SYMBIOSIS: PARASITISM 



93 



or other Phaseoleae or by Viciae such as Vicia, Ervum, or Pisum, the 

 bacteria of these again being useless to Trifolieae. We should have, in fact, 

 if these views be correct, different breeds of one and the same species of 

 bacterium (B. radicicola), comparable to the different races of brewers' 

 yeasts or to those described by Eriksson in the wheat rust-fungus Puccinia. 



Nobbe and Hiltner have attempted to turn their improved theories to 

 practical account by the introduction of ' Nitragin', a preparation manufac- 

 tured at the Hochst Chemical Works. Nitragin is a pure culture on gelatine 

 of bacteria from root-nodules a kind of living manure to be mixed with the 

 seed material or soil and strewn over the fields. There are eight different 

 sorts of nitragin on the market, suitable for peas, lupines, beans, &c., and it 

 is said to be very advantageous when first planting Leguminosae in virgin 

 soil, or in soil of poor quality, moors for instance, or where for many years 

 no Leguminosae have grown and the land is presumably destitute of nodule 

 organisms. The results are extremely inconstant and frequently very 

 difficult to judge, so that it is not surprising 

 that opinions are divided as to its efficacy *. 

 Instead of nitragin, ' leguminous earth,' i.e. 

 soil in which Leguminosae thrive well, is 

 sometimes used with advantage to ' inocu- 

 late ' new ground and bad earth such as 

 moorland. 



The remarkable association of bacteria 

 with leguminous plants is generally re- 

 garded as an instance of symbiosis, a 

 connexion from which both parties reap 

 advantage similar to the 'mutual' associa- 

 tion of alga and fungus in lichens. Here, as 

 is well known, the green or brown cells of 

 the alga are enclosed by the thickly matted filaments of the fungus (Fig. 20). 

 The alga supplies the metatrophic fungus with organic compounds and the 

 fungus repays the debt by providing the alga with the necessary moisture 

 and mineral salts, not to speak of the protection that its embrace involves. 

 This at least is the opinion of those who, following the ' symbiotic ' tendency 

 of the day, look upon a lichen as a co-operative concern. But it is more 

 than doubtful whether this view is correct. All the algae that live in lichens 

 are known to be able to live in a free state. They certainly do not need to 

 be supplied by the fungus with water and mineral salts, and as regards 

 protection it is difficult to see how the algal cells can be ' protected ' by 

 hyphae which cling round them tightly and send ' suckers ' into their interior 

 (Fig. 20, b). The fungus behaves in every way as a parasite which lives upon 



FlG. 20. Parasitism of Lichens, a, section 

 through the thallus of Xanthoria parietina 

 (from Schwendener) ; 6, algal cells surrounded 

 by fine fungous hyphae in Cladonia furcata 

 (from Bornet). The green algae dotted black. 

 Magn. a 500, b 950. 



* [See Dawson, Phil. Trans. 1899, Vol. 192, pp. 1-28.] 



