II 



plants, but also injure them by shading. Actually, however, this struggle for 

 room is the factor first manifested in each plant community and makes itself 

 felt in all field and forest plantations. In the grain field and in every forest 

 tract, the individual first growing most strongly chokes out its weaker neigh- 

 bors. It is the universal question of the strong driving hack the weak which 

 must find expression in all community life. 



The kind of community life just described in its relation to spacial sep- 

 aration can be termed neighborhood in distinction from the mutual influenc- 

 ing of organisms when united in space. A relationship of this latter kind 

 (symbiosis) must be the more intimate since one organism lives with the 

 other. De Bary (1866) distinguished a mutualislic symbiosis from an 

 antagonistic, according to whether the influence is mutually beneficial or 

 detrimental. The terms chosen by Vuillemin (1889) for this relationship 

 "symbiosis" and "antibiosis'" seem less fortunate to us. We find examples 

 of a mutualistic community also termed commensalism by van Beneden in 

 1878, as companionship at table, in the little bunches of roots of the sago 

 palm (Cycadeae) which occur on the surface of the soil, rigidly branching 

 like witches' brooms and which harbor numerous chains of Nostoc in the 

 large holes in their bark. The genus Gunnera shows similar conditions. 

 Further, the case is often mentioned in literature, in which a water plant, 

 Asolla caroliniana, resembling our Salvinia natans, in the axillary hollows of 

 the leaves, gives shelter to another Nostoc with longish members (Ana- 

 baena). The most accessible example of mutualism is ofi'ered by the struc- 

 ture of the lichen body, in v/hich fungus and alga remain connected per- 

 manently, to their mutual benefit, — Lichenism. 



In the same way may be explained the symbiosis of certain mycelia and 

 the roots of Fagus, Corylus, Castanea and some conifers, the so-called root 

 fungus or mycorrhisa w^hich is usually considered a necessary and universal 

 arrangement. In connection with the mycorrhiza should be mentioned the 

 protective device called Bacteriorhiza by Hiltner^ and Stormer (in 

 Beta and Pisum). Bacteria penetrate from the soil into the outer cell layers 

 of the roots, actually causing a browning of these layers, but otherwise not 

 especially disturbing the health of the plant. According to Hiltner, however, 

 these bacteria prevent the penetration of other injurious organisms (Phoma, 

 etc.). 



Finally we will consider the arrangement of root tubercles, which may 

 be found in different forms and grouping on the roots of the Lcguminoseae 

 and form those well-known grape-like bodies in aiders, which not infre- 

 quently may be observed as spherical nests of short branched roots as large 

 as one's fist. The organisms in the tubercles making the nitrogen of the air 

 available for the plant and described by the students of legumes as Rhic- 

 obium Leguminosarum Frank, or Bacillus radicicola Beijerinck, are bacteria 



1 Hiltner and Peters, Untersuchungen iiber die Keimlingskrankheiten der 

 Zucker- und Runkelriiben. Arbeiten d. Biolog-. Abt. am Kais. Ge.sundheitsamte. 

 Vol. IV. Part 3. 1904. 



