CONSTITUENTS OF PLANTS. 115 



Laurent, Bouillon, Lagrange). Ammonia* is also 

 found in this water, so that the same conditions 

 which sustain living beings on the land are com- 

 bined in this medium, in which a whole world of 

 other plants and animals exist. 



The roots of plants are constantly engaged in 

 collecting from the rain those alkalies which formed 

 part of the sea- water, and also those of the water of 

 springs, which penetrates the soil. Without alka- 

 lies and alkaline bases most plants could not exist, 

 and without plants the alkalies would disappear 

 gradually from the surface of the earth. 



When it is considered, that sea-water contains 

 less than one-millionth of its own weight of iodine, 

 and that all combinations of iodine with the metallic 

 bases of alkalies are highly soluble in water, some 

 provision must necessarily be supposed to exist in 

 the organization of sea- weed and the different kinds 

 of Fuci, by which they are enabled during their life 

 to extract iodine in the form of a soluble salt from 

 sea-water, and to assimilate it in such a manner, 

 that it is not again restored to the surrounding 

 medium. These plants are collectors of iodine, 

 just as land-plants are of alkalies ; and they yield 

 us this element, in quantities such as we could not 

 otherwise obtain from the water without the evapo- 

 ration of whole seas. 



* When the solid saline residue obtained by the evaporation of sea- 

 water is heated in a retort to redness, a sublimate of sal-ammoniac is 

 obtained. MARCET. 



I 2 



