FOOD OF MARINE PLANTS. 157 



\vw\ draw from a naked rock, the surface of which does not suffer 

 the slightest change — a plant which reaches a height of 360 feet 

 (Cook), and one of which, with its leaves and twigs, affords nour- 

 ishment to thousands of marine animals. These plants require 

 obviously only a fastening point in order to prevent a change of 

 place, or an arrangement by which their small specific weight is 

 compensated ; they live in a medium which conveys the neces- 

 sary nourishment to all their parts. Sea-water does not only con- 

 tain carbonic acid and ammonia, but also phosphates, and earthy 

 and alkaline carbonates, salts invariably found in the ashes of 

 marine plants, and indispensable for their growth. 



All our knowledge tends to prove that the conditions necessary 

 for the existence and duration of marine plants are the same as 

 those upon which the existence of terrestrial plants depends. 



But terrestrial plants do not live like marine plants, in a me- 

 dium containing all their elements, and surrounding every part 

 of their organism ; but their existence depends upon two media, 

 the one of which, the soil, contains constituents which are absent 

 from the other, the atmosphere. 



How is it possible, we may well ask, that there ever could 

 have been a doubt as to the part which the constituents of the soil 

 took in the growth of the vegetable world ? Yet, there was a 

 time when it was considered that the mineral constituents of 

 plants were not necessary and essential to their existence ! 



The same circulation exists on the surface of the earth as in 

 the sea ; there is an unceasing change — a perpetual destruction 

 and re-establishment of equilibrium. Practice in agriculture has 

 taught us that the amount of vegetable matters on a given sur- 

 face increases with the supply of certain substances, which were 



ORIGINAL CONSTITUENTS OF THE SAME SURFACE OF THE SOIL, and 



had been removed from it by means of plants. The excrements 

 of men and of animals arise from plants ; they are exactly the 

 materials which, during the life of the animal, or after its death, 

 obtain again the same form that they possessed as constituents of 

 the soil. 



We know that the atmosphere does not contain these materials, 

 and that it does not replace them ; we know further that, by their 

 removal from the soil, an inequality of production is occasioned, 



