VENUS'S FLY-TRAP. and as they stand at less than a right angle to each other, they have an excellent chance of capturing any intruder. The chief seat of the movement is near the mid-rib, but is not restricted to this part. Each lobe, when the lobes come together, curves inwards across its whole breadth, the marginal spikes alone not becoming curved. From the curving inwards of the two lobes, as they advance towards each other, the straight marginal spikes intercross by their apices at first, and ultimately by their bases. The leaf is then completely shut and encloses a shallow cavity. If made to. shut merely by the touching of one of the sensitive filaments, or by the inclusion of an object not yielding soluble nitrogeneous matter, the two lobes retain their inwardly concave form until they re-expand. The re-expansion, when no organic matter is enclosed, varies according to circumstances, a leaf in one instance being fully re-expanded in thirty-two hours. But the lobes, when soluble nitrogeneous matter is included, instead of remaining concave, thus containing within a concavity, slowly press closely together throughout their entire breadth, and as this takes place the margins gradually become a little everted, so that the spikes, which at first intercrossed, at last project in two parallel rows. So firmly do they become pressed together that, if any large insect has been caught, a corresponding projection is clearly visible on the outside of the leaf. When the two lobes are thus completely closed, they resist being opened, as by a thin wedge driven with astonishing force between them, and are generally ruptured rather than yield. If not ruptured, they close again with quite a loud flap. The slow movement spoken of, excited by the absorption of diffused animal matter, suffices for its final purpose, whilst the movement brought on by the touching of one of the sensitive filaments is rapid, and thus indispensable for the capturing of insects. Leaves remain shut for a longer time over insects, especially if the latter are large, than over meat. In many instances where they have remained for a long period over insects naturally caught, they were more or less torpid when they reopened, and generally so much so during many succeeding days that no excitement of the filaments caused the least movement. Vigorous leaves will sometimes devour prey several times, but ordinarily twice, or, quite often, once is enough to render them unserviceable. What purpose the marginal spikes, which form so conspicuous a feature in the appearance of the plant, subserve was unknown until the genius of Darwin solved the mystery. It was he that showed that elongated spaces between the spikes, varying from one-fifteenth to one-tenth of an inch in breadth according to the size of the leaf, are left open for a short time before the edges of the lobes come into contact, consequent upon the intercrossing of the tips of the marginal spikes first, thus enabling an insect whose body is not thicker than these measurements to escape, when disturbed by the closing lobes and the increasing darkness, quite easily between the crossed spikes. Moderately sized insects, if they try to escape between the bars, will be pushed back into the horrid prison with the slowly closing walls, for the spikes continue to close more and more until the lobes are brought into contact. Very strong insects, however, manage to effect their release. It would manifestly be a great disadvantage to the plant to remain many days clasped over a minute insect, and as many additional days or weeks in recovering its sensibility, inasmuch as a very small insect would afford but little nourishment. Far better would it be for the plant to wait until a moderately large insect was captured, and to allow the little ones to escape, and this advantage is gained by the slow intercrossing of the marginal spikes, which, acting like the large meshes of a fishing-net, allow the small and worthless fry to pass through. Touching any one of the six filaments is sufficient to cause both lobes to close, these becoming at the instant incurved throughout their entire breadth. The stimulus must therefore radiate in all directions from any one filament, and it must also be transmitted with considerable rapidity across the leaf, for in all ordinary cases, as far as the eye can judge, both lobes close at the same time. Physiologists generally believe that in irritable plants the excitement is transmitted along, or in close connection with, the fibrovascular bundles. Those in Dionæa seem at first sight to favor this belief, for they run up the mid-rib in a great bundle, sending off small bundles almost at right angles on each side, which bifurcate occasionally as they stretch towards the margin, the marginal branches from adjoining branches uniting and entering the marginal spikes. Thus a continuous zigzag line of vessels runs round the whole circumference of the leaf, while in the mid-rib all the vessels are in close contiguity, so that all parts of the leaf seem to be brought into some degree of communication. The presence of vessels, however, is not necessary for the transmission of the motor impulse, for it is transmitted from the apices of the sensitive filaments, which are hardly one-tenth of an inch in length, into which no vessels are seen to enter. Slits made close to the bases of the filaments, parallel to the midrib, and thus directly across the course of the vessels, sometimes on the inner and sometimes on the outer sides of the filaments, do not interfere with the transmission of the motor impulse along the vessels, and conclusively show that there is no necessity for a direct line of communication from the filament, which is touched towards the mid-rib and opposite lobe, or towards the outer parts of the same lobe. With respect to the movement of the leaves, the wonderful discovery made by Dr. Burdon Sanderson, and published in 1874, offers an easy explanation. There is, says this distinguished authority, a normal electrical current in the blade and footstalk, which, when the leaves are irritated, is disturbed in the same manner as is the muscle of an animal when contraction takes place. 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. 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, |