NUTRITION 43 



this in stiffening their bodies; but they can successfully be 

 brought to maturity without silica, although they will then 

 be comparatively soft, mechanically weak, though physio- 

 logically vigorous. 



From the few elements composing their food-materials 

 plants manufacture foods of the utmost variety. These are 

 present in the cells, either in solution or deposited in solid 

 form. The foods may be divided into two groups according 

 to the presence or absence of nitrogen as a component ele- 

 ment. The non-nitrogenous foods are the ones first formed 

 and the simpler — the sugars, starches, and oils; the nitro- 

 genous foods are elaborated from the former and are more 

 complex — the amides, proteids, nucleines, etc. Carbon, hy- 

 drogen, and oxygen are constituent elements of all the foods 

 in both groups, and carbon must be considered the charac- 

 teristic component element both of living organisms and of 

 their food. 



CARBON 



The source of carbon for all organisms except the nitrogen 

 bacteria and plants containing chlorophyll or its apparent 

 equivalent physiologically, bacterio-purpurin, is, directly or 

 indirectly, these color-containing plants. Since, howe\'er, the 

 purple-bacteria play a comparatively unimportant role as 

 food-producers, and their relations to carbon are like those 

 of green plants, we need not consider them. The nitrogen 

 bacteria will be discussed on page 68. 



The direct source of carbon for green plants, the ultimate 

 source for all organisms, is the carbon-dioxide of the air. 

 This is proved by the following facts : * 1. Green plants 

 cultivated in an atmosphere of normal composition, except 

 that every trace of carbon-dioxide has been removed, soon 

 cease to grow and finally die. 2. Green plants cultivated in 

 a soil of otherwise normal composition, but containing no 



" For details as to experiments demonstrating these facts consult Dar- 

 win and Acton's Practical Physiology of Plants, Moor's translation of 

 Detmer's, and the more recent "Laboratory Course in Plant Physiology" 

 by Ganong, and "Practical Text-book of Plant Physiology" by Mac- 

 Dougal. 



