NOURISHMENT OF PLANTS. 33 



2. Soluble in diluted muriatic acid : the earthy 

 salts (salts of lime and of magnesia, together with 

 salts of oxide of iron). 



3. Insoluble in water and acids : the silicates. 



Whether one or the other class of these salts pre- 

 dominates in a plant, may be ascertained, although 

 only in an approximate way, by treating its ashes 

 first with water, and then with diluted muriatic acid. 

 50 ounces of the ashes of potatoes contained 40 

 ounces of alkaline salts ; on the other hand, the same 

 quantity of the ashes of the tops, only 2 or 2J oun- 

 ces ; 50 ounces of ashes made from the young leaves 

 of trees contained 25 ounces of alkaline salts, but 

 when made from the old dry leaves, only 7J to 10 

 ounces. 



2. Whence do Plants obtain their Constitu- 

 ent Elements ? 



This inquiry follows directly from the first, and 

 leads us to consider the food of plants ; for we must 

 regard every substance which supplies the plant with 

 one or more of the elements necessary to the build- 

 ing up its body as a means of nutriment thereto. 

 Plants can absorb their nutriment only through the 

 pores — so fine as to be altogether invisible to the 

 naked eye — of their root-fibres and leaves. Hence 

 it follows, that every thing which can usefully con- 

 tribute to their nourishment must be either liquid or 

 aeriform^ since solid bodies cannot possibly penetrate 

 into their structure. From the results which have 



