310 STATE OF THE SAP IN WINTER. 



Explanation of tenfion of its veflels and fibres ; and the experiments of Bonnet 



whicTtTej^ces and Da Hamd ,eave litt,e g rounc ^ s of doubt, but that the new 

 of plants circu- matter which is added to the point of the root defcends from the 



late, their habi- cotyledons. The fuft motion therefore of the fluids in plants is 



tudes, changes, , ' 



&c. downwards, towards the point of the root ; and the veflels 



which appear to carry them, are of the fame kind with thofe 



which are fubfequently found in the bark, where I have, on a 



former occafion, endeavoured to prove that they execute the 



fame office. 



In the laft fpring I examined almoft everyday the progrefiive 

 changes which take place in the radicle emitled by the horfa 

 chefnut: I found it, at its fir It exiftence, and until it was fome 

 weeks old, to be incapable of abforbing coloured infufions, 

 when its point was taken oft', and I was totally unable to dii- 

 cover any alburnous tubes, through which the fap abforbed 

 from the ground, in the fubfequent growth of the tree, afcends: 

 but when the roots were confiderably elongated, alburnous 

 tubes formed; and as foon as they had acquired fome degree of 

 nrmnefs in their confidence, they appeared, to enter on their 

 office of carrying up the aqueous fap, and the leaves of the 

 plumula then, and not fooner, expanded. 



The leaf contains at leaft three kinds of tubes : the firft h 

 what, in a former Paper, I have called the central veflej, through 

 which the aqueous fap appears to be carried, and through which 

 coloured intufions readily pafs, from the alburnous tubes into 

 fhe leaf-ftalk. Thefe veflels are always accompanied by fpiral 

 tubes, which do not appear to carry any liquid : but there is 

 another veflel which appears to take its origin from the leaf, 

 and which defcends down the internal bark, and contains the 

 true or prepared fap. When the leaf has attained its proper 

 growth, it feems to perform precifely the office of the cotyledon ; 

 but being expofed to the air, and without the fame means to 

 acquire, or the fubftance to retain moiflure, it is fed by the al- 

 burnous lubes and central veflels. The true fap now appears to 

 be difcharged from the leaf, as it was previoufl y from the cotyle- 

 don, into the. veflels of the bark, and to be employed in the for- 

 mation of new alburnous tubes between the bate of the leaf and 

 the root. From thefe alburnous tubes fpring other central vef- 

 fels and fpiral tubes, which enter into and poflibly give ex- 

 iftence to, other leaves; and thus by a repetition of the fame 



procefs 



