10 THE FLOWERING PLANT. 



head is obtained in two ways — ( i ) by chemical analysis of plants ; 

 (2) by cultivating them in various solutions, and determining 

 which are most suitable. 



The essential elements (always found on analysis, and compounds 

 of which form a suitable food-solution) are the following : — Carbon, 

 hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 

 calcium, magnesium, and iron. The first six actually make up 

 the plant-body, while the remaining four have a beneficial in- 

 fluence upon the vital processes. For example, chlorophyll can- 

 not be formed when iron is absent. Carbon is obtained from 

 carbon dioxide ; hydrogen from water and ammonia or its com- 

 pounds ; oxygen from water and numerous salts ; nitrogen from 

 either ammonia and its salts, or else nitrates ; sulphur from sul- 

 phates ; phosphorus from phosphates ; potassium from various 

 compounds, especially chloride; calcium and magnesium as sul- 

 phate, phosphate, nitrate, and carbonate ; iron from numerous 

 compounds. 



Reception of food is followed by assimilation. That is to say, 

 the simple food- substances are built up step by step into the 

 complex compounds which constitute the plant-body. The first 

 .step consists in the formation of non-nitrogenous organic matter 

 from carbon dioxide and water, with liberation of oxygen. The 

 equation 



C0 2 + H 2 = CH 2 + 2 



carbon dioxide and water give non-nitrogenous matter and oxygen 



roughly represents this. It must not, however, be supposed that 

 the liberated oxygen is all derived from the carbon dioxide, though 

 half must be, while the other half comes fvom the water. The 

 oxygen in question passes off, at any rate to a large extent, into 

 Uie surrounding medium. It is most essential to remember that 

 this process has nothing whatever to do with the breathing of plants. 



This first step in assimilation, which bridges the gap between 

 inorganic and organic compounds, can only be effected by the 

 agency of chlorophyll in the presence of light. It has been 

 already mentioned that kinetic energy is liberated when chemical 

 compounds break down. Conversely, kinetic energy is converted 

 into potential when chemical compounds are built up. The neces- 

 sary kinetic energy appears to be obtained by the chlorophyll 

 from light -rays. 



When organisms contain no chlorophyll, they cannot live on 

 simple inorganic compounds only; hence the complex nature of 

 animal food. There is also no evolution of oxygen. 



The first visible product of assimilation is starch, which has 

 a chemical composition closely allied to that of cellulose. The 

 ultimate product of assimilation is protoplasm. 



