RETURN OP THE SAP. 37 



derable solubility, is peculiarly fitted for being applied to 

 the purposes of nourishment: it is accordingly hoarded in 

 magazines, with a view to future employment, being to ve- 

 getables, what the fat is to animals, a resource for the exi- 

 gencies that may subsequently arise. With this intention, 

 it is carefully stored in small cells, the coats of which pro- 

 tect it from the immediate dissolving action of the surround- 

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

 of its solution, when the dem.ands of the system require it. 

 The tuberous root of the potato, that invaluable gift of Pro- 

 vidence 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 inten- 

 tion of forming a permanent part of the vegetable structure, 

 constituting the basis of the woody fibre, and giving mecha- 

 nical support and strength to the whole fabric of the plant. 

 These latter structures may be compared to the bones of ani- 

 mals, composing, by their union, the solid frame work, or 

 skeleton of the organized system. The woody fibres do not 

 seem to be capable of farther alteration in the living vegeta- 

 ble, and are never, under any circumstances, taken up and 

 removed to other parts of the system, as is the case with nu- 

 tritive 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 unquestionably a powerful influence in deter- 

 mining certain chemical changes among the elements of or- 

 ganic products, and in imparting to them peculiar proper- 

 ties; for it is now a w^ell ascertained fact that a scarcely 

 sensible portion of any one ingredient is capable of pro- 

 ducing important dificrences in the properties of the whole 

 compound. An example occurs in the -case of gold, the 

 ductility of which is totally destroyed by the presence of a 

 quantity of either antimony or lead, so minute as barely to 

 amount to the two thousandth part of the mass; and even 

 the fumes of antimony, when in the neighbourhood of melt- 



