] 18 OBSERVATIONS ON THE STATE OP THE 



alburnum 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 should remain wholly unmixed. I must 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 through the external cellular, or more probably glandular 

 substance of the bark, and generates 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 this 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 ; but I have never been able to detect the 

 slightest degree of sweetness in decoctions of the sycamore wood in 

 winter. I am therefore inclined to believe that the saccharine matter 

 existing in the ascending sap is not immediately, or wholly, derived from 

 the fluid 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 always 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 thus 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 suffered to grow till the following winter, the alburnum acquires a 



