292 THE FOOD OF PLANTS 



nitrogen is presented to them in the form of ammonium nitrate, and they must 

 therefore be supplied with peptone or albumin ; on the other hand a few 

 organisms are actually able to assimilate free nitrogen (Sect. 68). The fact 

 that growth is possible under these circumstances shows that all the sub- 

 stances necessary for constructive metabolism can be produced, and that 

 a supply of energy is also obtained. A fungus, which can grow with formic 

 acid as its sole organic food, decomposes a portion of this nutriment in 

 respiratory and other processes, and so obtains the energy necessary for the 

 synthesis of carbohydrates, fats, and even proteids from formic acid, water, 

 and simple inorganic salts. The nitrite and nitrate bacteria can actually 

 commence their synthetic metabolism with carbon dioxide and water, just as 

 a green plant does, the necessary supply of energy being obtained by the 

 oxidation of ammonia to nitrous or to nitric acid (Sect. 63). It is not 

 impossible that organisms may be discovered which obtain the energy 

 necessary for the assimilation of carbon dioxide by the decomposition of 

 organic substances, as is indeed actually the case in those fungi which can 

 be fed with formic acid, for it is immaterial whether the energy is obtained 

 from the same compound that is assimilated or from another one. In 

 all cases every manifestation of energy of which the plant is capable is 

 dependent upon chemical changes of some kind or other '. 



With the exception of the nitrite and nitrate bacteria, no organisms 

 are known which can utilize chemical energy in the production of organic 

 food from carbon dioxide. Green plants, however, possess a mechanism 

 by means of which the energy of the rays of light can be employed 

 for this purpose, while the purple water-bacteria possess a compound 

 assimilatory pigment by means of which not only light but also dark heat 

 rays may be utilized in the assimilation of carbonic acid (Sect. 52), and it is 

 indeed possible that organisms may be discovered which are able to make 

 use of other forms of radiant energy for the assimilation of carbon from 

 carbon dioxide. Indeed, in certain cases electrical energy may be of service, 

 for organic and nutritive substances may be artificially produced by 

 synthesis under the influence of electrical forces. It is possible, therefore, 

 to distinguish between photosynthesis, thermosynthesis, chemosynthesis, 

 elcctrosynthesis, &c., according to the source from which the energy for 

 synthesis is obtained. 



Owing to their power of manufacturing organic food, green plants can 

 develop in water or in pure sand, provided the necessary inorganic salts are 

 supplied to them, whereas a fungus can only grow on such a medium when 

 sugar is added. Fungi, and other plants without chlorophyll (with the 

 exceptions mentioned above), either obtain their organic food from dead 



1 For details see Pfeffer, Studien z. Energetik, 1892. 



