232 BOTANY PART I 



The relation betweeu roots and certain Bacteria in the case of the Leguminosae is 

 better understood. It has long been known that peculiar outgrowths, the so-called 

 ROOT-TUBERCLES, are found on the roots of many Leguminosae (Bean, Pea, Lii]>in<', 

 Clover, etc.) (Fig. 204). These tubercles, of which a single bean plant may bear 

 4000, are caused by certain Bacteria (Bacillus radicicola, the two forms of which 

 are distinguished by HILTXER as Rhizobium radicicola and R. Beyerincki). These 

 Bacteria penetrate through the root - hairs into the cortex of the roots, and 

 there give rise to the tubercular growths. These tubercles become filled with 

 a bacterial mass, consisting principally of swollen and abnormally developed 

 (hypertrophied) BACTEUIOIDS, but in part also of Bacteria, which have remained 

 in their normal condition. The former seem to be eventually consumed by the 

 host plant, while the latter remain with the dead roots in the soil, to provide for 

 future reproduction. As the experiments of HBLLRIEGEL and the investigations of 

 NOBBE, BEYERINCK, HILTNER, and others prove, we have here a case of mutual 

 parasitism like those termed symbiosis by DE BARV. While the Bacterium lives 

 on carbohydrates and at first also on albuminous substances supplied by the host 

 plant, the latter profits by the power of fixing free nitrogen possessed by the 

 Bacteria. This is effected by means of an enzyme demonstrated by HILTNER. "While 

 the Bacteria remain alive they furnish a steady supply of nitrogenous substance to 

 the leguminous plant, and ultimately the remaining substance of the degenerated 

 Bacterioids is absorbed. When the large amount expended on nitrogenous 

 manures (Chili-saltpetre and ammonium sulphate) is borne in mind the agricultural 

 importance of this natural fixation of nitrogen will be evident. It has been 

 attempted to further it by infecting fields with soil rich in the bacteria or with 

 pure cultures of specially active forms (" nitragin"). The fact that the tubercle- 

 forming Leguminosae, unlike most plants, could flourish on ground poor in nitrogen, 

 and could accumulate stores of reserve proteids was known in the time of Pliny, 

 and these plants have long been known to form crops which enrich the soil by this 

 accumulation of nitrogen ( M ). 



If the soil in which such a leguminous plant grows contains a sufficiency of 

 nitrates, few tubercles are formed on the roots. A similar immunity against infec- 

 tion is obtained by plants which bear a number of actively functional tubercles. 

 In addition to the Leguminosae (in which order only Gleditschia triacanthos has as 

 yet been found free from tubercles) tubercle formation due to a symbiosis with a 

 lower organism is known in Elaeagnus and Alnus. According to NOBBE and 

 HILTXER, these plants can also utilise free nitrogen. These authors have also 

 shown experimentally that the same holds for Podocarpus, which possesses 

 mycorhiza. Nitrogen-fixing Bacteria such as Azotobactcr chroococcum are also found 

 according to BENECKEand KEUTNEU in the sea ; according to REIXKE they lodge in 

 numbers in the mucilaginous coatings of Sea-weeds. 



While among the higher plants only isolated forms have become 

 total parasites or saprophytes, while in others the parasitism or 

 saprophytism is occasional or partial, among the lower plants large 

 families with numerous genera and species are found completely 

 devoid of chlorophyll (Fungi and Bacteria), and wholly parasitic 

 or saprophytic in their mode of life. Of the Fungi and Bacteria 

 some are true parasites, and are often restricted to certain special 

 plants or animals, or even to distinct organs ; others, again* are 

 strictly saprophytic in their habit, while others may be either 



