10 



PLANT SOCIOLOGY 



commensals. All highly organized communities, on the other hand, 

 are composed of unequal commensals: only such are able to utilize to 

 the fullest extent the possibilities of the habitat. The low shrub, 

 moss, or lichen cover of a forest draws its nourishment from the upper 

 and middle horizons of soil ; the trees, from deeper layers. 



Commensalism begets competition, which becomes the more intense 

 the more nearly the life adjustments of the individual commensals 

 coincide and the more favorable the external inorganic conditions 

 are for plant life. Under unfavorable conditions of climate and soil 

 the competition even between species with similar requirements takes 

 on a milder form (Fig. 4). That competition is not wholly absent 



Fig. 4. — Open Stipa steppe in the northern limits of the Sahara, Djebel bou Arfa, with 

 root competition. {Photo by Daguin.) 



even among the open communities of deserts and high alpine rock 

 clefts and gravel slides is shown by many observations. 



Jenny-Lips (1930) found that there was active competition in a 

 gravel slide association, although the vegetation covered only 5 to 15 

 per cent of the surface. At a depth of 25 cm. the fine fibrous roots of 

 the herbaceous Thlaspi rotundifolium plant were intimately inter- 

 woven with those of an Athamanta plant standing 130 cm. away. In a 

 very open Stipa calamagrosiis association on the gravel slide there was a 

 horizon of very fine soil, only 5 cm. thick, lying under 15 cm. of coarse 

 gravel. This horizon of fine soil was filled with a mat of roots from 



