Thlaspi association on a gravel slide 

 at 2,250 m., Swiss Alps; 1, Thlaspi rohmdi folium; 2, Trisetum distichophyUum; 3, Viola 

 crniaia; 4, Leontodon montanus; and o, Silenc alpina; A, horizon of coarse gravel; B, 

 horizon of absorptive roots; and C, horizon of anchoring roots. {After Jenny-Lips.) 



Among others who have studied root layering are Kujala (1926), 

 who has described the layering in Finnish forests, and Braun-Blanquet, 

 who has investigated the phenomenon in the garigue of the Mediter- 

 ranean region (Fig. 25). Such aerial and subterranean layering 

 makes possible the coexistence of a number of ecologically different 

 types and permits the maximum utilization of a given area. 



Vitality {Gedeihen) . — The fact that a plant occurs often and plenti- 

 fully in a certain community does not necessarily warrant the conclu- 

 sion that it prospers there. A species may be a constant member of a 

 commiipity and yet only be tolerated and be quite meaningless in the 

 commui.al eeclo'?y. It completes its life cycle and produces seed 

 only in certain other communities. 



