266 
PHYSIOLOGY. 
BOOK II. 
Stock, and so identified itself with the horizontal system, it is 
gradually pushed outwards by the descent of woody matter 
from above through it; but in giving way it is constantly 
generating red matter from its horizontal system, through 
which the wood descends, which thus acquires a colour that 
does not properly belong to it. With regard to the instances 
of grafts overgrowing their stocks, or vice versa, it is obvious, 
that these are susceptible of explanation on the same principle. 
If the horizontal system of both stock and scion has an equal 
power of lateral extension, the diameter of each will remain 
the same; but, if one grows more rapidly than the other, 
the diameters will necessarily be different : where the scion 
has a horizontal system that developes more rapidly than that 
of the stock, the latter will be the smaller, and vice versa. 
It is, however, to be observed, that in these cases plants are 
altogether in a morbid state, and will not live for any consi- 
derable time. 
Another case was, that if a large ring of bark be taken 
from the trunk of a vigorous elm or other tree, without being 
replaced with any thing, new beds of wood will be found 
in the lower as well as upper part of the trunk ; while no 
ligneous production will appear on the ring of wood left ex- 
posed by the removal of the bark. Now this is so directly at 
variance with the observations of others, that it is impossible 
to receive it as an objection until its truth shall have been 
demonstrated. It is well known, that if the least continuous 
portion of liber be left upon the surface of a wound of this 
kind, that portion is alone sufficient to establish the communi- 
cation between the upper and lower lips of the wound ; but, 
without some such slight channel of union, it is contrary to 
experience that the part of a trunk below an annular incision 
should increase by the addition of new layers of wood until 
the lips of the wound are united, unless buds exist upon the 
trunk below the ring. 
Those who object to the theory of wood being generated by 
the action of leaves, either suppose, 1st, that liber is deve- 
loped by alburnum, and wood by liber ; or, 2dly, that " the 
woody and cortical layers originate laterally from the cambium 
furnished by pre-existing layers, and nourished by the descend- 
