94 



BACTERIA AND THE NITROGEN CYCLE 



the alga. As a parasite it must establish a close connexion between its 

 tissues and those of its host, and, since it cannot crawl into the alga as the 

 tape-worm does into man, it winds its hyphae around the cells. Enclosed 

 thus in the light, well-ventilated thallus of the fungus, the algae continue to 

 live, but they draw no nourishment from it. The advantage is entirely on 

 the side of the fungus, which is a true parasite. 



The association between the leguminous plants and the bacteria of 

 their root-nodules, paradoxical as it appears at first sight, is of a precisely 

 similar character; the plant is parasitic upon the micro-organism. To 

 render this view more comprehensible, we will consider in detail the 

 development of the nodules. The fine root-hairs of a young leguminous 

 plant, destitute as yet of nodules, push themselves everywhere into the 

 crannies between the particles of soil, taking up water and mineral salts and 



secreting fluids which dissolve and 

 render assimilable the substances with 

 which the cell-wall comes into contact. 

 There can be no doubt that some of 

 the secretions of the root-hairs act 

 chemotactically upon many of the 

 bacteria that swim in the water-filled 

 crevices of the soil ; and these would be 

 attracted furthermore by organic sub- 

 stances issuing from wounds or scratches 

 upon the root-hairs or upon the epi- 

 dermis of the root. Asparagin, one of 

 the most powerfully chemotactic com- 

 pounds known, is present in large 

 quantities in the tissues of germinating 

 plants, and must necessarily escape 

 wherever the cell-walls are injured. 

 It is quite possible that this substance 

 acts as a ' bait ' to attract the bacteria 

 which would swarm into a torn root- 

 hair just as they do into a capillary filled with asparagin (Fig. 21, b). There 

 are reasons, too, for thinking that the plant facilitates the entrance of the 

 micro-organisms by a softening of the cell-membrane of the root-hairs. 

 This much is certain, that the bacteria are chemotactically attracted to the 

 plant, and that, nourished by the organic substances offered to them, they 

 multiply rapidly and force their way from the surface of the root inwards. 

 Here again by softening its cell-walls the plant seems to smooth their path, 

 and in thick columns the bacteria zoogloea* pushes on from cell to cell 



Fig. 21. Invasion of leguminous roots by bacteria, 

 a, Cell from the integument of root of the pea with 

 nucleus and so-called 'infection thread,' a broad 

 stream of bacteria zoogloea that pushes its way 

 through the cell walls (from Prazmowski) ; b, end of 

 a root-hair of the pea ; at the right, particles of earth, 

 and on the left a mass of bacteria have gathered. 

 Inside the hair, protoplasm mixed with bacteria which 

 are pushing their way in a thin stream upward. 

 (From B. Frank.) 



* The so-called ' infection-thread,' first observed penetrating the root-hairs by Marshall Ward 

 (see Phil. Trans. 1887, Vol. 178, p. 545). 



