Is deposited during IVinicr. 317" 



Coloured infusions, when its point was taken oft, and T was 

 totally unable to discover any alburnous tul)es, through 

 which the sap absorbed from the ground, in the subsequent 

 growth of the tree, ascends : but when the roots were con- 

 siderably elonsrated, alburnous tubes formed ; and' as soon 

 as they had acquired some degree of firmness in their con- 

 sistence, they appeared to enter on their office of carrying 

 up the aqueous sap, and the leaves of the plumula then, 

 and not sooner, expanded. 



The leaf contains at least three kinds of tubes : the first is 

 what, in a former paper, I have called the central vessel, 

 through which the aqueous sap appears to be carried, and 

 through which coloured infusions readily pass, from the 

 alburnous tubes into the leaf-stalk. These vessels are al- 

 ways accompanied by spiral tubes, which do not -appear to 

 carry anv liquid : but there is another vessel which appears 

 to take its origin from the leaf, and which descends down 

 the internal bark, and contains the true or prepared sap. 

 When the leaf has attained its proper growlh, it seems to' 

 perform precisely the office of the cotyledon ; but being ex- 

 posed to the air, and without the same means to acquire, or 

 the substance to retain moisture, it is fed by the alburnous 

 tubes and central vessels. The true sap now appears to 

 he discharged from the leaf, as it was previously from the 

 cotyledon, into the vessels of the bark, and to be employed 

 in the formation of new alburnous tubes between the base of 

 the leaf and the root. From these alburnous tubes spring 

 other central vessels and spiral tubes, which enter into, and 

 possibly give existence to, other leaves ; and thus by a re- 

 petition of the same process the young tree or annual shoot 

 continues to acquire new parts, which apparently are formed 

 from the ascending aqueous sap. 



But it has been proved by Du Hamcl that a fluid, similar 

 to that which is found in the true sap vessels of the bark, 

 exists also in the alburnum, and this fluid is extremely ob- 

 vious in the fig, and other trees, whose true sap is white, or 

 coloured. The vessels, which contain this fluid in the al- 

 burnum, are in contact with those which carry up the 

 aqueous sap ; and it does not- appear probable that, in a 

 body so porous as wood, fluids so near each other §houId 

 remain wholly unmixed. I nnist therefore conclude, that 

 when the true sap has been delivered from the cotyledon or 

 leaf into the returning or true sap vessels of the bark, one 

 portion of it secretes throutrh the external cellular, or more 

 probably glandular substance of the bark, and gctierates a 



Vol.i!2. No. S8. Sept. 1S03. X new 



