178 METABOLISM 



nourished in an autotrophic or a heterotrophic manner. Since the decom- 

 position of carbon-dioxide is dependent on the presence of chlorophyll we may 

 conclude from the absence of that colouring matter that the organism is of 

 necessity obliged to fall back on organic materials containing carbon. Experi- 

 mental observations on a large number of Bacteria and Fungi have fully 

 confirmed this conclusion in the great majority of cases. On the other hand, 

 the constant occurrence of an organism in soil which is rich in organic material 

 suggests that it may live heterotrophically even though chlorophyll be present. 

 Although the plant be dependent on nitrogen, sulphur, phosphorus, or other 

 ash constituents in an organic form, it does not always follow that it also 

 requires the carbon to be in an organic combination. The most remarkable 

 condition is where another living organism, animal, or plant serves as a sub- 

 stratum rich in organic nutriment, where, in a word, the mode of life is 

 ■parasitic. A large number of Fungi as well as certain higher plants, e. g. 

 Lathraea and Orobanche, exhibit this type of heterotrophic life ; in these, 

 chlorophyll is for the most part wanting. A colourless parasite must, so far 

 as nutrition is concerned, obviously behave just like the colourless member 

 of an autotrophic plant, e. g. the root. It might, therefore, be thought that it 

 would serve the purpose if we were to treat such parasites as an appendix to 

 our study of autotrophic plants. In reality, however, we know very little 

 indeed as to their nutrition ; we are much better acquainted with the behaviour 

 of certain Fungi and Bacteria which live on dead organic matter. Whilst 

 parasites are always present on certain definite plants, often on a single 

 species or variety, many saprophytes, that is to say, heterotrophic organisms 

 which live on dead organic materials, appear on the most varied substrata, and 

 thus are specially adapted for the study of the nutrients which make life 

 possible for them. We shall begin with these plants and especially with their 

 dependence on carbon in the organic form. [Benecke has recently published 

 (1904) an important and comprehensive memoir on the nutrition of Fungi.] 



The requirements of Mould-fungi as regards mineral matters have already 

 been referred to ; it will be sufficient, therefore, to note here that essentially the 

 same substances are needed by them as by higher plants. The only difference 

 is that the Fungi require only one of the alkaline earths, calcium or magnesium, 

 whilst higher plants require both. In order to study the sources of carbon 

 employed by our ordinary moulds we may select a nutritive solution which, in 

 addition to minerals, contains nitrate of ammonia to meet the demand for 

 nitrogen, adding to the solution different bodies containing carbon, and 

 introduce a few spores of Aspergillus niger or of Penicillium glaucum. Accord- 

 ing to the way in which the fungus grows we may readily draw conclusions as 

 to the nutritive value of the source of carbon supplied. We are able to 

 determine, for example, that sugar is an excellent nutrient, but that many acids, 

 such as formic or oxalic, are very inferior nutrients or have no nutritive value 

 at all. Exhaustive studies on this question have been carried out by Pasteur 

 (i860 and 1862), Nageli (1879 ^^^ 1882), and Reinke (1883). These investi- 

 gators have shown that an extraordinarily large number of carbon compounds 

 may serve as nutrients to Fungi, e. g. carbohydrates, alcohols, organic acids, 

 fats, amido-compounds, peptones, &c. We give below a summary by way of 

 showing how varied in character these compounds are ; the materials are 

 arranged in descending order of their value as nutrients : — 



Nageli (1882) gives the following series a propos of Fungi : — i. sugar ; 

 2. mannite, glycerine, leucin ; 3. tartaric acid, citric acid, succinic acid, aspa- 

 ragin ; 4. acetic acid, ethyl-alcohol, quinic acid ; 5. benzoic acid, salicylic acid, 

 propylamin ; 6. methylamin, phenol. 



Pfeffer (Phys. 1, 372), as a result of later experiments, rearranged the series 

 as follows: — i. sugar ; 2. peptone ; 3. quinic acid; 4. tartaric acid ; 5. citric 

 acid ; 6. asparagin ; 7. acetic acid ; 8. lactic acid ; 9. ethyl-alcohol ; 10. ben- 

 zoic acid ; 11. propylamin ; 12. methylamin ; 13. phenol ; 14. formic acid. 



