Plant Food 35 



shall see later on, through the pores of the leaves, these pores 

 performing the part of both lungs and mouths for the plant. 

 Carbon is one of the most important substances present, build- 

 ing up portions of all the plant-tissues, and usually forming 

 one-half of the dried plant by weight. 



Hydrogen, another element present in every organic com- 

 pound, is obtained from water, H2O, which is taken up in large 

 quantities by the roots, and also, to a slight extent, from 

 ammonia, NH3. 



Oxygen. — The sources of this element are to some extent 

 the two compounds HjO and CO2 already noted, and also the 

 various oxygen salts of the metals, such as sulphates, phos- 

 phates, nitrates, &c. Besides this, the free oxygen of the 

 atmosphere enters the plant, combining with the tissues, as 

 we shall have to notice further on under the head of Respira- 

 tion. 



Nitrogen, which is an essential element of protoplasm, the 

 albuminoids, and allied bodies, is obtained from ammonia, 

 NH3, and nitric acid, HNO3, which latter, with the metals, 

 forms the salts known as nitrates. It must be borne in mind 

 that, although nitrogen exists to a very large extent in the air 

 in a free or uncombined state, yet that plants are only able to 

 absorb it from its compounds. 



Sulphur is taken up as sulphates, that is, salts of sulphuric 

 acid, H2SO4 ; probably chiefly calcium sulphate or gypsum, 

 CaS04. This element also is required for the protoplasm and 

 albuminoids. 



Phosphorus is obtained from phosphates, principally calcium 

 phosphate, Ca3,2P04. 



Silicon is taken in as silica, SiO,, which is often largely 

 present in the soil ; whilst the sources of chlorine are the 

 chlorides of the various metals, especially of sodium, the 

 chloride of which is common salt. The metals mentioned 

 above are all obtained as salts combined with the acids we have 

 noticed 



c. Method of introduction of the food substances into the 

 plant. — Before any food can be absorbed it is necessary for it 

 to be in a state of solution. The water present in the soil 



