OF THE ALBURNOUS VESSELS OF TREES. 135 



important purposes in hot climates, where the dews are abundant, and 

 the soil very dry ; for the moisture the dews afford may thus be con- 

 veyed to the extremities of the roots : and Hales has proved that the 

 leaves absorb most when placed in humid air ; and that the sap descends, 

 either through the bark or alburnum during the night. 



If the inverted action of the alburnous vessels in the decorticated space 

 be admitted, it is not difficult to explain the cause why some degree of 

 growth takes place below such decorticated spaces on the stems of trees ; 

 and why a small portion of bark and wood is generated on the lower lip 

 of the wound. A considerable portion of the descending true sap 

 certainly stagnates above the wound, and of that which escapes into the 

 bark below it, the greater part is probably carried towards, and into, the 

 roots ; where it preserves life, and occasions some degree of growth to 

 take place. But a small portion of that fluid will be carried upwards by 

 capillary attraction, between the bark and the alburnum, exclusive of the 

 immediate action of the latter substance, and the whole of this will stag- 

 nate on the lower lip of the wound ; where I conceive it generates the 

 small portion of wood and bark, which Hales and Duhamel have 

 described. 



I should scarcely have thought an account of the preceding experi- 

 ments worth sending to you, but that many of the conclusions I have 

 drawn in former memoirs appear, at first view, almost incompatible with 

 the facts stated by Hales and Duhamel, and that I had one fact to com- 

 municate relative to the effects produced by the stagnation of the 

 descending sap of resinous trees, which appeared to lead to important 

 consequences. I have in my possession a piece of a fir-tree, from which 

 a portion of bark, extending round its whole stem, had been taken off 

 several years before the tree was felled ; and of this portion of wood, one 

 grew above, and the other below, the decorticated space. Conceiving 

 that the wood above the decorticated space ought to be much heavier 

 than that below it, owing to the stagnation of the descending sap, I 

 ascertained the specific gravity of both kinds, taking a wedge of each as 

 nearly of the same form, as I could obtain, and I found the difference 

 greatly more than I had anticipated, the specific gravity of the wood 

 above the decorticated space being 0'590, and of that below only 0'491 : 

 and having steeped pieces of each, which weighed a hundred grains, 

 during twelve hours in water, I found the latter had absorbed 69 grains, 

 and the former only 51. 



The increased solidity of the wood above the decorticated space, in 

 this instance, must, I conceive, have arisen from the stagnation of the 



