The facts just given plainly show that the glands have the power of absorption, for otherwise it is impossible that the leaves should be so differently affected by non-nitrogenous and nitrogenous bodies, and between these latter in a dry and damp condition. It is surprising how slightly damp a bit of meat or albumen need be in order to excite secretion and afterwards slow movement, and equally surprising how minute a quantity of animal matter, when absorbed, suffices to produce these two effects. It seems hardly credible, and yet it is certainly a fact, that a bit of hard-boiled white of egg, first thoroughly dried, then soaked for some minutes in water and rolled on blotting paper, should yield in a few hours enough animal matter to the glands to cause them to secrete, and afterwards the lobes to close. That the glands have the power of absorption is likewise shown by the very different lengths of time (as we shall presently see) during which the lobes remain closed over insects and other bodies yielding soluble nitrogenous matter, and over such as do not yield any. But there is direct evidence of absorption in the condition of the glands which have remained for some time in contact with animal matter. Thus bits of meat and crushed insects were several times placed on glands, and these were compared after some hours with other glands from distant parts of the same leaf. The latter showed not a trace of aggregation, whereas those which had been in contact with the animal matter were [page 300] well aggregated. Aggregation may be seen to occur very quickly if a piece of a leaf is immersed in a weak solution of carbonate of ammonia. Again, small cubes of albumen and gelatine were left for eight days on a leaf, which was then cut open. The whole surface was bathed with acid secretion, and every cell in the many glands which were examined had its contents aggregated in a beautiful manner into dark or pale purple, or colourless globular masses of protoplasm. These underwent incessant slow changes of forms; sometimes separating from one another and then reuniting, exactly as in the cells of Drosera. Boiling water makes the contents of the gland-cells white and opaque, but not so purely white and porcelain-like as in the case of Drosera. How living insects, when naturally caught, excite the glands to secrete so quickly as they do, I know not; but I suppose that the great pressure to which they are subjected forces a little excretion from either extremity of their bodies, and we have seen that an extremely small amount of nitrogenous matter is sufficient to excite the glands.
Before passing on to the subject of digestion, I may state that I endeavoured to discover, with no success, the functions of the minute octofid processes with which the leaves are studded. From facts hereafter to be given in the chapters on Aldrovanda and Utricularia, it seemed probable that they served to absorb decayed matter left by the captured insects; but their position on the backs of the leaves and on the footstalks rendered this almost impossible. Nevertheless, leaves were immersed in a solution of one part of urea to 437 of water, and after 24 hrs. the orange layer of protoplasm within the arms of these processes did not appear more aggregated than in other speci- [page 301] mens kept in water, I then tried suspending a leaf in a bottle over an excessively putrid infusion of raw meat, to see whether they absorbed the vapour, but their contents were not affected.
Digestive Power of the Secretion.*—When a leaf closes over any object, it may be said to form itself into a temporary stomach; and if the object yields ever so little animal matter, this serves, to use Schiff’s expression, as a peptogene, and the glands on the surface pour forth their acid secretion, which acts like the gastric juice of animals. As so many experiments were tried on the digestive power of Drosera, only a few were made with Dionaea, but they were amply sufficient to prove that it digests, This plant, moreover, is not so well fitted as Drosera for observation, as the process goes on within the closed lobes. Insects, even beetles, after being subjected to the secretion for several days, are surprisingly softened, though their chitinous coats are not corroded,
[Experiment 1.—A cube of albumen of 1/10 of an inch (2.540 mm.) was placed at one end of a leaf, and at the other end an oblong piece of gelatine, 1/5 of an inch (5.08 mm.) long, and
* Dr. W.M. Canby, of Wilmington, to whom I am much indebted for information regarding Dionaea in its native home, has published in the ‘Gardener’s Monthly,’ Philadelphia, August 1868, some interesting observations. He ascertained that the secretion digests animal matter, such as the contents of insects, bits of meat, &c.; and that the secretion is reabsorbed. He was also well aware that the lobes remain closed for a much longer time when in contact with animal matter than when made to shut by a mere touch, or over objects not yielding soluble nutriment; and that in these latter cases the glands do not secrete. The Rev. Dr. Curtis first observed (‘Boston Journal Nat. Hist.’ vol. i., p. 123) the secretion from the glands. I may here add that a gardener, Mr. Knight, is said (Kirby and Spencer’s ‘Introduction to Entomology,’ 1818, vol. i., p. 295) to have found that a plant of the Dionaea, on the leaves of which “he laid fine filaments of raw beef, was much more luxuriant in its growth than others not so treated.” [page 302]
1/10 broad; the leaf was then made to close. It was cut open after 45 hrs. The albumen was hard and compressed, with its angles only a little rounded; the gelatine was corroded into an oval form; and both were bathed in so much acid secretion that it dropped off the leaf. The digestive process apparently is rather slower than in Drosera, and this agrees with the length of time during which the leaves remain closed over digestible objects.
Experiment 2.—A bit of albumen 1/10 of an inch square, but only 1/20 in thickness, and a piece of gelatine of the same size as before, were placed on a leaf, which eight days afterwards was cut open. The surface was bathed with slightly adhesive, very acid secretion, and the glands were all in an aggregated condition. Not a vestige of the albumen or gelatine was left. Similarly sized pieces were placed at the same time on wet moss on the same pot, so that they were subjected to nearly similar conditions; after eight days these were brown, decayed, and matted with fibres of mould, but had not disappeared.
Experiment 3.—A piece of albumen 3/20 of an inch (3.81 mm.) long, and 1/20 broad and thick, and a piece of gelatine of the same size as before, were placed on another leaf, which was cut open after seven days; not a vestige of either substance was left, and only a moderate amount of secretion on the surface.
Experiment 4.—Pieces of albumen and gelatine, of the same size as in the last experiment, were placed on a leaf, which spontaneously opened after twelve days, and here again not a vestige of either was left, and only a little secretion at one end of the midrib.
Experiment 5.—Pieces of albumen and gelatine of the same size were placed on another leaf, which after twelve days was still firmly closed, but had begun to wither; it was cut open, and contained nothing except a vestige of brown matter where the albumen had lain.