384 THE FOOD OF PLANTS 



them similar metabolic products. At the same time a supply of energy is obtained, 

 but this ever present necessity is by no means the sole or even the most important 

 factor in determining the character of metabolism. 



The complex organic substances which are primarily liberated by the decom- 

 position of vegetable and animal remains afford an appropriate food supply for many 

 heterotrophic plants, and there is probably not a single organic product of meta- 

 bolism or decomposition which does not provide more or less suitable food- 

 material for some lowly organism or other. This is, however, not absolutely 

 necessary in the balance of nature, for living plants are able to assimilate or 

 decompose substances (such as oxalic acid, &c.) ; which in themselves alone are 

 incapable of affording an adequate food supply. 



As regards the commoner mould-fungi (PenidlHum glaucum, Aspergillus niger), 

 the nutritive values of different carbon-compounds have the following order when 

 supplied one at a time, and with ammonium nitrate as a source of nitrogen : grape 

 and cane-sugar, peptone, albumen, quinic acid, tartaric and citric acids, asparagin, 

 acetic acid, butyric acid, ethyl alcohol, benzoic acid, propylamine, methylamine, 

 phenol, formic acid. The order is, however, different with other organisms, but 

 in most fungi development is most rapid upon sugar and peptone. These results 

 in all cases refer only to the total amount of growth upon the given solution. 



The influence of varying conditions. A particular succession of nutritive values 

 holds good only under given cultural conditions, for a change in the latter may 

 reverse the positions of two or more substances. Thus Thiele ' has found that the 

 maximal temperature for the growth of PenidlHum glaucum on grape-sugar lies at 

 3i c C, on glycerine at 3.6C, on salts of formic acid at 35 C C. Hence at 33C the 

 feebly nutritive formic acid appears to be a better food-material than sugar. 

 Similar changes are caused by alterations in the concentration of the nutrient 

 solution employed, and in this respect Aspergillus niger behaves in part in 

 a precisely opposite manner to PenidlHum. 



Under unfavourable conditions no development is possible, even though a 

 supply of the most suitable carbon-compound is assured, and hence it is not always 

 easy to say whether an apparently non-nutritious substance may not under special 

 circumstances serve as nutrient material to a particular plant. In the absence 

 of an appropriate source of nitrogen growth necessarily ceases, while the presence 

 of oxygen renders the development of obligate anaerobes impossible. Similarly 

 many fungi can withstand an acid, and in some cases a strongly acid medium, 

 whereas a comparatively feeble acidity inhibits the development of most bacteria, 

 for these usually require neutral or weakly alkaline media. Again, over-concentration 

 retards growth, while strong poisons cannot be presented in sufficient amount to 

 serve as food-material - owing to the injurious action of even comparatively dilute 



1 Thiele, Die Temperaturgrenzen d. Schimmelpilze. 1896, pp. 10, 36. 



2 Example of limits of acidity in Sects. 85, 86. See also Nageli, Die niederen Pilze, 1877, p. 31. 

 On the limits of concentration, see Eschenhagen, Uber d. Einfluss v. Lbsungen versch. Concentr. auf 

 Schimmelpilze, 1891. Cf. also Thiele, I.e. , and Sect. 73. {Penicillium glaucum will grow upon 

 i per cent, solutions of Atropin, Muscarin, and of Eserin sulphate, if supplied with inorganic salts, 

 and also even upon 2 per cent, solutions though much more slowly.] 



