296 THE FOOD OF PLANTS 



at least certain that the ash constituents, as well as nitrogen, become of 

 importance only when actually assimilated. Sulphur takes part in the 

 formation of the molecule of proteid compounds, and many remarkable 

 phosphorized proteids exist. A plant may contain large quantities of 

 uncombined nitrates or ammonium salts, and frequently the elements of the 

 ash may in part be present in the form of inorganic compounds. Many 

 of the organic metabolic products contain neither nitrogen nor any ash 

 constituent, for in the processes of metabolism not only carbon, but also 

 nitrogen and the mineral constituents, may be dissociated again as in- 

 organic compounds from the organic substances with which they were 

 united. 



It is one of the aims of physiology to trace all the changes which an 

 absorbed food material may undergo, and the first thing necessary is to 

 determine empirically which are the essential elements, and which com- 

 pounds of them form most suitable food. From this point of view it is 

 permissible to treat separately of the ash constituents as a group apart, 

 for most plants absorb them in inorganic form, or are at least able to 

 do so if necessary. Nitrogen also may be absorbed either in the form of 

 an inorganic salt or of an organic compound. 



The very unequal nutritive values of different carbon compounds show 

 that every molecular combination is not of equal value in the nutrition 

 of a particular plant. The inorganic salts absorbed are for the most part 

 highly oxidized, and hence the energy necessary for their assimilation by 

 the protoplasm must be derived from the oxidation of pre-existent carbon 

 compounds by respiration, or from intracellular molecular decompositions 

 which involve a liberation of energy. Carbon compounds do not, however, 

 furnish the sole source of energy in all cases, for the energy necessary for the 

 synthesis of proteids from carbon dioxide, water, &c. by the nitrite and 

 nitrate bacteria, is obtained by the oxidation of ammonia to nitrous acid, 

 or of nitrous to nitric acid, while certain other bacteria obtain energy by 

 the oxidation of sulphur or sulphuretted hydrogen. 



It is extremely difficult to determine precisely what part an clement 

 plays in the metabolism of a plant, for it may be utilized in a variety of 

 ways ; indeed it is not always easy to say to what extent a particular 

 substance serves as food material at all. If every element which is of 

 use to the plant is regarded as having a nutritive function, then both the 

 non-essential sodium and silicon must be admitted as forming part of the 

 food of plants, for these elements arc certainly not entirely useless. Such 

 carbon compounds must also be included as remain intact under normal 

 conditions, and are consumed only when the plant is starved. Nutritive 

 substances may be replaced by others of this character ; but we under- 

 stand by an essential element one without a certain minimum of which the 

 plant is unable to exist. 



