178

NATURE AND PROPERTIES OF TIMBER.

[sect. X.

Wood appears to be composed of various vessels, which, in the living tree, convey thefluids necessary to its growth; between those vessels there are cells interposed. There isnothing of the character of solid fibres in wood, except the thin membraneous coats of thecells and vessels, which adhere so slightly together in recently formed wood, that it is easyto separate them. The vessels in the growing tree are intended to convey a watery fluidcalled the sap, from the roots to the leaves; when it arrives at the leaves it undergoes certainchanges, and returns through the bark ;* * * § and the bark being expanded by this accessionof moisture, rises from the wood, and leaves a cavity that becomes filled with the propersap, which gradually hardens and forms a new layer of wood. The rising sap flows chieflythrough the annual rings next the bark; and from the experiments of Mr. Knight,f it ap-pears that the sap during its ascent dissolves some portion of a substance that had been de-posited in the vessels of the wood during the preceding winter, for the nourishment of thebuds, leaves, and young wood ; hence the flowing sap is more dense in the upper than inthe lower part of a tree. Dr. Darwin draws a like conclusion from the debarked oaks pro-ducing leaves. %

In trees, as the leaves expand the sap ceases to flow, and the bark again adheres to thewood; and from the middle of June to the middle of August, there appears to be a pausein vegetation; but after this period the sap again begins to flow, and the bark whichadhered so closely in the preceding months may be separated nearly as easily as in thespring.

315.—The sap which rises through the wood, from the roots, is very different in itsnature from that which descends through the bark to form a new layer of wood. Thatwhich ascends is nearly as liquid as water, and is called the common sap. It has in generala sweetish taste, and contains sugar and mucilage. It always contains an acid, sometimesin a free state, sometimes combined with lime or potash. When this sap is left to itself, itsoon ferments and becomes sour ; and when the proportion of sugar is considerable, it willundergo the vinous fermentation.§

The descending sap, called the proper sap, differs so considerably in different trees, andis so difficult to procure in a separate state, that its properties have not been much exa-mined. It is always less liquid, and contains a much greater proportion of vegetable mat-ter than the common sap. It is also very probable that trees of the same kind produceproper sap of different qualities in different climates, as we find that the facts establish-ed respecting timber, the growth of one climate, are not applicable to the same species oftimber grown in another climate.

* Art. Anatomy, Vegetable, Supplement to the Encyclopaedia Britannica, p. 292.

t Philosophical Transactions , 1805.

+ Phytologia, p. 159. Probably the fact may be owing entirely to the fluid being reduced by evaporation.

§ The properties of the different kinds of sap that have been examined are given in Dr. Thomson s System of Che-mistry, iv. 209— 213 .