DIV. ii PHYSIOLOGY 261 



also on albuminous substances supplied by the host plant, the latter profits by 

 the power of fixing free nitrogen possessed by the bacterioids. The bacterioids 

 furnish a steady supply of combined nitrogenous substance to the leguminous 

 plant. It has been calculated that Lupins are able in this way to obtain 200 kg. 

 of nitrogen per hectare. 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 "). A marked increase in the crop of Serradella is obtained in this way. 

 If the soil in which a Leguminous plant is grown contains a sufficiency of nitrates, 

 the plants may live at their expense ; since the presence of nitrates exerts an 

 injurious influence on Bacillus radicicola, practically no nodules are formed under 

 such circumstances. 



Besides the Leguminosae, Elaeagnus and Alnus are able to utilise free atmo- 

 spheric nitrogen when their roots bear nodules ; these are due to infection by 

 another of the lower organisms. A species of Podocarpus which has a mycorrhiza 

 can also utilise atmospheric nitrogen. It is thus not improbable, though as yet 

 unproved, that other mycorrhizas may have a similar significance. The roots 

 not only of the phanerogamic plants without chlorophyll, referred to on p. 256, 

 but also of most green plants living in the humus soil of woods and heaths, 

 especially the trees, stand in close relation to Fungi ("). 



The fungal hyphae are sometimes found within the root occurring in tangled 

 groups in the cells of definite cortical layers, while individual filaments extend into 

 the soil. In other plants the hyphae invest the outer surface of the young roots 

 with a closely-woven sheath. The former is called endotrophic, the latter ecto- 

 trophic mycorrhiza, but the extreme forms are connected by intermediate conditions. 

 The fungi of the endotrophic mycorrhiza are in part digested by the cells of the 

 root, and thus all the substances liberated will be available for the phanerogamic 

 plant. This is not known in the case of ectotrophic mycorrhiza. STAHL regards the 

 significance of fungal infection of the flowering plant to lie in the active absorption 

 of nutritive salts from the soil by the fungus. The advantage to the fungus is 

 obviously, at least in the cases in which it infects green plants, the provision of 

 carbohydrates which it obtains. It is probable that the consortia of Fungi and 

 Algae which are called Lichens can be ranked here as regards their physiology of 

 nutrition ( 39a ). 



More recently swellings which are due to infection by bacteria have been dis- 

 covered in the leaves of tropical plants belonging to the Rubiaceae and Myrsinaceae. 

 While, however, in the case of the Leguminosae the infection always depends on 

 accidental meeting of the bacteria and the flowering plant, in these families the 

 bacteria are present in the embryo of the plant. When they are artificially kept 

 from the egg-cell the development of Ardisia is abnormal. It is quite probable 

 that in these cases also an assimilation of free nitrogen takes place C 40 ). 



C. ASSIMILATION or OTHER SUBSTANCES 



Sulphuric acid most nearly resembles nitrogen since it also is used 

 in the construction of proteids which contain about J-1J per cent 

 of S. It is still uncertain where and under what conditions its 

 assimilation occurs ; we only know that a reduction of acid radicals 

 must take place in the process. In some plants sulphur is combined 

 in other substances besides proteids. 



