28 UMBILICAL VESSELS, Secr.ll IL'4.

expand the buds. Its exiftence in the fap-wood as well as in the roots 1s fhewn from the pullulation of oak-trees, which have been fripped’cf their bark, and alfo from the expanfion of the eyes of a vine-fhoot, when it is cut from the tree, and planted in the earth, as defcribed in Sect. XV. 1. 3.

This fuggefts to us the reafon why the wood of trees is fo much fooner fubjeét to decay, when they are felled in the vernal months; becaufe the fugar, which the fap-wood then contains, foon runs into fermentation, and produces what is called the dry rot; whence the cuftom has prevailed of debarking oaks in the fpring, and felling them in the autumn; and it is probable that the wood of all other trees would laft much longer, if it was thus managed, as the growth of the new leaves would exhauft the fugar of the fap-wood.

Sweet juices for a fimilar purpofe of expanding the buds of herba- ceous plants are depofited during the autumn in their roots, as in tur- nep, beet, tragaposon; or in the knots or joints of the fem, as in graffes, and the fugar-cane; which like the farina and oil in feeds, and the dulcet mucilage of fruits, and the honey of flowers, were defigned for the food of the young progeny of plants, but become the fufte- nance of mankind!

Às the faccharine matter which is thus depofted in the roots, or iñ the alburnum, or in thé joints of plants, muft be diluted by the moifture abforbed from the earth by their roots, we underftand why the leaves of the lower branches of trees are firft expanded, as is feen diftintly in the hawthorn hedges in April, as thefe muft firft receive the afcending fap-juice, as was obferved by Dr. Walker in his ac- count of the maple,

4 The force of the rifing fap from a vine-ftump in the bleeding feafon, as difcovered by Dr. Hales, is at fome times equal to the whole preffure of the atmofphere, which is about fourteen pounds on a iquare inch of furface. This great power in raifing the fap he af- crbes to capillary attraction, and to the variations of heat durin o the

day