248 PRINCIPLES OF GENERAL PHYSIOLOGY 



require the supply of some food containing these elements. For example, the 

 hydrochloric acid of the gastric juice must have chlorine. To form the haemoglobin 

 of the blood corpuscles, iron is necessary ; since a number of these corpuscles are 

 regularly broken up, new ones must be formed. Probably most of the iron 

 required is obtained from the debris of the old cells, so that comparatively little 

 further supply is needed. 



To avoid misapprehension, it must be mentioned here that recent investigations 

 have shown the necessity of minute quantities of certain organic substances, whose 

 nature is as yet not understood. Details will be found below. 



THE CHEMICAL COMPLEXITY OF THE FOOD-STUFFS REQUIRED 



The green plant is able to obtain its carbon from the carbon dioxide of the air, 

 its hydrogen from water, and its nitrogen from nitrates in the solutions bathing 

 its roots. It is possible, therefore, to grow such plants as the bean, or better, the 

 wallflower, from the seed to flowers and fruit, with its roots immersed in a solution 

 containing merely potassium nitrate and some other inorganic salts, sulphates and 

 phosphates of calcium. A trace of iron must be present. But this growth is only 

 possible in the light and it is by the aid of radiant energy from the sun that the 

 assimilation of carbon is made possible. 



The methods by which instructive experiments of this kind can be performed will be found 

 in the works of Darwin and Acton (1894, pp. 51-55) and of Macdougal (1901, pp. 223-232), 

 in addition to many other textbooks of practical physiology of plants. 



The green colouring matter, chlorophyll, by means of which the carbon assimila- 

 tion of the green plant is effected, has been said to be the most interesting substance 

 in existence, and, beyond doubt, the mechanism by which alone the higher animals 

 themselves are enabled to maintain their life is of the utmost importance. The 

 question will be discussed in Chapter XIX. 



When we investigate the fungi, many of them highly organised plants, but 

 devoid of chlorophyll, we find, as would be expected, that they cannot obtain 

 their supply of carbon from carbon dioxide alone. Sugar appears to be their 

 best source of carbon, but most carbon compounds, unless poisonous, suffice, with 

 the exception of the very simplest ones, such as formic acid and urea. 



What is perhaps more remarkable is that the higher fungi are unable even 

 to use nitrates as a source of nitrogen, which the green plant is able to do. 

 These higher fungi require ammonium salts, amines, or amino-acids ; although 

 urea cannot afford them carbon, it suffices as a source of nitrogen. Moulds and 

 certain bacteria, lower fungi, are able to obtain nitrogen from nitrates, so that 

 this capability is not entirely limited to the green plant, and it is not necessarily 

 connected, as might be thought, with the use of the sun's energy for the assimila- 

 tion of carbon. At the same time, we must remember that the obtaining of 

 nitrogen from nitrates is common to all green plants, whereas it is only a few 

 of the simplest fungi that possess it, and we find, moreover, especially amongst 

 the bacteria, very specialised requirements as to the chemical nature of their 

 food-stuffs. I may instance the fact that it was found impossible to cultivate 

 the tubercle bacillus with success until glycerol was added to the medium. On 

 the other hand, there are some bacteria which possess the very remarkable 

 aptitude of using methane as a source of carbon (Sohngen, 1905). In this connection 

 we may note that, although certain bacteria are able to utilise particular 

 substances for food, it does not follow that this food is that on which they 

 thrive best. In want of better, they can put up with it. 



The animal organism, even in its lowest forms, the protozoa, is satisfied with 

 nothing less complex than glucose as source of carbon. As regards nitrogen, 

 the requirements appear to be different according to the purpose to which it 

 is to be put, growth, maintenance, or source of energy. The experiments of 

 Grafe (1912) appear to indicate that ammonium salts, in presence of excess 

 of carbohydrate, may replace wear and tear in the dog and pig, although no 

 tissue is laid on. Further facts bearing on this question will be found in the 

 section on " Protein Metabolism " below, together with its probable explanation. 



