176 LIFK : OUTLINKS OF GENERAL BIOLOGY 



succeeds because it is a firm, not a single plant. 'I'lie whole system 

 of the plant is interix^net rated with fungus threads, which extend 

 from root to stem, from leaves to Hower, and rest against the enve- 

 lopes of the seed, at hand to infect the next generation. Sterilised 

 seeds will germinate and put forth a couple of leaves. But there the 

 seedling stops without even developing roots. It is probable that tlu 

 fungus is able to capture atmospheric nitrogen. A similar partnershij' 

 occurs in some other Ericaceae. It nmst be noticed, however, that 

 Miss Ka\Tier's careful conclusions must be reconsidered, by hersell 

 first of all. in the light of Kniuls::n's recent criticisms. 



Of great practical importance is the symbiosis between certain 

 microbes {Bacillus radicicola) and plants belonging to the order 

 Leguminosa'. The motile rodlet-like bacteria have a widespread 

 occurrence in the soil, and enter the roots by the root-hairs. They 

 develop into minute gelatinous threads (zooglcca) consisting of 

 numerous bacteria, and these spread into the outer cells of the root. 

 There they get free from the gelatinous cord, and in the abundant 

 food they change from rodlets into forked or branched shapes, which 

 in other bacteria are generally regarded as degenerative. The outer 

 cells of the root are stimulated to form galls or tubercles, which are 

 often the size of peas. The bacteria are benefited, for they feed on 

 the abundant carbohydrate reserves in the root; and the green 

 plants are benefited, for their partners are able to fix the free 

 atmospheric nitrogen, some compound of which is eventually ab- 

 .sorbed by the green plant. The evidence of this is now manifold: 

 the Leguminous plant increases in nitrogenous compounds far 

 Ix'yond what can be accounted for by the mineral composition of 

 the soil; in sterilised soil no tubercles are formed and the plants 

 do not grow vigorously; the fixation of atmospheric nitrogen by 

 /?. radicicola and some others has been experimentally demonstrated. 

 The story reads like the conversion of a parasite into a symbion. 

 When the bacilli iirst enter the plant, they multiply rapidly and 

 show great vigour, and they produce some disorganisation in the 

 cells of the root, from which they are absorbing carbohydrate 

 material. But the plant .soon recovers itself, being enriched by the 

 nitr()genf)us comj)ounds which the bacilli are providing. Soon, 

 however, the bacilli become inert, and eventually they decay, 

 leaving more nitrogenous material at the disposal of the green plant. 

 An individual tubercle lasts only for a year; and if the Leguminous 

 plant is more than an annual, there must be fresh infection of new 

 rootlets. It is probably from the air dissolved in the absorbed soil- 

 water that the bacilli obtain the free nitrogen with which they start, 

 and it has been shown that the nitrogen-fixation is facilitated by an 

 abundance of sugar and hindered by an abundance of nitrates or 

 of proteins. What the bacilli do is not to be confused with the r61e 

 of other .soil-bacteria, e.g. Nitro.somonas, which oxidise ammonia ^ 



