RETURN OF THE SAP. 37 



With this intention, it is carefully stored in small 

 cells, the coats of which protect it from the imme- 

 diate dissolving action of the surrounding watery 

 sap, but allow of the penetration of this fluid, and 

 of its solution, when required by the demands of the 

 system. The tuberous root of the potatoe, that in- 

 valuable gift of Providence to the human race, is a 

 remarkable example of a magazine of nutritive 

 matter of this kind. 



The lignin, on the contrary, is deposited with 

 the intention of forming a permanent part of the 

 vegetable structure, constituting the basis of the 

 woody fibre, and giving mechanical support and 

 strength to the whole fabric of the plant. These 

 latter structures may be compared to the bones 

 of animals ; composing by their union the solid 

 framework, or skeleton of the organized system. 

 The woody fibres do not seem to be capable of 

 further alteration in the living vegetable ; and they 

 are never, under any circumstances, taken up and 

 removed to other parts of the system, as is the 

 case with nutritive matter of a more convertible 

 kind. 



The sap holds in solution, besides carbonaceous 

 matter, some saline compounds, and a few earthy 

 and metallic bases ; bodies which, in however 

 minute a quantity they may be present, have un- 

 questionably a powerful influence in determining 

 certain chemical changes among the elements of 

 organic products, and in imparting to them peculiar 

 properties; for it is now a well ascertained fact 

 that a scarcely sensible portion of any one in- 

 gredient is capable of producing important dif- 

 ferences in the properties of the whole compound. 



