SAP OF TREES DURING WINTER. 117 



that the new matter which is added to the point of the root descends 

 from the cotyledons. The first motion therefore of the fluids in plants 

 is downwards, towards the point of the root ; and the vessels which 

 appear to carry them are of the same kind with those which are subse- 

 quently found in the bark, where I have, on a former occasion, endeavoured 

 to prove that they execute the same office. 



In the last spring I examined almost every day the progressive changes 

 which take place in the radicle emitted by the horse-chestnut : I found 

 it, at its first existence and until it was some weeks old, to be incapable 

 of absorbing coloured infusions when its point was taken off, and I was 

 totally unable to discover any alburnous tubes through which the sap 

 absorbed from the ground, in the subsequent growth of the tree, ascends ; 

 but when the roots were considerably elongated, alburnous tubes formed ; 

 and, as soon as they had acquired some degree of firmness in their 

 consistence, 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 

 always accompanied by spiral tubes, which do not appear to carry any 

 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 growth, it 

 seems to perform precisely the office of the cotyledon ; but being exposed 

 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 be 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 repetition 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 Duhamel 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 obvious in the fig, and other trees, whose true 

 sap is white or coloured. The vessels which contain this fluid in the 



