THE FOOD OF PLANTS. 269 



tains substances found in but one or two other 

 plants known to man. Among the rarer bodies are 

 the newly discovered metals caesium and rubidium, 

 and how or where the plant obtains them is indeed 

 a mystery, as the most delicate chemical tests have 

 failed to detect these elements in soils. In common 

 garden beets, also, the same substances have been 

 found. Copper has frequently been observed in 

 vegetable products used for food, and what is very 

 singular, the metal has recently been discovered in 

 the feathers of birds, and some of the tints in the 

 plumage are due to its presence. The fluorine 

 which is found in the enamel of teeth, in men and 

 animals, comes from plants, as does also the man- 

 ganese which accompanies iron in the blood. Alu- 

 minium, the metal which within a few years has 

 been regarded with special interest, as of great 

 service in the arts, has been found in certain spe- 

 cies of Lycopodium, and zinc has been found in the 

 Viola, caliminaria. a plant common in some sections 

 of France. Bromine and iodine are found in the 

 marine algoe, or sea weeds, and for a long time the 

 entire amount of these important substances em- 

 ployed in medicine and the arts was derived from 

 sea plants cast on shore by the waves. 



The organic constituents of plants, elaborated or 

 formed from combinations of the elements, car- 

 bon, hydrogen, nitrogen, and oxygen, make up the 



