<.STA*E OF THE SAP IN WINTER. 311 



procefs the young tree or annual (hoot continues to acquire Explanation of 

 1 J ° _ ,. the manner m 



new parts, which apparently are formed from the atcendmg ^- lc h the juices 



aqueous fap. of P ,ant ? *j*" 



n . . . ~ tt , . , n -j r -i i late, their habi- 



But it has been proved by Du Hamel that a fluid, nmirar to tudeS) c h an ges, 



that which is found in the true Cap veflels of the bark, exifts &c. 

 alfo in the alburnum, and this fluid is extremely obvious in the 

 fig, and other trees, whofe true fap is white, or coloured. The 

 veflels, which contain this fluid in the alburnum, are in con- 

 tad with thofe which carry up the aqueous fap ; and it does 

 not appear probable that, in a body fo porous as wood, fluids fo 

 near each other fhould remain wholly unmixed. I mutt there- 

 fore conclude that when the true fap has been delivered from 

 the cotyledon or leaf into the returning, or true fap veflels of 

 the bark, one portion of it fecreles through the external cel- 

 lular, or more probably glandular fubftance of the bark, and 

 generates a new epidermis, where that is to be formed; and 

 that the other portion of it fecretes through the internal glan- 

 dular fubflance of the bark, where one part of it produces the 

 new layer of wood, and the remainder enters the pores of 

 the wood already formed, and fubfequently mingles with the 

 afcending aqueous fap ; which thus becomes capable of af- 

 fording the matter neceflary to form new buds and leaves. 



It has been proved in the preceding experiments on th.e 

 afcending fap of the fyeamore and birch, that that fluid does 

 not approach the buds and unfolding leaves in thefpring, in the 

 ftate in which it is abforbed from the earth : and therefore we 

 may conclude that the fluid, which enters into, and circulates 

 through the leaves of plants, as the blood through the lungs of 

 animals, confifts of a mixture of the true fap or blood of the 

 plant with matter more recently abforbed, and lefs perfectly 

 aflimilated. 



It appears probable that the true fap undergoes a confider- 

 able change on its mixture with the afcending aqueous fap ; 

 for this fluid in the fyeamore has been proved to become more 

 fenfibly fweet in its progrefs from the roots in the fpring, and 

 the liquid which flows from the wounded bark of the fame 

 tree is alfo fweet ; but I have never been able to detect the 

 flighteil degree of fweetnefs in decoctions of the fyeamore 

 wood in winter. I am therefore inclined to believe that the 

 faccharine matter exifting in the afcending fap is not immedi- 

 ately, or wholly, derived from the fluid which had circulated 



through 



