270 FOOD OF PLANTS. [BOOK n. 



CHAPTER XII. 



OF THE FOOD OF PLANTS. 



PLANTS acquire food both by their roots and leaves ; and there 

 is reason to believe that there is little difference in the power 

 possessed by these organs of taking it up. The largest part 

 of the food of plants is, however, derived from the earth, and 

 the air lying in its interstices, and is introduced into their 

 system through the roots. The latter are, however, incapable 

 of absorbing anything solid ; fluid and gaseous matter only 

 can pass through their spongelets. It is exclusively in the 

 form of gaseous matter or of water that the nutritive ingre- 

 dients of the soil are received by roots ; not, however of pure 

 water, which in fact does not exist in nature, but of water 

 holding various solid matters in solution, the most remarkable 

 and abundant of which are, potash, soda, magnesia, lime, 

 alumina, silex, phosphates, and sulphates. 



Soil in its natural state is filled with the remains of organic 

 bodies, which decompose, and yield ammonia, or become con- 

 verted into carbonic acid. In proportion to the abundance 

 of these is soil fertile. Ammonia, and the carbonic acid 

 incessantly forming below the surface of the earth, enter 

 freely into the roots ; mixing with water and such other prin- 

 ciples as may already have been formed there, they become 

 sap, ascend the stem, partly decompose to a certain extent 

 as they pass along, giving, apparently, oxygen to the spiral 

 vessels, which convey it into other parts of the system. 

 When sap reaches the leaves, it liberates its oxygen more 

 completely, and leaves its carbon to unite with the tissue of 

 vegetation, or to enter into new combinations with water, 

 atmospheric air, or other elements that it finds itself in 

 contact with : whence proceed the gummy, amylaceous, 

 resinous, oily, and other products peculiar to the vegetable 

 kingdom. 



