J 



28 



UMBILICAL VESSELS. 



Sect. III. II. 4. 



pand the buds. Its exift 



the fap-wood as well as in th^ 



roots is {hewn from the puUulation of oak-trees, which have be^n 

 Gripped of their bark, and alfo from the expanfion of the eyes of 

 vine-lhoot, when it is cut from the tree, and plaHted in the earth, s 



t 



defcribed in Sedl. XV 



3 



This fuggefts to us the reafon why the wood of trees is fo much 

 fooner fubjed to decay, when they are felled in the vernal months i 

 becaufe the fugar, which the fap-wood then contains, foon ru 

 fermentation, and produces what is called tlic dry rot ; whence the 

 cuftom has prevailed of debarking oaks in the fpring, and felling 



them in the autumn 



and 



probable that the wood of 



th 



trees would lafl much longer, if it was thus managed, as the growth 

 of the new leaves would exhaufl 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, tragap 



or in the knots or joints of the ftem, as in 



grafles, and the fugar- cane; which hke the farina and oil in feeds, and 

 the dulcet mucilage of fruits, and the honey of flowers, were defigncd 

 for the food of the young progeny of plants, but become the fujfte- 



of mankind 



th 



As the faccharine matter which is thus depofited in the roots, or in 

 B alburnum, or in the joints of plants, muft be diluted by the 

 moifture abforbcd from the earth by their roots, we underl!and why 

 the leaves of the lower branches of trees are firfl: expanded, as is (gcii 

 diftinaiy in the hawthorn hedges in April, as thefe muft firfl receive 



the afcending fap-j 



of the mapl 



was'obferved by Dr. Walk 



in 



h 



ac- 



feafo 



4. The force of the rifing fap from a vine-ftump in the bleed 



as difcovered by Dr. Hales 



fome times eq 



to th 



whole prelfure of the atmofphere, which is about fourteen pound 



fquare inch of furface 



This great power in raifing the fap he af 



cribes to capillary attradion, and to the variations of heat during the 



d ay 



i» 



