136 ROTATION OF CROPS. 



the carbon which appeared as sugar in the former case would 

 have been applied in the formation of other constituents of the 

 tree, in the event of its having had a free and unimpeded circu- 

 lation. When the soil is frozen in winter, there cannot be an 

 absorption of alkalies by the roots ; but notwithstanding this, it 

 cannot be doubted that during the day the evergreen and the 

 leaves of firs and pines must absorb continually from the air car- 

 bonic acid, which will be constantly decomposed by the action of 

 the light. When circulation is unimpeded, the carbon of this 

 carbonic acid may perhaps be converted into wood or into other 

 constituents of the plant ; but, in the absence of the conditions 

 necessary for this conversion, it may now secrete resin, balsam, 

 and volatile oils. In the generation of the sugar, or in that of 

 resin and volatile oil in the firs and pines, all the constituents of 

 the leaves must take part ; and hence we cannot suppose that 

 their alkalies, their lime, &c, are either accidental, or that they 

 are unnecessary to the exercise of this vital function. 



For the conversion of the carbon or carbonic acid into sugar, 

 it is necessary that certain conditions exist in the plant itself, in 

 addition to the external circumstances (such as heat and air). 



We furnish the conditions essential to the formation of starch, 

 or of sugar, when we supply to the leaves — that is, to the organs 

 destined for the absorption and assimilation of the carbonic acid 

 — their necessary constituents. 



The sap of such plants as are rich in sugar or in starch, and 

 also the sap of most woody plants, contains much potash and 

 soda, or alkaline earths. We cannot suppose that these are mere 

 accidental ingredients ; on the contrary, we must believe that 

 they serve some purposes of the plants, and that they assist in 

 the formation of certain of their constituents. It has already 

 been mentioned, that they exist in the plants in a state of com- 

 bination with certain organic acids. These acids are so far 

 characteristic of certain genera, that they are never absent from 

 them. Hence the organic acids themselves must assist in some 

 of the vital functions. Now, when it is remembered that unripe 

 fruits, such as grapes, are unfit to eat on account of their acidity ; 

 that these fruits possess the same power as the leaves of absorb- 

 ing carbonic acid, and of giving off oxygen on exposure to light 



