SOCIAL LIFE AMONG PLANTS 9 



exist there (Fig. 3). Most efficient is the protection afforded, uncon- 

 sciously of course, by spiny shrubs for their companion plants. In 

 open pastures individuals or clumps of Crataegus often shelter seedlings 

 of oak and maple that are entirely absent from adjacent grazed wood- 

 lands. In closely grazed mountain valleys there are often young 

 spruces, pines and larches surrounded by a protective circle of Juni- 

 perus. It is striking that in the plains between Ujda and Taourirt 

 (eastern Morocco), populated by thousands of sheep, the last remnant 

 of a tree is the Pistacia atlantica, confined to the protection of the 

 impenetrable spiny armor of the Jew's-thorn. 



The list of plants requiring protection might easily be extended. 

 They represent the least intimate stages of dependence; but even 

 among them there may be mutual relations of protected and protector. 

 Such a list might include protected plants changing the soil conditions 

 to the advantage or disadvantage of the protector; forests smothered 

 by invasion of peat moss; raw humus of the forest floor hindering 

 natural reproduction; more favorable moisture conditions for seedUngs 

 provided by a layer of moss; or the exhaustion of the soil moisture by 

 the more deeply rooted protected tree to the detriment of the protecting 

 shrub. Sukatschev ("Esquisses phytosociologiques, " vol. I, 1921) 

 states that in the Pinus cemhra region of Lake Baikal Hypnum schreheri 

 and Hylocomium splendens in places become so luxuriant that natural 

 reproduction of the pine is prevented, and the forest degenerates into a 

 mossy scrub community of Ledum palustre. 



From these considerations it is evident that dependency plays an 

 important but secondary role in the life of plant communities; the 

 really decisive role is taken by the second type of social life, 

 commensalism. 



Commensal Unions. — According to Van Beneden, we understand 

 by commensal organisms those which enter into competition separately, 

 and their common relation consists in the fact that they simultaneously 

 utilize the various life conditions of a given habitat. Le commensal est 

 simplement un compagnon de table. The relation between the com- 

 mensals rests upon the struggle for space, light, and nutrients. The 

 struggle for existence goes on between similar commensals when 

 different species have the same or nearly the same requirements and 

 becomes most intense between individuals of one and the same species. 

 But the "table companions" may have different needs, either because 

 they use different nutrients or because their organs use different layers 

 of soil or air. In such cases we have to do with unequal commensals. 



The most simply organized plant communities, such as plankton and 

 many lichen and moss communities, form a round table of equal 



