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; but when viewed as a transparent object under the microscope, minute specks of light were transmitted, and this was not the case with the unaffected parts of the same leaves.These effects, however, merely show that the secreted fluid is highly injurious or poisonous to leaves; for nearly the same effects were produced in from one to two days on various kinds of young leaves, not only by artificial pancreatic fluid, prepared with or without thymol, but quickly by a solution of thymol by itself.On one occasion leaves of Corylus were much discoloured by being kept for eighteen hours in pancreatic fluid, without any thymol.With young and tender leaves immersion in human saliva during rather warm weather, acted in the same manner as the pancreaticfluid, but not so quickly.The leaves in all these cases often became infiltrated with the fluid.
Large leaves from an ivy plant growing on a wall were so tough that they could not be gnawed by worms, but after four days they were affected in a peculiar manner by the secretion poured out of their mouths.The upper surfaces of the leaves, over which the worms had crawled, as was shown by the dirt left on them, were marked in sinuous lines, by either a continuous or broken chain of whitish and often star-shaped dots, about 2 mm.in diameter.The appearance thus presented was curiously like that of a leaf, into which the larva of some minute insect had burrowed.But my son Francis, after making and examining sections, could nowhere find that the cell-walls had been broken down or that the epidermis had been penetrated.When the section passed through the whitish dots, the grains of chlorophyll were seen to be more or less discoloured, and some of the palisade and mesophyll cells contained nothing but broken down granular matter.These effects must be attributed to the transudation of the secretion through the epidermis into the cells.
The secretion with which worms moisten leaves likewise acts on the starch-granules within the cells.My son examined some leaves of the ash and many of the lime, which had fallen off the trees and had been partly dragged into worm-burrows.It is known that with fallen leaves the starch- grains are preserved in the guard-cells of the stomata.Now in several cases the starch had partially or wholly disappeared from these cells, in the parts which had been moistened by the secretion; while it was still well preserved in the other parts of the same leaves.Sometimes the starch was dissolved out of only one of the two guard-cells.The nucleus in one case had disappeared, together with the starch-granules.The mere burying of lime-leaves in damp earth for nine days did not cause the destruction of the starch-granules.On the other hand, the immersion of fresh lime and cherry leaves for eighteen hours in artificial pancreatic fluid, led to the dissolution of the starch- granules in the guard-cells as well as in the other cells.
From the secretion with which the leaves are moistened being alkaline, and from its acting both on the starch-granules and on the protoplasmiccontents of the cells, we may infer that it resembles in nature not saliva, but pancreatic secretion; and we know from Fredericq that a secretion of this kind is found in the intestines of worms.As the leaves which are dragged into the burrows are often dry and shrivelled, it is indispensable for their disintegration by the unarmed mouths of worms that they should first be moistened and softened; and fresh leaves, however soft and tender they may be, are similarly treated, probably from habit.The result is that they are partially digested before they are taken into the alimentary canal.I am not aware of any other case of extra- stomachal digestion having been recorded.The boa- constrictor is said to bathe its prey with saliva, but this is doubtful; and it is done solely for the sake of lubricating its prey.Perhaps the nearest analogy may be found in such plants as Drosera and Dionaea; for here animal matter is digested and converted into peptone not within a stomach, but on the surfaces of the leaves.