SYMBIOSIS 139 



which arc partial parasites, while others have become more or less 

 completely dependent upon the host without as yet losing their chloro- 

 l)hyll, with the exception of Geosiphon which contains Nostoc as 

 jiroducent. This is almost certainly the relation that exists between 

 blue-green algae and higher plants, as in the coaction of Anabaena 

 with the water fern, Azolla, and Nostoc or its relatives in Si)hagnum, 

 certain liverworts, the Cycadaceae, and an occasional angiosperm, as 

 Gunnera. 



The outstanding symbiont among the bacteria is Pseud omonoas 

 {Rhizohium) radicicola, which produces the characteristic root- 

 tubercles of nearly all Fabaceae and is probably directly concerned 

 with fixation of atmospheric nitrogen. Similar nodules are to be 

 found in Alnus, Elaeagnus, Myrica, and Ceanothus (Neger, 1913), 

 and in Casuarina and Podocarpus (McLuckie, 1923), and in most 

 cases they are also to be ascribed to the same bacterium, though rarely 

 a fungus may be concerned. Since Pseudomonas seems to be an oblig- 

 atory endoparasite and the legume gains the boon of an added supply 

 of nitrogen, this symbiosis constitutes one of the relatively few con- 

 vincing examples of mutualism among plants, though even here the 

 host grows normally in the absence of its symbiont. A somewhat 

 similar physiological relation is thought to obtain between certain 

 soil algae, Nostoc, Cylindrospermum, etc., and nitrogen-fixing bacteria, 

 especially Azotobacter. Bacteria and yeasts may also exhibit certain 

 characteristic types of symbiotic relation in the fermentation of kephir 

 and ginger beer. 



The lichen has long been cited as the chief example of mutualism 

 between alga and fungus, but when allowance is once made for the 

 microscopic size and nature of the host, the essential relation is one of 

 parasitism in varying degree. This is clearest in the higher forms 

 especially, in which the fungus sends a haustorium into the algal 

 host, finally destroying it, but destruction of the host is the general 

 result in practically all cases, it seems. Probably all the host algae 

 can, and many of them do, live independently, and this is true also 

 of a considerable number of the fungus parasites among the less inte- 

 grated forms. As would be expected, a score or more of different fungi 

 occur on the same species of algal host, and several of them may 

 have two hosts enwrapped in the thallus. Furthermore, over a hun- 

 dred species of lichens contain in addition to the proper host a second 

 or rarely a third alga near the surface, always blue-green and produc- 

 ing a peculiar structure, the cephalodium, but of unknown function. 

 Finally, there may be a secondary fungus, apparently parasitic on 

 the lichen, but actually deriving its food also from the host algae. 



