Accepting the explanation as satisfactory, we arrive at the conclusion that the tubes or pitchers of the Sarracenias have the power of secreting a limpid fluid, in addition to the honey-like secretion of the lip, and that this fluid is collected at the bottom of the tube, into which the captured insects fall. Having sipped the nectar which was spread at the mouth, a harmless but seductive lure, spread upon treacherous ground, we have seen that the structure of the tube was favourable to the retention of the insects, and that numbers of them gradually, but surely, found their way to the bottom. Here another secretion is stored, which doubtless possesses some properties of its own, and has some function to perform. In order to determine this, Dr. Mellichamp proceeded to the investigation of the fluid secretion of the pitchers.
"By draining every leaf plucked of its few drops of juice, I collected about half an ounce of the secretion in a vial, with which I made careful experiments in testing its intoxicating effect upon insects. My subjects were chiefly house-flies. About half a drachm to a drachm of the secretion was placed in a small receptacle, and the flies thrown in from time to time, the liquor not being deep enough to immerse them completely, but enabling them to walk about in it without swimming and the risk of being drowned. Some twenty flies were experimented with. At first the fly makes an effort to escape, though apparently he never uses his wings in doing so—the fluid, though not seemingly very tenacious, seems quickly to saturate them, and so clings to them, and clogs them, as to render flight impossible. A fly, when thrown in water, is very apt to escape, as the fluid seems to run from its wings, but none of these escaped from the bath of the Sarracenia secretion. In their efforts to escape they soon get unsteady in their movements, and tumble sometimes on their backs; recovering, they make more active and frantic efforts, but, very quickly, stupor seems to overtake them, and they then turn upon their sides either dead (as I at first supposed) or in profound anæsthesia.
"I had no doubt, from the complete cessation of all motion, and from their soaked and saturated condition, that they were dead, and, like dead men they were 'laid out,' from time to time, as they succumbed to the powerful liquor; but to my great surprise, after a longer or shorter interval—from a half-hour to an hour or more—they indicated signs of returning life by slight motions of the legs and wings, or body. Their recovery was very gradual, and eventually, when they crawled away, they seemed badly crippled and worsted by their truly Circean bath. After contact with the secretion, the flies which were first thrown in became still, seemingly dead, in about half a minute, but whether from exposure to the air, or exhausted by action on these insects, the liquor did not seem to be so intoxicating with those last exposed to its influence. Anæsthesia or intoxication certainly did not occur so quickly; it took from three to five minutes generally, and in one rebellious subject it took at least ten minutes for him to receive his coup-de-grace. A cockroach thrown in succumbed almost immediately, as did also a small moth, and much more slowly a common house-spider. On the recovery of the latter it was almost painful to witness his unsteady motions, and seeing him dragging his slow length along.
"Without doubt, therefore, the secretion found in the tubes of Sarracenia is intoxicating, or anæsthetic, or narcotic, or by whatever word you may prefer to indicate that condition to which these insects succumb. I forgot to mention that while experimenting as above I also threw several flies in water—a few escaped, one remained for some hours, still 'paddling' and undrowned. A large 'blue fly' was also repeatedly immersed in a weak solution of gum-arabic (in imitation of the fluid of Sarracenia) but he remained unhurt all night, and I liberated him in the morning."
Having thus far recapitulated the results of most of the experiments which have been undertaken in connexion with the Sarracenias, it is necessary to see how matters stand with them in relationship to their insectivorous proclivities. They possess pitchers, or receptacles capable of holding and retaining insects. These receptacles are furnished with glands which excrete around the mouth of the pitcher saccharine juice peculiarly attractive to insects, although of no special service to the plant. Insects seduced by this nectar congregate around the mouths of the pitchers, the sides of which are so constructed that they present no obstacles to downward precipitation. The inner lining of these pitchers consists of four zones, the lowest present hairs, in considerable numbers, pointing downwards, so that insects which have once fallen down are unable to get out again, in fact, they become veritable traps. Fluid accumulates at the bottom of these vessels, in which imprisoned insects are drowned, whether or not it is intoxicating in its properties matters but little. The whole structure and adaptation is that of a "fly-catcher." Numerous insects have constantly been found at the bottom of the pitchers. Thus far, although the evidence is circumstantial, it seems to indicate insectivorous propensities, but no experiments yet instituted have demonstrated the presence of an acid secretion, which should aid in the digestion of the captured insects. That the plant has the power of catching insects will not be denied, but as yet there is no proof that it has the power of digesting them. As Mr. W. H. Gilburt has said, "The pitchers contain fluid, but nothing corresponding to a digestive fluid has been detected in them; so that if the insects which perish in the pitcher are of any value to the plant, and afford any nutriment, it must be simply by maceration, and the glands can be regarded as absorbent only. Unfortunately, however, the evidence of absorbent glands is faulty."
We have met with no indications in any of the details of experiments, of aggregation of protoplasm, which, as we have seen elsewhere, accompanies the absorption of animal matter. Hence, for the present, the admission of the Sarracenias into the circle of insectivorous plants can only be tentative. There may be strong presumption that it would not have become a fly-catcher, on such an extensive scale, if such a proceeding were not in some way beneficial to the plant. That, unless insects are of some service, it may be regarded as a waste of power, to secrete a honeyed juice around the open jaws of death, and lure the unoffending flies to certain destruction, in wanton mischief. The fact, perhaps, would be better stated in such terms as would indicate, that hitherto the observations are incomplete, and, although strongly suspected, the Sarracenias can only be charged with destroying insects, and not with devouring them.
It is manifestly difficult to suggest any plausible theory to account for the presence of insects, nay, more, the special adaptation for the capture of insects, if we reject the carnivorous hypothesis. We could not accept the suggestion that they simply store up insects for certain insectivorous birds, nor that insects are collected in their receptacles in order to furnish food to some larvæ, which are developed from the eggs of other insects, which are deposited there by choice or chance. The only feasible theory would be one which can associate the decaying insects in some manner with the roots, by demonstration that the nitrogenous matter is conveyed from the pitchers by decay or puncture at the base, or some such means, whereby the generous fluid may be conveyed into the soil, and absorbed by the plant in the ordinary manner.
"It must be quite certain that the insects which go on accumulating in the pitchers of Sarracenia are far in excess of its needs for any legitimate process of digestion. They decompose, and various insects, too wary to be entrapped themselves, seem habitually to drop their eggs into the open mouth of the pitchers, to take advantage of the accumulation of food. The old pitchers are consequently found to contain living larvæ and maggots, a sufficient proof that the original properties of the fluid which they secreted must have become exhausted. And Barton says that various insectivorous birds slit open the pitchers with their beaks to get at their contents."
At one of the meetings of the Scientific Committee of the Royal Horticultural Society, during 1879, it was reported that a cultivator of Sarracenias in this country had one species, the pitchers of which furnished such an attraction to flies, that they soon became completely gorged; and thus, as the pitchers were destroyed, the plants could not be successfully cultivated until a device was discovered, which consisted in blocking the mouths of the pitchers with cotton-wool, which had the desired effect, and afterwards the cultivation of that species proceeded satisfactorily. This fact would certainly indicate that animal food is not essential to at least one species.
In an interesting communication on carnivorous plants, from an entomological point of view, Professor C. V. Riley (American Department of Agriculture) has described two insects, and given details of their life-history, which live in, and are parasitic upon, the contents of the pitchers of Sarracenia. He has shown that these insects flourish upon the ruin of the many victims of the honeyed lure which these pitchers present, and as a summary of his observations he concludes thus:—
"1. There is no reason to doubt, but every reason to believe, that Sarracenia is a truly insectivorous plant, and that by its secretions and structure it is eminently fitted to capture its prey.
"2. That those insects most easily digested (if I may use the term), and most useful to the plant, are principally ants and small flies, which are lured to their graves by the honeyed paths; and that most of the larger insects, which are not attracted by sweets, get in by accident and fall victims to the peculiar mechanical structure of the pitcher.
"3. That the only benefit to the plant is from the liquid manure resulting from the putrescent captured insects.
"4. That the parasitic moth is a mere intruder, its larva sharing the food obtained by the plant.
"5. That the fly (which also breeds in the pitchers) has no other connexion with the plant than that of a destroyer, though its greatest injury is done after the leaf has performed its most important functions. Almost every plant has its peculiar insect enemy, and Sarracenia, with all its dangers to insect life generally, is no exception to the rule."
Another plant, very similar to the Sarracenias, is found at an elevation of 5,000 feet on the Sierra Nevada of California. "It has pitchers of two forms; one, peculiar to the infant state of the plant, consists of narrow, somewhat twisted trumpet-shaped tubes, with very oblique open mouths, the dorsal lip of which is drawn out into a long, slender, arching, scarlet hood that hardly closes the mouth. The slight twist in the tube causes these mouths to point in various directions, and they entrap very small insects only. Before arriving at a state of maturity the plant bears much larger, nearly erect pitchers, also twisted, with the lip produced into a large inflated hood that completely arches over a very small entrance to the cavity of the pitcher. A singular orange-red, flabby, two-lobed organ hangs from the end of the hood, right in front of the entrance, which, according to Professor Asa Gray, is smeared with honey on its inner surface. These pitchers are crammed with large insects, especially moths, which decompose in them, and result in a putrid mass." It is a curious fact that the change from the slender, open-mouthed, to the inflated, close-mouthed pitchers, is absolutely sudden in each individual plant. No intermediate stages are to be found, so that the young pitchers almost represent those Sarracenias which have open mouths and erect lids, and the mature pitchers represent those other Sarracenias which have closed mouths and globose lids.