194 THE FOOD AND NUTRITION OF PLANTS. 



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 



RESULTS. 



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

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

 and nitrogen (partly in the form of ammonia or of other com- 

 pounds) from the air, or dissolved them from the remains of for- 

 mer vegetation 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). 

 Daring its ascent, its properties are often more or less altered, 

 chiefly by dissolving the soluble organized matter it meets with ; 

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

 sensible properties in different species. This dissolved -portion is 

 already elaborated food, and may therefore be immediately em- 

 ployed in vegetable growth. But the crude sap itself is merely 

 raw material, unorganized, mineral matter, as yet incapable of 

 forming a part of the living structure. Its conversion into organ- 

 ized matter constitutes the process of 



344. Assimilation (12, 15), or what, from an analogy with an 

 animal function, is usually termed Vegetable Digestion. To un- 

 dergo this important change, the crude sap is attracted into the 

 leaves, or other green parts of the plant, which constitute the ap- 

 paratus of vegetable digestion, 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 matter of plants, upon which, or 

 in connection with which, the light exerts its wonderful action, is 

 first developed. When plants are made to grow in insufficient 

 light, as when potatoes throw out shoots in cellars, this green- 

 matter is not formed. When light is withdrawn, it is soon decom- 

 posed ; as we see when Celery is blanched by heaping the soil 

 around its stems. So, also, the naturally greenless leaves of plants 

 parasitic upon the roots or stems of other species (135) have no 

 direct power of assimilation, but feed upon and grow at the ex- 



