OF THE WOOD. 45 



bark at different periods tiirough the spring and sum- 

 mer, the same accurate observer found a great difference 

 in the thickness of the k^yer of new wood produced 

 under it, which was always less in proportion as the 

 operation was performed kuer in the season. 



That the bark or liber produces wood seems therefore 

 proved beyond dispute, but some experiments persuad- 

 ed Du Hamel tliat in certain circumstances, the wood 

 was capable of producing a new bark. This never hap- 

 pened in any case but wh.en the whole trunk of a tree 

 was stripped of its bark. A Cherry-tree treated in this 

 manner exuded from the whole surface of its wood in 

 little points a gelatinous matter, which gradually ex- 

 tended over the whole and became a new bark, under 

 which a layer of new wood was speedily formed. Hence 

 Mirbel concludes, vol. 1. 176, that the alburnum and 

 the wood are really the origin of the new layers of wood, 

 by producing first this gelatinous substance, or matter 

 of organization, which he and Du Hamel call cambium, 

 and which Mirbel supposes to produce the liber or 

 young bark, and at the same time, by a peculiar arrange- 

 ment of the vascular parts, the alburnum or new wood. 

 His opinion is strengthened by the observation of a 

 tribe of plants to be explained hereafter, Palms, Grasses, 

 &c. in which there is no real bark, and in which he 

 finds that the woody fibres do actually produce the cam- 

 bium. Dr. Hope's experiment will scarcely invalidate 

 this opinion, because it may be said the cambium had 

 already in that case formed the liber. 



This matter will be better understood when we come 

 to speak of Mr. Knight's experiments on the course of 

 the sap. 



