176 BIOLOGY OF PLANTS. 



most widely diffused substance in nature, nearly all rocks 

 containing traces of it. Iron is found in many seeds. 



Manganese is nearly as widely disseminated, but is found 

 in still less quantities, both in the soil and in the organs of 

 plants. 



8. Phosphoric acid has been found in all plants hitherto 

 examined, and always in combination with alkalies and alka- 

 line earths. The seeds of different grains form a large quan- 

 tity of phosphate of magnesia. This acid is derived from 

 the soil, and is an ingredient in all lands capable of cultiva- 

 tion. Phosphoric acid has also been detected in all mineral 

 waters. Sulphuret of lead (galena) contains crystallized 

 phosphate of lead. Phosphate of alumina often encrusts rock 

 crystals. Phosphate of lime is found in many rocks, and 

 even in volcanic holders. There can be no doubt but that 

 this acid is developed in the soil, and supplies phosphate of 

 lime to plants, and plants furnish it to the bones and brains 

 of animals. 



9. Sulphuric, nitric and carbonic acids, combined with 

 potash and other alkalies, and common salt, or chloride of 

 sodium, are found in some plants. 



Nitrate of potash is formed during the fermentation of ma- 

 nures. Sulphuric acid is formed from the sulphuret of iron 

 which is found in most rocks. 



Common salt, (chloride of sodium,) must come from the 

 sea, or from animal manures, as it could not be retained in 

 the soil, owing to its solubility. Very small quantities of ox- 

 ide of copper, and some metallic fluorides are absorbed by 

 some plants, although we cannot afhrm, that they are ne- 

 cessary to their growth. 



10. Some plants also take up small quantities of iodine 

 and bromine in the form of iodides and bromides ; but wheth- 

 er they are necessary to the growth, cannot be fully ascer- 

 tained, although it is probable they are, since such plants are 

 never found away from the sea, the source of these sub- 



