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On the Office of the Heart Wood of Trees. By f . A. Knight, Esq. 

 F.R.S. In a Letter addressed to the Right Hon. Sir Joseph Banks, 

 Bart. G.C.B. P.R.S. Read February 5, 1818. [Phil. Trans. 

 1818, p. 137.] 



As all trees that afford timber live many years before their albur- 

 num becomes converted into heart wood, it does not seem probable 

 that that substance should execute any important office in the vege- 

 table. Mr. Knight, however, is not disposed to coincide in opinion 

 with some writers, who consider the heart wood as a wholly lifeless 

 substance ; and he now thinks that, in common with the alburnum 

 and bark, it becomes a winter reservoir of that organizable matter 

 which the tree expends in its vernal germination ; that every species 

 of tree and perennial plant contains within itself during winter all the 

 matter employed in forming its early foliage and shoots ; and that it 

 is owing to the presence or absence of such reservoir that the lives 

 of plants become annual, biennial, and perennial. The annual wholly 

 exhausts itself in feeding its flowers and seeds, it forms no reservoir, 

 and therefore perishes. A biennial fills its reservoir in one season, 

 and exhausts it in the following. In the tree, as in the biennial, 

 part of the sap descends in the spring to form roots, and part as- 

 cends to produce buds, but it also forms a new layer of bark upon 

 the whole surface of its alburnum. 



The alburnum and bark of trees not appearing to Mr. Knight to 

 contain as much organizable matter as appeared thus expended, and 

 observing much soluble matter in the heart wood, he was led to ex- 

 amine the relative quantity of fluid in them. He found that in a 

 vigorous oak, 40 years old, 1000 parts of alburnum gave, in Decem- 

 ber, 469, and of the heart wood 500. The experiment was repeated 

 upon similar pieces of the same tree in April, and 1000 of alburnum 

 lost 532, and 1000 of heart wood 507. In a poplar 80 years old, 

 1000 of alburnum in December lost 535, and heart wood 626 : in 

 March the loss was 557, and 684. This abundance of fluid in the 

 heart wood was first observed by M. Coulomb, who regarded it as 

 ascending from the earth; and concluded that the sap of trees chiefly 

 passes up near the medulla, through the heart wood. Mr. Knight 

 considers this conclusion erroneous ; for when he divided the albur- 

 num of an oak tree in winter, there was no life in the part above the 

 wound in the ensuing spring ; and in June the leaves faded rapidly 

 after a similar experiment. These and other considerations induce 

 Mr. Knight to reject the hypotheses which assume the ascent of the 

 sap through the heart wood, and lead him to believe that the fluid 

 which affords the organizable matter required for the annual con- 

 version of alburnum into heart wood is derived from the bark, and 

 that it passes inwards during the latter part of the summer and 

 autumn through the convergent cellular processes, to return in part 

 through the same passages, when a new layer of bark is to be formed 

 in the spring. 



