unless they could digest the cellulose forming the cell-walls; for it is
well known that all other nutritious substances are almost completely
withdrawn from leaves, shortly before they fall off. It has, however,
now been ascertained that some forms of cellulose, though very little or
not at all attacked by the gastric secretion of the higher animals, are
acted on by that from the pancreas. {35b}
The half-decayed or fresh leaves which worms intend to devour, are
dragged into the mouths of their burrows to a depth of from one to three
inches, and are then moistened with a secreted fluid. It has been
assumed that this fluid serves to hasten their decay; but a large number
of leaves were twice pulled out of the burrows of worms and kept for many
weeks in a very moist atmosphere under a bell-glass in my study; and the
parts which had been moistened by the worms did not decay more quickly in
any plain manner than the other parts. When fresh leaves were given in
the evening to worms kept in confinement and examined early on the next
morning, therefore not many hours after they had been dragged into the
burrows, the fluid with which they were moistened, when tested with
neutral litmus paper, showed an alkaline reaction. This was repeatedly
found to be the case with celery, cabbage and turnip leaves. Parts of
the same leaves which had not been moistened by the worms, were pounded
with a few drops of distilled water, and the juice thus extracted was not
alkaline. Some leaves, however, which had been drawn into burrows out of
doors, at an unknown antecedent period, were tried, and though still
moist, they rarely exhibited even a trace of alkaline reaction.
The fluid, with which the leaves are bathed, acts on them whilst they are
fresh or nearly fresh, in a remarkable manner; for it quickly kills and
discolours them. Thus the ends of a fresh carrot-leaf, which had been
dragged into a burrow, were found after twelve hours of a dark brown
tint. Leaves of celery, turnip, maple, elm, lime, thin leaves of ivy,
and, occasionally those of the cabbage were similarly acted on. The end
of a leaf of _Triticum repens_, still attached to a growing plant, had
been drawn into a burrow, and this part was dark brown and dead, whilst
the rest of the leaf was fresh and green. Several leaves of lime and elm
removed from burrows out of doors were found affected in different
degrees. The first change appears to be that the veins become of a dull
reddish-orange. The cells with chlorophyll next lose more or less
completely their green colour, and their contents finally become brown.
The parts thus affected often appeared almost black by reflected light;
20