58 



I'lfYSIOLOGY 



CHAP. 



carbonic acid or the carbonates of the soil to form carbohydrates, 

 and free nitrogen to form proteins. No less interesting are the 

 various forms of RhizoHum leguminosarum, studied by Hellziegel, 

 Nobbe, Beyerinck, Franck and others, which penetrate the root- 

 hairs of the common Leguminosae (beans, peas, lupins, trefoils, etc.), 

 and produce hypertrophy in the form of nodules or tubercles 



containing 



a fungoid mass, consisting 



of bacteria for the most part of excep- 

 tional size, with a less number of normal 

 form and proportions (Fig. 13). Accord- 

 ing to the said authors, the rhizobium 

 lives in symbiosis with the leguminous 

 plant. The latter provides the bacterium 

 with carbohydrate ; and the bacterium, 

 by conversion of the free nitrogen into 

 an organic form, provides the leguminous 

 plant with the nitrogenous compounds 

 required for the synthetic formation of 

 proteins, thus promoting the general 

 welfare of the plant. 



In the greater number of cases, 

 however, the assimilation of carbon is an 

 anabolic function of green plants, which 

 are capable of reducing the carbonic acid 

 of the air by means of chlorophyll, under 

 the influence of the luminous rays of 

 the sun (particularly of the less refran- 

 gible red and yellow rays) ; and the 

 assimilation of nitrogen is, generally 

 speaking, due in plants to reduction of 

 the nitrates contained in the humus, 

 and not to intake of free nitrogen. The 

 clearest demonstration of this fact is 

 afforded by the cultivation of green 

 plants in artificial solutions which, with 

 the exception of carbon, contain all the 

 chemical elements that participate in the formation of living 

 matter, in the form of combinations of salts. The formula given 

 by Sachs for this artificial nutrient fluid is as follows : 



PIG. 13. Root of I "(')(' faiiia. 

 (Noll.) Shows root-hairs copi- 

 ously provided with nodules thi- 

 rhizobium. 



Water . 



Potassium nitrate 

 Sodium chloride . 

 Potassium sulphntc 

 Magnesium sulphate 

 Calcium phosphate 

 Ferrous sulphate . 



gr. 



1000-0 

 1-5 

 0-5 

 0-5 

 0-5 

 0-5 

 0-005 



If a grain of maize is placed in this solution to germinate, the 



