THE ELEMENTARY CONSTITUENTS OF PROTOPLASM 45 



as well as by Schloesing and Laurent, that the power of a leguminous 



crop to enrich the soil with nitrogen was dependent on the presence 



on the roots of certain small nodules which had been described long 



before by Malpighi (Fig. 14). They showed also that the production of 



these nodules took place only as a result of infection. Beans grown in 



sterilised sand produced a plant free from nodules, which, however, 



grew very scantily unless nitrogenous manure were 



added to the sand. Such a crop derived the nitrogen 



for its growth from the added nitrogen, the total 



amount of which in the soil was therefore diminished 



by the crop. If, however, the sterilised sand were 



treated with an infusion of root nodules from another 



plant without the addition of any combined nitrogen 



at all, the beans developed nodules on their roots and 



grew luxuriantly, and at the termination of their 



growth the soil was richer in nitrogen than at the 



commencement. On microscopic examination the 



protoplasm which makes up these nodules is found 



to be swarming with small rods (Fig. 15), and it was 



shown by Beyerinck that these rods are bacteria and 



can be cultivated in media apart altogether from the 



plant. We have thus an example of a class of 



bacteria which, like those of humus, are able to 



assimilate the free nitrogen of the atmosphere, but, 



unlike them, can only effect this assimilation in a 



condition of symbiosis, i.e. living in the growing 



tissues of a leguminous plant. Similar nodules have 



been described on the roots of other plants which 



can grow in a soil free from combined nitrogen, e.g. FlG - }*- .^ )ot f 



& . . vetch with nod- 



conifers, but it is in the legummosae that their ules. 

 presence is most widespread. 



The source of the combined nitrogen, which can be built up by 

 plants into proteins and utilised in this form by animals, is thus not 

 only the ammonium nitrite produced by the agency of electric dis- 

 charges in the atmosphere, but also the free nitrogen of the atmosphere 

 assimilated by various types of bacteria. 



Sulphur is found in all soils in the form of sulphates, generally of 

 lime. As sulphates it is taken up by plants. In the plant cell a process 

 of deoxidation takes place at the expense of the energy derived either 

 from the starch or, in the case of bacteria, from other ingredients of 

 their food-supply. It is built up, together with nitrogen, carbon, and 

 hydrogen, to form sulphur derivatives and amino- acids such as cystine, 

 and these, together with other amino-acids, are synthetised to form 

 proteins. Practically the whole of the sulphur taken in by animals is 



