64 OF THE INORGANIC CONSTITUENTS OF PLANTS. 



CHAPTER VII. 



Of the Inorganic Constituents of Plants.* 



Carbonic acid, water, ammonia, and sulphates, are necessary 

 for the existence of plants, because they contain the elements 

 from which their organs are formed ; but other substances are 

 likewise requisite for the formation of certain organs destined 

 for special functions peculiar to each family of plants. Plants 

 obtain these substances, as they do the sulphur they contain, 

 from inorganic nature. In the ashes left after the incineration 

 of plants, the same substances are found, although in a changed 

 condition. 



Many of the inorganic constituents vary according to the soil 

 in which the plants grow, but a certain number of them are in- 

 dispensable to their development. All substances in solution in 

 a soil are absorbed by the roots of plants, exactly as a sponge 

 imbibes a liquid, and all that it contains, without selection. The 

 substances thus conveyed to plants are retained in greater or 

 less quantity, or are entirely separated when not suited for 

 assimilation. 



Alkaline and earthy phosphates form invariable constituents 

 of the seeds of all kinds of grasses, of beans, peas, and lentils. 



These salts are introduced into bread along with the flour, and 



* " Many authors," says Saussure, " consider that the mineral ingredi- 

 ents of plants are merely accidentally present, and are not at all necessary 

 to their existence, because the quantity of such substances is exceedingly 

 small. This opinion may be true as far as regards those matters which 

 are not always found in plants of the same kind ; but there is certainly no 

 evidence of its truth with those invariably present. Their small quantity 

 does not indicate their inutility. The phosphate of lime existing in the 

 animal body does not amount to the fifth part of its weight, yet no one 

 doubts that this salt is necessary for the formation of its bones. I have 

 detected the same compound in the ashes of all plants submitted to exami- 

 nation, and we have no right to suppose that they could exist without it." 

 (De Saussure, p. 241.) 



