52 PLANT PHYSIOLOGY 



which our usual chemical methods appear remarkably rough. Take, for 

 example, the case where 



181, 1. 9, after 1895 read ; vid. summary by EMMERLING in LAFAR'S Mykologie, 

 I, p. 429). 



1. 15, for is known to be rapidly read may be regarded as almost 

 1. 36 P. 182, 1. 32, for and we saw . . . glucose acts best, read and we have 

 proved that the nitrogen can be assimilated in an inorganic combination. 

 We have now to ask whether this is generally true. Our answer is ' by no 

 means '. Certain Fungi are obviously heterotrophic so far as their nitrogen 

 requirements are concerned ; they either directly require proteid, peptone, 

 amino-acids, acid-amides, &c., or, at least, thrive better with these compounds 

 than with inorganic ones. On the other hand, there are also organisms which 

 are certainly autotrophic, and exist better with inorganically than with 

 organically combined nitrogen. In the case of those that are preferably 

 heterotrophic, as also in the case of those that are preferably autotrophic, the 

 further question arises whether they select one definite nitrogenous compound 

 from amongst others presented to them. It is impossible offhand to answer 

 this question in relation to the fungus that has been most frequently investigated, 

 viz. Aspergillus niger, for the literature on the subject, though very extensive, 

 is often very contradictory ; it is very much less easy to formulate any general 

 rules on the subject. Still, it appears to us permissible to separate Fungi and 

 Bacteria into a number of classes in accordance with their preference for 

 certain sources of nitrogen, on the lines adopted by BEIJERINCK (1890) and 

 FISCHER (1903, p. 96). One must not, however, imagine that by classifying 

 an organism in one of these groups we are expressing a generally valid con- 

 clusion. The nutritive value of a source of nitrogen is naturally dependent on 

 many accessory factors, e.g. the reaction of the culture solution and the nature 

 of the source of carbon presented at the same time. Since cultures in which 

 the nitrogen is present as nitrate become gradually more and more alkaline, 

 Fungi which are able to produce acids metabolically will grow better in these 

 cultures than those which do not possess that power. These latter will, how- 

 ever, grow eventually in the presence of nitrate if we artificially combine the 

 alkali which arises. Conversely, culture solutions containing ammonium are 

 generally acid, and since various Fungi can tolerate acids in quite variable 

 degrees, the nutritive value of the source of nitrogen in this case will be deter- 

 mined by another subsidiary factor, viz. the power of the fungus to resist 

 these acids. As an example of the effect of the source of carbon on the meta- 

 bolism of nitrogen we may note that in FISCHER'S experiments (1897, p. 53) 

 Bacillus colt, B. subtilis, and B. pyocyaneus were able to live on nitrate in the 

 presence of glucose ; if glucose were replaced by glycerine then B. pyocaneus 

 alone could employ the nitrate, while the others required ammonia to meet 

 their nitrogen wants. These facts limit very much the value of the following 

 classification, which attempts to arrange Fungi and Bacteria in accordance 

 with the suitability of the source of nitrogen : 



1. Nitrogen-organisms. These forms prefer the free nitrogen of the air to 

 any nitrogenous compound (these will be treated of in Lecture XVIII). 



2. Nitrate-organisms. These grow just as well as or even better with 

 nitric acid than with other compounds. Among Fungi of this type we have 

 Alternaria tennis, Mucor racemosus, Aspergillus glaucus (LAURENT, 1889), 

 Monilia Candida (WENT, 1901), and among Bacteria, foecal Bacteria (JENSEN, 

 1898), Bac. pyocyaneus, and B. fluorescens. 



3. Nitrite-organisms. BEIJERINCK (1893) has shown that Bac. perlibratus 

 grows best with nitrite, and corresponding statements have been made for 

 certain Fungi by WINOGRADSKI (1899) and RACIBORSKI (1906). 



