57 
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 inter- 
nal part, the heart-wood . The alburnum is white, 
and full of moisture, and in young trees and annual 
shoots it reaches even to the pith. 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 trachece , and the false 
tracheae . t 
The tubes, which he has called simple tubes, 
seem to contain the resinous or oily fluids peculiar 
to different plants. 
The porous tubes likewise contain these fluids ; 
and their use is probably that of conveying them 
* Fig. 3. represents the result of the experiment on the maple. 
Journal de Physique, September 1811, page 210 
f Fig. 4, 5, 6, and 7. represent MirbeFs idea of the simple 
tubes, the porous tubes, the tracheae, and the false tracheae. 
