3 1 8 State of the true Sap of Trees dvn-'mg Wi7iter. 



new epidermis, where that is to be formed ; and that the 

 other portion ol' it secretes through the internal glandular 

 substance of the hark, where one part of it produces the. 

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

 the wood already formed, and subsequently mingles with, 

 the ascending aqueous sap ; which tihus becomes capable 

 of affording the matter necessary tO; tbrni new buds and 

 leaves. 



It has been proved in the preceding experiments on the 

 ascending sap of the sycamore and birch, that that fluid- 

 does not approach the buds and unfolding leaves in the 

 spring, in the state in which it is absorbed 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, consists of a mixture, 

 of the true sap or blood of the plant with matter mure re- 

 cently absorbed, and less perfectly assimilated. 



It appears probable that the true sap undergoes a consi- 

 derable change on its mixture with the ascending aqueous 

 sap; for this fluid in the sycamore has been, proved to be- 

 come more sensibly sweet in. its progress- from the roots ia 

 the spring, and the liquid which flows from the wounded 

 bark of the same tree is^ also sweet ; hut I have never been 

 able to detect the slightest degree of sweetness in decoctions 

 of the sycamore wood ia winter. I am therefore inclined 

 ~to believe that the saccharine matter existing in the ascend- 

 ing sap is not immediatelv, or wholly, derived from the 

 iiuid which had circulated through the leaf in the preceding 

 year ; but that it is generated by a process similar to that of 

 the germination of seeds, and that the same process is al- 

 ways going forward during the spring and summer, as long 

 as the tree continues to generate new organs. But towards 

 the conclusion of the summer I conceive that the true sap. 

 simply accumulates in the alburnum, and thu» adds to the 

 specific gravity of winter-felled wood, and increases the 

 quantity of its extractive matter. 



I have some reasons to believe that the true sap descends 

 through the alburnum as well as through the bark ; and I 

 have been informed that if the bark be taken from the 

 trunks of trees in the spring, and such trees be suffcretl to 

 grow till the following winter, the alburnum acquires a great 

 degree of hardness and durability. If subsequent experi- 

 ments prove that the true sap descends through the albur- 

 iHim,.it will be easy to point out the cause why trees con- 

 tinue tp regelate after all communication between the leaves 



and 



