1838] 



FARMERS' REGISTER, 



591 



It has been shown by ihe experiments of Mr. 

 Knight, and those made by other phyi^iologists, 

 that the cap descending through the "bark alter 

 being modified in the leaves, is the principal cause 

 of the growth of the tree ; tlius, if the bark is 

 wounded, the principal formation of new bark is 

 on the upper edge of the wound ; and when the 

 wood has been removed, the formation of new 

 wood takes place immediately beneath the bark: 

 and every vessel and passage in the bark and wood 

 of trees seems capable of carrying fluids in different 

 and opposite directions, though more readily and 

 copiously in one direction than in others, which 

 offers something analogous to the anastomosis of 

 vessels in animal bodies. A fact noticed by M. 

 Palisot de Beauvois, is explained on this principle. 

 That gentleman separated different portions of cor- 

 tical layers from the rest of the bark in several trees, 

 and found that in most instances the separated bark 

 grew in the same manner as the bark in its natural 

 state. The experiment was tried with most suc- 

 cess on the lime-tree, the maple, and the lilac ; the 

 layers of bark were removed in August 1810, and 

 in the spring of the next year, in the case of the 



maple and the lilac, small annual shoots were pro^ 

 duced in the parts where the bark was insulated.* 



The wood of trees is composed of an external 

 part, called alburnum or sap-wood, and of an in- 

 ternal part, the heart-wood. The alburnum is 

 white, and full of moisture, and in young trees 

 and annual shoots it reaches even to the pilh. 

 The alburnum is the great vascular system of the 

 vegetable through which the sap rises, and the 

 vessels in it extend from the leaves to the minutest 

 filaments in the roots. 



There is in the alburnum a membranous sub- 

 stance, composed of cells, which are constantly 

 filled with the sap of the plant; and there are in 

 the vascular system several different kinds of 

 tubes; Mirbel has distinguished four species — the 

 simple tubes, the porous tubes, the tracheae, and 

 ihe, false trachcce.] 



* Fig. 3. represents the result of the experiment on 

 the maple. Journal de Phisique, September ISll, p. 

 210. 



t Fig- 4, 5, 6, and 7. represent Mirbel's idea oi the 

 simple tubes, the porous tubes, the tracheae, and ths 

 I false tracheae. 



