ll exogenous plants, or those which increase in size on the
outside of the stem, is brought about by the descent of certain
formative tissue called cambium, elaborated by the leaves and descending
between the old wood and the bark, where it is formed into alburnum or
woody matter. Some think that it is also formed by the roots and ascends
from them as well as descending from the leaves. Be this as it may,
there is no doubt about its descent. In such comparatively soft-wooded,
free growing plants as the abutilon the descent of the cambium is very
free and in considerable quantity, so that the stock would soon be
inclosed in a layer of it descending from the graft. When being
converted into woody matter it also forms adventitious buds which under
certain favorable circumstances will emit shoots of the same character
as the graft from which it was derived. The graft is such cases may be
said to inclose the stock in a tube of its own substance, leaving the
stock unaffected otherwise. The variegated shoot in this case was in
reality derived from the downward growth of the graft and not from the
original stock, which was not therefore contaminated by the graft. In
cases where the stock is of much slower growth than the graft, or the
graft is inserted upon a stock of some other species, the descending
cambium does not inclose the stock, but makes layers of wood on the stem
of the graft, which thus, as is frequently seen, overgrows the stock,
sometimes to such an extent as to make it unsightly. Nobody ever saw an
apple shoot from a crab stock, a pear from a quince stock, or a peach
shoot from a plum stock. This is one of the arguments in favor of the
view that cambium also rises from the roots.

Again, to show that the stock is not affected by the graft, or the graft
by the stock, except as to root power, let any person graft a white beet
upon a red beet, or contrariwise, when about the size of a goosequill,
and when they have attained their full growth, by dividing the beet
lengthwise he will find the line of demarkation between the colors
perfectly distinct, neither of them running into the other.

The theory that leaf variegation is a disease has been held by many
distinguished botanists and is in nowise new. But this theory has been
controverted, and we think successfully, by other botanists, and it is
not now accepted by the more advanced vegetable physiologists. There are
now so many acute and industrious students and obs