ON THE GROWTH OF VEGETABLES. 115 
has received the name of the generating zone, becomes the seat of 
a double formation, the one cortical, the other ligneous. Fig. 137, 
which we have given to show the different elements of the stem of 
a tree, serves also to show its mode of development. That part 
included in the lines marked 1, represents the wood and bark of 
the first year, that marked 2 the wood of the second year. The 
generating layer separates these two elements, and is indicated 
by the letter ¢. Cells, fibres, and vessels result from the transfor- 
mation of the delicate elements of this generating tissue. In 
other words, at this point the transposition of caméium, or bark 
from without to within, and of wood from within to without, takes 
place. The medullary rays are continued without interruption or 
modification across the new beds, forming itself anew, and, without 
VP RMN VP FB TEM M 
Fig. 137.—Horizontal and t ion of the trunk of an Elm 
n 
being in connection with the pith, prolonging itself, in short, to 
the bark. What passes in the second year is repeated during the 
third and fourth. A very useful consequence results from this. 
We are enabled by observing certain characters to distinguish the 
Successive layers which form themselves year by year. The age 
of a tree is thus, so to speak, inscribed upon its own sutface. In 
the oak, for example, it is very easy to distinguish the annual 
layers. The transformation of the generating zone in the wood 
12 
