190 THE FOOD AND NUTRITION OF PLANTS. 



searches seem to show that these crops exhaust the soil less than 

 the cereal grains, in part at least, on account of the large quantity 

 of organic matter, rich in nitrogen, which they leave to be incor- 

 porated with the soil. The theory of rotation in crops, founded by 

 De Candolle on the assumption that excretions from the roots of a 

 plant accumulate in the soil until in time they become injurious 

 to that crop, but furnish appropriate food for a different species, 

 is entirely abandoned as an explanation ; and even the fact that 

 such excretions are formed, at least to any considerable extent, is 

 not made out. That they could accumulate and remain in the soil 

 without undergoing decomposition is apparently impossible. 



Sect. III. Assimilation, or Vegetable Digestion, and its 



Eesults. 



342. We have reached the conclusion, that the universal food of 

 plants is rain-watex*, which has absorbed some carbonic acid gas 

 and nitrogen (partly in the form of ammonia or of other compounds) 

 from the air, or dissolved them from the remains of former vegeta- 

 tion in the soil, whence it has also taken up a variable (yet more or 

 less essential) quantity of earthy matter. 



343. This fluid, imbibed by the roots, and carried upwards 

 through the stem, receives the name of sap or crude sap (79), 

 Upon its introduction into the plant, this is at once mingled with 

 some elaborated sap or soluble organized matter it meets with ; 

 thus becoming sweet in the Maple, &c., and acquiring different 

 sensible properties in different species. This latter is already elab- 

 orated food, and may therefore be immediately employed in vegeta- 

 ble growth. But the crude sap itself is merely raw material, unor- 

 ganized or mineral matter, as yet incapable of forming a part of the 

 living structure. Its conversion into organized matler constitutes 

 the pi'ocess of 



344. Assimilation, or what, from an analogy with animal life, is 

 usually termed Vegetable Digestion. To undergo this important 

 change, the crude sap is attracted into the leaves, or other green 

 parts of the plant, which constitute the apparatus of assimilation, 

 where it is exposed to the light of the sun, under which influence 

 alone can this change be effected. Under the influence of solar 

 light, the fabric is itself constructed, and the chlorophyll, or green 



