Page:Philosophical Transactions - Volume 095.djvu/115

Rh the true Sap of Trees is deposited during Winter. loi

vessels, which contain this fluid in the albumuni, are in contact ivith 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 should remmn wholly unmixed. I must there- fore 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 through the external cellular, or more probably ^ndidar substance of the bark, and gene- rates a new epidermis, where that is to be formed ; and that the other portion of it secretes through the internal glandular substance 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 subsequently mingles with the ascending aqueous sap; which thus becomes capable of affording the matter necessary to form 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 more recently absorbed, and less perfectly assimilated.

It appears probable that the true sap undergoes a considerable change on its mixture with the ascending aqueous sap ; for tliis fluid in the sycamore has been proved to become more sensibly sweet in its progress from the roots in the spring, and the liquid which flows from the wounded bark of the same tree is also sweet ; buti have never been able to detect the slightest degree of