80 THE SAP OF PLANTS. 



has inferred, that the water out 'of which plants are formed already 

 contains the necessarj' chemical principles. To this it is objected that 

 plants grown in water alone never arrive at perfection, or mature their 

 seeds. But this is not strictly true ; they do perfect their seeds : but 

 it is not surprising that crude water should be insufficient for purposes 

 which are fully answered by water properly mixed and tempered.' 



" That the extractive matter contained in earth was the real food of 

 plants was long ago stated by Woodward and Kylbel ; and most phy- 

 siologists liave adopted this opinion. But it has been estimated that a 

 plant, when dried, does not derive more than a twentietii part of its 

 weight from extractive matter and carbonic acid dissolved in water. 

 Now, supposing tiiis calculation to be not far from the truth, it serves 

 to show that extractive matter and carbonic acid are not alone sufficient 

 for the nutriment of plants. 



" Nevertheless, if neither extractive matter nor carbonic acid can be 

 considered to constitute exclusively the food of plants, it is at least quite 

 certain that they not only cannot exist without the latter, but tiiat it 

 forms by far the greater part of their food. It is well known that roots 

 cannot perform their functions unless within the reach of the atmo- 

 sphere. This arises from the necessity for their feeding upon carbonic 

 acid, which, after having been formed by the oxygen of the atmosphere 

 combining with the carbon in the soil, is then received into the system 

 of the plant, to be impelled upwards, dissolved in the sap till it reaches 

 the leaves, where it is decomposed by light, the oxygen liberated, and 

 the carbon fixed. It has also been ascertained that, feed plants as you 

 will, they will neither grow nor live, whether you offer them oxygen, 

 hydrogen, azote, or any other gaseous or fluid principle, unless carbonic 

 acid be present. 



" The course which is taken by the sap, after entering a plant, is 

 tlie next subject of consideration. Tiie opinion of the old botanists was, 

 tiiat it ascended from the roots between the bark and the wood ; but 

 this has been long disproved by modern investigators, and esoecially by 

 the experiments of Mr. Knight. If a trunk be cut tiirough in the 

 spring, at the time the sap is rising, tiiis fluid \vill be found to exude, 

 more or less, from all parts of the surface of the section, except the 

 hardest heart -wood, but most copiously from the alburnum. If a branch 

 be cut through at the same season, it will be found that, while the 

 lower face of the wound bleeds copiously, scarcely any fluid exudes 

 from the upper face ; from which and other facts it has been fully 

 ascertained that the sap rises through the wood, and chiefly through 

 the alburnum. Observations of the same nature have also proved tliat 

 the sap descends through the liber. But the sap is also diffused laterally 

 through the cellular tissue, and this with great rapidity, as will be 

 apparent upon placing a branch in a coloured infusion, which will 

 ascend and descend in the manner just ^tated, and will also disperse 

 itself laterally in all directions round the principal channels of its 

 upward and downward route. 



" With regard to the vessels through which this universal diffusion 

 of the sap takes place, it has already been stated that its upward course 

 is always through the woody fibre, and probably also througii the ducts, 

 and that it passes downwards through the woodv fibre. But there can 



