COMt^OSITION OF PLANTS. 



229 



roots replace completely to the soil, whatever it had lost by 

 nourishing the plants. Potatoes, when planted in a field which 

 had previously been fallow, keep back the winter crop. For 

 this reason, they are rather reared in a field which had before 

 been under crop ; dung is applied after them, and then peas(^ 

 are sown. 



III. Moi^c intimate Cmstituents of Vegetables. 



351. 



It is not possible to state precisely how every one of the 

 more intimate constituents of plantsjs formed from the com- 

 mon sap ; but in a great many instances we can give a dis- 

 tinct explanation, and in others, where the component parts 

 are still unknown to us, we must satisfy ourselves with pro- 

 bable conjectures. 



The first matter formed from the common sap, is the or- 

 ganizing mucilaginous matter,'or the forming juice, respect- 

 ing which we remarked above (271), that it contains the two 

 primitive forms, namely, spherules or vesicles, and ray-shaped 

 or straight-lined bodies. The mucilaginous matter is a taste- 

 less and inodorous fluid, which, without undergoing the ace- 

 tous fermentation, passes, after a considerable lapse of time, 

 into a kind of putrefactive decomposition. When treated 

 with mineral acids, it forms oxalic and saccho-lactic acid. By 

 dry distillation, mucous acid, with a burnt smell, is disengaged 

 from it, and from this ammonia is formed by fixed alkali. We 

 thus see that the common sap, when it passes into mucilage, 

 changes its constituent principles, and that azote in particular 

 is disengaged during this process. With respect to the acids 

 which mucilage exhibits, they can scarcely be considered as 

 present in the living plants, because they are first disengaged 

 by means of mineral acids and distillation. Hoivever, as car- 

 bonic acid consists of 72 parts of oxygen, and 28 of carbon, 

 the oxalic acid, which we find as a constituent part of some 

 plants, may easily be derived from it; for this acid con- 

 sists of 70 parts of oxygen;, 26 of carboa, and 2 of hydrogen. 



