FOOD-ELEMENTS. 105 



quantity is derived mainly from the decomposition of car- 

 bonic dioxide (CO2), taken into tlie leaves through the 

 stomates from the surrounding air, of which it constitutes 

 on an average only four hundredths of one per cent. Hy- 

 drogen is present in much smaller quantity than car- 

 bon. It is probably derived mainly from the decomposition 

 of water (H2O). Oxygen forms the largest proportiou, 

 after carbon, of the weight of the dried vegetable substance. 

 It is introduced into the plant in excessive quantities, in the 

 form of water, carbonic dioxide, and oxygen-salts, and also 

 absorbed directly from the atmospheric oxygen. The small 

 quantity of nitrogen is obtained from the compounds of 

 ammonia (NH3) and nitric acid. Sulphur is furnished 

 by the salts of sulphuric acid. 



134. The elements, carbon, hydrogen, oxygen, nitrogen, 

 and sulphur, compose the greater (combustible) part of the. 

 substance of plants. They are constituents of cellulose, 

 and the albuminoids which form protoplasm. "They 

 mainly form the organized and organizable part of the 

 plant, and of every individual cell. Their imjportance, 

 therefore, lies in the fact that they furnish the chief mate- 

 rials for the construction of the plant.'' Iron is indispen- 

 sable for the production of the chlorophyll, though extra- 

 ordinarily small quantities suffice. If large quantities of 

 solutions of iron become distributed in the tissues of plants, 

 the cells quickly die. The importance of this food-element 

 is great, since without chlorophyll no assimilation takes 

 place. It has also been demonstrated that the presence of 

 potassium is necessary to the assimilating activity of chlo- 

 rophyll. If it is absent, the plants do not increase in 

 weight, but behave as if absorbing only pure water. 



