Intelligence in Plants and Animals / Being a New Edition of the Author's Privately Issued "Soul and Immortality."

by Thomas G. Gentry

After contraction has endured for a greater or less time, dependent upon circumstances which we do not well understand, re-expansion of the leaves is effected at an insensibly slow rate, whether or not any object is enclosed, both lobes opening in all ordinary cases at the same time, although each lobe may act to a certain extent independently of the other. The re-expansion is not determined by the sensitive filaments, for these may be cut off close to their bases, or be entirely removed, and re-expansion occur in the usual manner. It is believed that the several layers of cells forming the lower surface of the leaf are always in a state of tension, and that it is owing to this mechanical state, aided probably by fresh fluid being drawn into the cells, that the lobes begin to separate as soon as the contraction of the upper surface diminishes.

Six known genera, Drosophyllum, Roridula, Byblis, Drosera, Dionæa and Aldrovanda comprise the Droseraceæ, all of which capture insects. The first three genera effect this purpose solely by the viscid fluid secreted from their glands, and the last, like Dionæa, which has already been described, through the closing of the blades of the leaf. In these last two genera rapid movement makes up for the loss of viscid secretion. But of all the genera none is more interesting than the typical Sundews.

Growing in poor peaty soil, and sometimes along the borders of ponds where nothing else can grow, certain low herbaceous plants, called Droseras, abound. So small and apparently insignificant are they, that to the ordinary observer they are almost unnoticed. But they have peculiarities of structure and nature that readily distinguish them. Scattered thickly over their leaves are reddish bristles or tentacles, each surmounted by a gland, from which an extremely viscid fluid, sparkling in the sunlight like dew, exudes in transparent drops. Hence the common name of Sundew by which the half-dozen species found in the United States east of the Mississippi River are known. A one-sided raceme, whose flowers open only when the sun shines, crowns a smooth scape, which is devoid of tentacles. Drosera rotundifolia, our commonest species, has a wide range, being indigenous to both Europe and America. In the United States it extends from New England to Florida and westward, and is occasionally associated with Drosera longifolia, a form with long strap-shaped leaves, but whose distribution is mostly restricted to maritime regions, from Massachusetts to Florida.

ROUND-LEAVED SUNDEW.
Leaves Acting as Stomachs.

All of the species are remarkably similar in habits, capturing insects, and digesting and absorbing the soft parts, a circumstance which explains how these plants can flourish in an extremely poor soil where mosses, which depend almost entirely upon the atmosphere for their nourishment, only can live. Although the leaves of the Droseras at a hasty glance do not appear green, owing to the purple color of the tentacles, yet the superior and inferior surfaces of the blade, the stalks of the central tentacles, and the petioles contain chlorophyll, rendering the best of evidence that the plants obtain and assimilate carbon dioxide from the air. But when the poverty of the soil where these plants grow is considered, it is at once apparent that their supply of nitrogen would be exceedingly small, or quite deficient, unless they had the power of obtaining it from some other source. From captured insects this important element is largely obtained, and thus we are prepared to understand how it is that their roots, which consist of only two or three slightly divided branches, from one-half to one inch in length, and furnished with absorbent hairs, are so poorly developed. From what has been stated it would seem that the roots but serve to imbibe water, but there is no doubt that nutritious matters would also be absorbed were they present in the soil.

With the edges of its leaves curled so as to form a temporary stomach, and with the glands of its closely-inflected tentacles pouring forth their truly acid secretion, which dissolves animal matters that are subsequently absorbed, Drosera may be said to feed like an animal. But, unlike an animal, it drinks by means of its roots, and largely, too, for it would not be able to supply its glands with the necessary viscid fluid. The amount needed is by no means an inconsiderable quantity, as two hundred and seventy drops may sometimes be exposed during a whole day to a glaring sun. Such a profuse exudation implies preparations for hosts of insect visitors. In this Drosera has not miscalculated. Its bright pink blossoms and brilliant, glistening dew lure vast numbers of the smaller kinds, and the larger ones, too, to certain death. But the wholesale destruction of life that goes on is much in excess of what the plant requires for food. While the smaller flies remain adherent to the leaves, affording them the needed aliment, the larger insects, after death, fall around the roots, where they decay and fertilize the soil with nitrogen, which doubtless through the proper channels makes its way into the body of the plant, thus helping to give it tone and vigor. There are times when these plants work better than at others, but whether this is caused by the electrical condition of the atmosphere, or the amount of its contained moisture, is a question which science has not positively determined.

Drosera longifolia folds it leaves entirely around its victim, from the apex down to the petiole after the manner of its vernation, but in Drosera rotundifolia, whose marginal tentacles are longer, the tentacles simply curve around the object, the glands touching the substance, like so many mouths receiving nourishment. Experimented upon with raw beef, the tentacles of healthy leaves, from within to without, but in periods of time varying from six to eight or nine hours, clasp firmly the beef, almost concealing it from view. Equally vigorous leaves, however, made no move towards clasping a bit of dry chalk, a chip of flint, or a lump of earth. Bits of raw apple cause a curving of the tentacles, but very few of the glands are seen touching them. It would seem, therefore, that these plants are really carnivorous, preferring animal substances, which they, by the aid of some ferment analogous to pepsin, which is secreted by the glands, are able to absorb. A minute quantity of already soluble animal matter is the exciting cause, and this must be taken in by the glands, or there is no secretion of the fermenting material.

In all ordinary cases the glands alone are susceptible to excitement. When excited, they do not themselves move or change form, but transmit a motor impulse to the bending part of their own and adjoining tentacles, and are thus carried towards the centre of the leaf. Stimulants applied to the glands of the short tentacles on the disc indirectly excite movement of the exterior tentacles, for the stimulus of the glands of the disc acts on the bending part of the latter tentacles, near their bases, and does not first travel up the pedicels to the glands, to be then reflected back to the bending place. Some influence, however, does travel up to the glands, causing them to secrete most copiously, and the secretion to become acid, just such an influence as that which in animals is transmitted along the nerves to glands, modifying their power of secretion, independently of the condition of the blood-vessels. Over organic substances that yield soluble matter the tentacles remain clasped for a much longer time than over those not acted upon by the secretion, or over inorganic objects. That they have the power of rendering organic substances soluble, that is, that they have the power of digestion, is no longer a question of dispute. They certainly have this power, acting on albuminous compounds in exactly the same manner as does the gastric juice of mammals, the digested matter being afterwards absorbed. In animals the digestion of albuminous compounds is effected by means of a ferment, pepsin, together with weak hydrochloric acid, though almost any acid will serve, yet neither pepsin nor an acid by itself has any such power. It has been observed that when the glands of the disc are excited by the contact of any object, especially of one containing nitrogeneous matter, the outer tentacles and often the blade become inflected, the leaf thereby becoming converted into a temporary cup or stomach. The discal glands then secrete more copiously, the secretion becoming acid, and, moreover, some influence being transmitted by them to the glands of the exterior tentacles, causing them to emit a more abundant secretion, which also becomes acid. This secretion is to a certain extent antiseptic, as it checks the appearance of mould and infusoria, and in this particular acts like the gastric juice of the higher animals, which is known to arrest putrefaction by destroying the microzymes.

With animals, according to Schiff, mechanical irritation excites the glands of the stomach to secrete an acid, but not pepsin. There is strong reason to believe, too, that the glands of Drosera, which are continually secreting viscid fluid to replace the losses by evaporation, do not secrete the ferment proper for digestion when mechanically irritated, but only after absorbing certain matters of a nitrogeneous nature. The glands of the stomachs of animals secrete pepsin only after they have absorbed certain soluble substances designated peptogenes, showing a remarkable parallelism between the glands of Drosera and those of the stomach in the secretion of their appropriate acid and ferment.