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Secr, I, T3 UMBTILICAL' VESSELS 25

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Thefe umbilical veffels, which fupply the buds of plants with nou- rifhment in the early fpring, and unfold their foliage, have been much attended to by Dr. Hales and Dr. Walker(Edinb. Phil, Tranfa®, Vol.I.) The former obferved, that the fap from the ftump of a vine, which he had cut off in the beginning of April, arofe twenty-one feet high in glafs tubes affixed to it for that purpofe, but which in a

ew weeks ceafed to bleed. Dr. Walker alfo marked the progrefs of

the afcending fap in various‘branches of trees, and: obferved, that in cold weather it ftopped many hours in a day, as well as in the night, and found likewife as foon as the leaves became expanded, that the wounded trees ceafed to bleed.

The veflels, which conveythefap-juice with fuch amazing force, are fituated in or compofe the alburnum, or fap-wood, of the trunk or root of the tree; norisit furprizing, that fome of it when prefled by fo high a column fhould exfude into the cells between the alburnum and bark, as in thefe cells much fap-juice was obferved by Dr.Walker, and this accounts for the great eafe with which the barks of willows and of oaks are feparated in the fpring from their wood. The abforb- ent mouths of thefe fap-veflels open externally in the moift earth on the roots of trees, and alfo into the air on their trunks; and thus mix the aqueous fluids, which they thus imbibe, with the faccharine and mucilaginous materials depofited previoufly in the alburnum of thefe roots and trunks.

3. This afcending fap-juice during the fpring feafon is in fome trees{o fweet, that it is ufed in making wine, as that of the birch- tree in this country; and fugar is procured in fuch quantity from a maple in Penfylvania, that from each tree five or fix pounds of good fugar have been made annually without deftroying it, Rufh, on Sugar Maple. Philips, London. This fugar is depoñited I believe in the fap-wood of the trunk and roots of trees, as in the manna-afh, and is diflolved in the fpring by the moifture, which is drank up by the abforbents from the earth and atmofphere, and forcibly carried on to

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