THE AIR-VESICLES OF BLADDERWORTS (UTRICULARIA). BY J. B. SCHNETZLER. [IN England we have, amongst our wild flowers, three Bladderworts-Utricularia vulgaris, or Bladderwort, growing in stagnant water; U. intermedia, a rare plant; and U. minor, or lesser Bladderwort; and many of our readers will be glad to repeat the observations contained in the following paper, which is translated from the " Archives des Sciences."] The genus Utricularia, or Bladderworts, comprises aquatic plants found in the stagnant water of ditches, marshes, etc. The leaves are submerged, and divided into fine threads, furnished with vesicles, or utricles (asci), to which De Candolle ascribes the following characters : These utricles are rounded, and furnished with a species of movable operculum, or lid. In the youth of the plant, they are full of mucus heavier than water, and the plant, weighed down by them, remains at the bottom. Towards the season of flowering, the leaves secrete a gas which enters the utricle, and drives out the mucus, opening the lid for its escape. The plant is thus supplied with a quantity of air-vessels, which elevate it gradually, and cause it to float on the surface. The process of flowering takes place in the free air; and when it is finished, the leaves again secrete mucus, which replaces the air in the utricles, weigh down the plant, and cause it to descend again to the bottom of the water, where it ripens its seed in the situation in which they should be sown.* In spite of the labours of Göppert,† Benjamin,‡ Schleiden, § Schacht, Reinsch, there is not yet a complete agreement of botanists as to the origin and morphological signification of these aerial vesicles. Before the publication of the works cited, they were usually regarded as a modification of the parenchyma of the leaves, which follows the numerous ramifications of the veins, under the form of a narrow band, and which by dilating from time to time produced the utricles. Schleiden, who studied the development of these little. organs, saw them appear at the angles of the division of the leaf under the form of little bodies like horns (cornets), supported on short pedicels. The lower side of the horn, and the *De Candolle, "Physiologie Vegetale," t. xi., p. 87. Ibid., 1848, p. 17. § "Grundzüge der Wissenschaft, Botanik," 388, iv. Auff. Beitrage zur Anatomie," etc. "Denkslinflen der K. Bair. Bot. Gesettschoft," 1859, B. iv. 153. lower margin of its opening, which itself scarcely increased in size, developed themselves much more than other parts; so that the complete utricle formed a small rounded body, laterally compressed, prolonging itself on the upper surface or on one side of the pedicel, and producing on the other side an opening in the form of a funnel, projected into the interior of the utricle, and having its exterior aperture closed by a fringe of hair attached to the upper margin. The interior surface of the funnel is adorned with differently shaped and elegant hairs, disposed in regular order. All the interior surface of the utricle is likewise covered with hairs composed of two cells, each of which is prolonged into two appendages of unequal length (Schleiden, loc. cit.). Benjamin explains the formation of these utricles by supposing an arrest of development in certain segments of the leaves; instead of elongating themselves, they increase in breadth; a constriction takes place, forming a narrow neck, and they appear as little globular bodies, attached by a short pedicel to the vein of the leaf. According to Benjamin, we can follow these phases of formation by examination of a single leaf from its base to its summit. The utricle, at first filled with protoplasm, becomes, by the rapid absorption of this fluid, a reservoir of air, and, stretching in all directions, gradually assumes its ultimate form, which somewhat resembles a stomach, the pedicel taking the place of the pylorus, and the opening of the caudiac orifice. The mouth of the utricle he represents as a valve opening inwards. . . . Schacht regards those organs of the Bladderworts, which most botanists consider leaves, as leaf-bearing branches, which, in their young stages, are rolled up, like the fronds of ferns; under this crook, their leaves are formed in succession, and in their axils small conical bodies appear, composed of little cells, like the beginning of a bud. These small bodies soon exhibit at their rounded extremities little cavities, produced by an arrest of the development of their cells, the margins of which grow, and the little cellular body, at first sessile, afterwards exhibits a prolongation at its base in the form of a pedicel. The lateral walls of the young utricle develop more and more, and the air cavity becomes bigger, the margins of the lateral walls incline towards each other, and fold inwards, while the original aperture closes. The original opening is, in fact, a valve, formed by a fold of the margin of the aperture; and the beard which, according to Schleiden, closes the opening, is found later on the external surface. . . . Thus Schacht considers the utricles as modifications of the ramifications of the axis, and not of the leaves. In April, 1867, I studied the formation of the utricles of the lesser Bladderwort (Utricularia minor), in the Marsh of Jogny, below Vevey. Schacht, who studied the formation of the utricles in the common Bladderwort (U. vulgaris), admits their formation in the axils of the leaves, and considers them analogous to buds. We see, indeed, between the ramifications of the leaves little bodies appear, composed of conical cells, with their free extremities slightly rounded. These little bodies, at first sessile, soon raise themselves on pedicels, the cells of which afterwards differentiate themselves into an external layer, corresponding with the layer of parenchyma, which follows the veins of the divided leaves, while the interior cells of the pedicel put themselves in communication with the cells that form the tissue of the veins, of which they at last appear to be a continuation. Whilst the pedicel thus becomes a prolongation of a leaf segment, the little globular body which it supports appears to us as a portion of the parenchyma of the same leaf. The walls of the little cellular body, whose extremity becomes hollowed out as a little cup, continue to grow while those of the hollow remain stationary. They at length unite, and close the cavity. In the utricles thus formed in the lesser Bladderwort, there may be seen, towards the so-called embouchure, certain prolongations, or feather-divided appendages, like the capillary segments of leaves, properly so called; so that a perfect utricle looks like an expansion of the leaf parenchyma, supported on a vein which prolongs and ramifies itself beyond the utricle. The end, at first open, afterwards closes by two unequal folds of the walls, and thus form a sort of funnel covered with hairs, at the bottom of which the folds show themselves as two dark bands, bearing linear hairs, while those at the mouth are usually capitulate. If the utricles, at the commencement of their formation, show themselves at the angles of the leaf segments, their position is by no means constant, when we examine them at a more advanced stage, in which the leaf itself is modified. The "globules with pedicels" of Benjamin are often found a good way from the angles of the segments; on the lesser Bladderwort they may even be seen at the extremities of the leaf divisions. We cannot therefore infer from their position any analogy with buds. From the foregoing remarks it will be understood that the globules with pedicels" of Benjamin, and the small hornshaped bodies of Schleiden, are only intermediate phases of the utricles. An anatomical examination of the perfect utricle confirms this view. The walls of the utricle are composed of two layers of angular cells, which have at first a clear green colour. In VOL. XII.-NO. III. Р the intercellular canals small conical cells are seen at an early period, which terminate inside and out by a little rounded cell. The interior cell forms, at a later stage, the base of the quadrifid hairs spoken of by Schacht. This author does not mention the exterior cells, which are always seen in great numbers, even on the young utricles of the lesser Bladderwort, under the form of small flattened globes, which, at a subseqnent period, are often split in two. These globules are also met with on other parts of the segmented leaves, when they appear like little mushrooms, with their stems buried in the cellular tissues. The external globule becomes filled in time with a brown substance. Schleiden* observed these flattened cells on the exterior of the utricles, but he does not mention those on the leaves; their presence on the leaf, properly socalled, appears to me an additional proof that the utricle is only a modification and expansion of the parenchyma. The quadrifid hairs which garnish the interior of the utricles bear some resemblance to the stellate hairs often found on the inner surface of the air-vessels of the water-lilies. The intercellular spaces of the leaves of the Bladderwort contain much gas, which makes them look black under the microscope; the black band thus occasioned is prolonged through a pedicel as far as the utricle. In plants exposed to light I have often observed a strong disengagement of oxygen gas, bubbles of which rose through the water for a considerable time, forming an almost continuous thread. These gas bubbles were disengaged at the angle of two leaf segments, not far from the utricle. Similar bubbles are also disengaged at the ends of the capillary segments of the leaves. As to the mushroom-shaped cells, of which the pileus, a little constricted in the middle, is often divided into two, they seem to me to occupy the place of stomata, and to act as glands. They exhibit, in fact, a great analogy with the glands often found at the base of viscid leaves of Pinguicula vulgaris, which terminate in a brown, rounded pileus, like that of a small mushroom, whilst the stems are colourless, like those of the Bladderworts. The mucilage which covers the surface of the leaves of Pinguicula correspond also with that which fills the cavity of the young utricles. We have already seen that the utricles exhibit at their commencement a very pale green colour, which, at a later period, becomes deeper. The Bladderworts taken from the Marshes of Jogny on the 18th of October, 1866, still exhibited a number of green utricles; but the greater part were dark violet or blue. In these coloured utricles the angular cells of the interior layer, which are usually hexagonal, contain a coloured liquid, Grundzüge," 4th Edition, 397. passing from rose-lilac to violet-blue, giving the cells the aspect of painted glass, surrounded by silver threads. The cells which closed the intercellular canals were either red or dark blue, and surrounding them were other cells of reddish tint. At the same period I found in the segments of the leaves, by the side of green cells containing grains of chlorophyll, cells filled with a pale red fluid. The cells of the external layer of the utricles contained chlorophyll grains, grouped along their walls, whilst the interior was colourless. The cells of mushroom form had their buttons always brown. The change of colour in the wall-cells of the utricles in which we see the green pass into rose, lilac, violet, and blus, depends evidently on a chemical action which has some relation to their contents and functions. It must be observed that the colouration of the interior cells is due to a liquid, whilst the granules of chlorophyll have disappeared, or did not exist. These granules appear to have been exposed to a dissolving action, and to an agency which has changed their colours. The red colour of cellular liquids is usually ascribed to a free acid, and the blue tint to the presence of an alkali. In the utricles of the lesser Bladderwort all the transitions may be seen, from bright red to dark blue. The cavity of the utricles contains at the beginning a mucilaginous liquid of a neutral reaction, and it is in this liquid that, at a later period, we see a little bubble of gas, which gradually increases as the liquid diminishes. We may easily satisfy ourselves of the presence of this liquid by changing the position of the utricle, when the gas bubble will be seen to reach the highest level by passing through a viscous matter, which opposes a certain resistance to its passage. In the month of June and July the vesicles are nearly filled with air. The plant then rises to the surface of the water, and the stalk which, in the lesser Bladderwort, bears from two to five pale yellow flowers, stands up in the air, and two unilocular anthers spread their pollen over the stigma of the pistil, out of contact with the water. The ascensional force thus developed is very considerable. Reinsch* estimates the mean contents of a utricle as 2.57 cubic millimetres, and its weight as 0.6 milligrammes, and the ascensional force of a single utricle will be equal to 1,964 milligrammes. There are about 597 utricles on a principal stem, giving an ascending or floatation power of 0-778 grammes for an entire plant. Reinsch reckons a total of 4:44 grammes. (Reckoning four branches it would be 3.112 grammes.) Now the weight of the head of flowers which rise above the water is 0.295 grammes; there is thus a considerable excess of power capable of maintaining all the flowers above the water # Mickroscop." |