58 COURSE OP THK SAP. 



to the leaf itself, but are returned by another set of ves 

 sels, as Mr. Knight has demonstrated, into the new layer 

 of bark, which they nourish and bring to perfection, 

 and which they enable in its turn to secrete matter for a 

 new layer of alburnum the ensuing year. It is presum- 

 ed that one set of the returning vessels of trees may 

 probably be more particularly destined to this latter 

 office, and another to the secretion of peculiar fluids in 

 the bark. See Phil. Trans, for 1801, p. 337. In the 

 bark principally, if I mistake noi, the peculiar secretions 

 of the plant are perfected, as gum, resin, &c. each un- 

 doubtedly in an appropriate set of vessels. From what 

 has just been said of the office of leaves, we readily per- 

 ceive why all the part of a branch above a leaf or leaf- 

 bud dies when cut, as each portion receives nourish- 

 ment, and the means of increase, from the leaf above it. 

 By the above view of the vegetable ceconomy, it ap- 

 pears that the vascular s} stem of plants is strictly annual. 

 This, of course, is admitted in herbaceous plants, the 

 existence of whose stems, and often of the whole indi- 

 vidual, is limited to one season ; but it is no less true 

 with regard to trees. (4) The layer of alburnum on the 



(4) [The effect of girdling trees, as practised in new settle- 

 ments in the United States, is readily explained on the theory of 

 Mr. Knight. In this operation a circle of bark and also of the 

 alburnum or outer wood is removed from around the trunk. A 

 check is thus put both to the ascent and descent of the sup, and 

 the tree dies in consequence sooner or later. Sometimes how- 

 ever the sap ascends through a trunk which has been girdled, 

 and the tree puts out leaves in the Ciisuing sunmier. This lact 

 is not explained by the principles here laid down, but agrees with 

 a subsequent paper of Mr. Knight {Phil. Trans. 1808) in which 



