THE NUTRITION OF HETEROTROPHIC PLANTS 181 



stances which our ordinary chemical reagents fail to differentiate ; that 

 of two bodies which have an entirely similar constitution and differ 

 only in the spacial arrangement of their atoms, the one is assimilated easily, 

 the other with difficulty or not at all. A well-known and typical example 

 of the behaviour of such ' stereoisomeric ' bodies has been given by PASTEUR 

 (1858, 1860). He cultivated Penicillium in optically inactive racemic acid and 

 showed that it was resolved into dextro- and laevo-tartaric acids, and that the 

 dextrotartaric acid was used up first. Numerous similar examples have been 

 discovered since then (PFEFFER, 1895), and it has been shown that many, 

 though not all, organisms prefer definite optically active substances. There is, 

 for example, a bacterium which behaves in exactly the reverse way to Peni- 

 cillium, and which prefers the laevotartaric acid (PFEFFER, 1895), while Bacillus 

 subtilis appears to have no preference for either. Similarly BUCHNER (1892) 

 has observed that fumaric acid forms a good nutrient for Aspergillus and 

 Penicillium, while the stereoisomeric maleic acid is known to be rapidly 

 poisonous. Reference should also be made here to lactic acid and many of the 

 glucoses, which we shall take a later opportunity of discussing. 



Interesting as these conclusions are we do not obtain by their study any 

 deeper insight into the reasons for the unequal metabolic value of nearly 

 related bodies nor for the similar treatment of very different bodies ; and 

 yet elucidation of these facts must be obtained if we are to reach a clear under- 

 standing as to the mode of assimilation of food materials. Meanwhile our 

 experiments with stereoisomeric bodies offer us valuable suggestions in other 

 respects. They show, for example, how well the power of selection is developed 

 in Fungi. Aspergillus is able to distinguish not only between dextro- and laevo- 

 tartaric acids but also between entirely distinct substances. Out of a nutritive 

 solution containing much glucose and some glycerine, it selects first of all 

 the more valuable food-material, viz. the glucose. It may be said indeed that, 

 in the presence of glucose, glycerine is not employed at all. The converse, 

 however, does not hold good ; the slightest traces of dextrose are greedily 

 absorbed, although glycerine be present in quantity. Similarly, PFEFFER 

 (1895) has shown that glycerine is excluded from metabolism in the presence 

 of peptone and lactic acid in the presence of dextrose. 



Let us glance now at the requirements of heterotrophic organisms for 

 nitrogen. In the nutritive solutions we have employed hitherto we have in 

 general presented the nitrogen in the form of nitrate of ammonia, and we saw 

 that the requirements for nitrogen were met in this way, and that proteid was 

 undoubtedly manufactured. We have now to ask whether this is the only and 

 the best form in which one may offer nitrogen to Fungi. One very important 

 question is whether nitrate will act without ammonia, and whether Fungi, like 

 autotrophic plants, prefer a nitrate to an ammonium salt. As a matter of 

 fact, research has shown that different Fungi and Bacteria behave in totally dif- 

 ferent ways in relation to nitrogen, so that they have to be arranged in several 

 groups (compare BEIJERINCK, 1890, FISCHER, 1903, p. 96 ; [BENECKE, 1904]). 



1. Nitrate Organisms. These thrive in the presence of nitrates quite as 

 well as, if not better than, along with other compounds. To this group belong 

 the Fungi Alternaria tenuis, Mucor racemosus, Aspergillus glaucus (LAURENT, 

 1889) ; and among Bacteria : foecal Bacteria (JENSEN, 1898), Bacillus pyocyanus, 

 and Bacillus ftuorescens. Some employ nitrites, e. g. Bacillus perlibratus 

 (BEIJERINCK, 1893) and a fungus described by WINOGRADSKY (1899). 



2. Ammonia Organisms. These develop in the presence of nitrates but 

 thrive much better with ammonia. To this series belong, e. g. Eurotiopsis, 

 Aspergillus niger, yeast and Bacillus subtilis. 



3. Amide Organisms. Bacillus perlibratus, Bacillus typhi and Rhizopus 

 oryzae grow better with asparagin than with ammonia. Other acid amides 

 and amino-acids appear to operate in a similar manner. 



