« НазадПродовжити »
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.
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-bluo, 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 blue, 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
at the period of fecundation. After this is completed the utricle gradually fills with liquid ; the specific gravity of the plant increases, and it descends slowly with its fruit, sinking below the level of the water, and the seeds fall from their capsule in the soil in which they are to germinate. We find amongst authors a difference of opinion as to the position of the Bladderworts in the water before flowering. Some regard them as attached to the soil by slight roots; others, like Reinsch, consider them to be floating plants. They are at first really attached to the soil at the bottom of the water; but the air vesicles which develop on their leaves gently drag them out of the mud, and in this action I see the true uso of the utricle, for the entire plant floats very well in the water, and rises to the surface.
I placed a tuft of Bladderwort while the vesicles were still green in a large vessel of water, and found this to be the
The water snails in the same vessel eat up all the vesicles, and the plant still floated.
Bladderworts are not the only plants in which movements are produced by disengagements of gas. In Hottonia, Aldrovanda and Trapa natans, we observe at the flowering season slow movements which displace the entire plant, while in other aquatic species, such as Nymphea, Vallisneria, Ranunculus aquatilis, etc., it is only certain parts which elongate themselves. In the Bladderwort and Aldrovanda it is the air vesicles which diminish the specific gravity, uproot it from the soil, and cause it to ascend. In the Hottonia, air cells are found amongst the leaflets, and in the petioles of Trapa natans air cavities are formed before inflorescence.
Sometimes a plant cannot completely detach itself from the soil, and the grains of pollen are then preserved from contact with the water by another method, and one conspicuous instance may be cited of an evolution of gas, which, instead of moving the plant, plays a more direct part in the process of fecundation. In the Lake of Escoubous, at the top of the High Pyrenees, 2,052 metres above the sea-shore, a remarkablo variety of Ranunculus aquatilis grows, and form extensive beds, anchored to the bottom of the water by rootlets, which push their way among a thick carpet of dark green tremelloid ulva. In this situation, contrary to the laws which determine aquatic plants to seek the free air to accomplish their inflorescence and reproduction, it remains constantly submerged, far from the banks, where the sharpness of the frosts might destroy it, and far also from great depths, where it would not find light enough for its growth. It spreads out its finely divided leaves, and its white corollas, gilt at the bottom, and the processes of fecundation and reproduction take place without moving to the surface. An air bubble, produced by a vegetative process, is detained amongst the petals, and in this bubble the anthers deposit their pollen.*
The evolution of gas in close cavities, which we see in a certain number of aquatic plants, before the opening of their flowers, is evidently connected with what is called vegetable respiration. During this process, the plant not only takes carbonic acid from the air or the water; it absorbs oxygen at all parts, which combines with certain vegetable matter, and forms carbonic acid. The chemical action of solar light excites the decomposition of the carbonic acid, which is absorbed, as well as of that which the plant forms, the carbon being combined with the elements of water and nitrogenous bodies, while the oxygen is discharged. Stomata appears to play an important part in respiration, although, according to the researches of Duchartre, there is no fixed relation between the number and size of the stomata, and the quantity of gas which the plants disengage under solar influence.
In certain trees of a dry and coriaceous tissue, there is an inverse relation between the number of stomata and the feebleness of the gaseous evolution; but that which proves that the
gas evolved by the plant does not come from the stomata only, is that we see it disengaged from the cells of the epidermis of the upper surface of leaves of plants which have no stomata in that position, when we plunge them under water. We have noticed a similar evolution of gases from the submerged leaves of Bladderworts. In aquatic plants which are entirely submerged, the leaves have no stomata, and absorption and .exhalation take place from the whole surface of the epiblema. The experiments of MM. Cloez and Gratiolet show that the decomposition of carbonic acid by the green parts of submerged plants only takes place under the influence of light. In darkness, contrary to what takes place in aerial plants, no carbonic acid is produced. A certain temperature is also necessary for the process. It does not begin below 15° (C), when the temperature is increasing, and cannot continue below 10°, when it is decreasing. The gas evolved by the plant contains a little nitrogen besides the oxygen.
If we proceed to apply the preceding observations to the leaves of the Bladderworts, we find them in water which is usually very rich in carbonic acid, which is absorbed by the leaves; and, under the influence of light, oxygen, and a little nitrogen are disengaged. These gases are also found in the aeriferous canals which traverse the leaf segments, and they escape from different points as small bubbles.
We have seen these bubbles escape through the walls of the utricles, which
* Guérin “ Dict. d'Hist. Nat.” t. vii. p. 465.