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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 use 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 case. 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 remarkable 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. viii. p. 465.

appear to oppose some resistance to their transit, and may be stretched in consequence. The utricles, floating freely in the water, become the seat of endosmotic and chemical actions, especially when the surrounding water has a temperature of from 16° to 15° (C). The utricles enclose at the beginning a mucilaginous liquid, in which a gas-bubble soon appears, and increases in size: this is the oxygen evolved under the influence of light and heat. The plant disengages itself from the soil, and mounts towards the surface; the secretion of gas becomes more abundant, and the flower-stems are raised above the water. The oxygen secreted in the air-vesicles seems to exercise a chemical influence in changing the colour of the cell-walls, which become rose, lilac, and blue. The colouration of the envelope affects the internal processes of the cell. know that in organs not coloured green, like the petals of a flower, there is no evolution of oxygen, but an absorption of oxygen and an evolution of carbonic acid. This gas does not leave the utricle, but is probably assimilated there; the utricle becomes again filled with mucilage, and water which it absorbs augments in weight and causes it to descend. Thus the utricles have a respiratory as well as a hydrostatic function.

GRUITHUISEN'S CITY IN THE MOON.-JUPITER'S SATELLITES.— OCCULTATIONS.

BY THE REV. T. W. WEBB, A.M., F.R.A.S.

WE will now, in continuation of our subject, direct our attention to the region lying S. and S.W. of the great clefts recently described.-Dionysius (25), a small crater (14 miles across, Lohrm.), lying on the shore of the M. Tranquillitatis, and having perhaps 3800 f. of depth, or more than enough to hide the peak of Snowdon, is chiefly remarkable on account of its brilliancy, amounting to 7° for its interior, and 9° for its wall. Yet it is not to be seen in the earth-shine, probably, as B. and M. remark, on account of its small dimensions. It would be an interesting and not unpromising investigation, to ascertain whether this is the true cause, and attended with no great trouble, as it would not be difficult to select other small craters which are perceptible on the dark side, and whose magnitude would be comparable with that of Dionysius. I have never made the attempt, but trust some of my readers may have at once sufficient instrumental power and leisure to take it in hand. We have seen that changes of colour, of

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which little explanation can be given, frequently come on with the advancing lunar day; and there is no antecedent improbability in the idea that similar periodical alternations may be in progress during the lunar night. Dionysius has been selected by Birt as a standard of magnitude among craters of a similar character.-Ariadæus, and Ariadæus a, are a pair of craters at the W. end of the great cleft which bears that name.* Silberschlag, a crater 9 miles across, and of 8 of brightness, appears, with those already mentioned, in the diagram of this region in our August number. There also we shall find Agrippa (26), a fine ring, somewhat elliptical, 27 miles broad, carrying a peak S., and interrupted by a little crater N.; in the steepest parts its interior slope amounts to 60°; and it ranges on the W. 6900 f., and 1000 f. more on the E., above the interior. It has several terraces, and a central hill.Godin (27) has a narrow but very steep ring of 8° light, equally deep with Agrippa (Schr. makes it deeper), and of a somewhat quadrilateral form: its breadth is 23 miles. It is connected with Agrippa by ridges running in an oblique S.S. W. direction, a peculiarity of which other instances might be given. Several very defined craters lie in the neighbourhood. E. of Godin we find a small but very brilliant crater, Rhæticus b, which attains 9° of luminosity. Rhaeticus itself, which will be found in our recent diagram, is an irregular ring, chiefly distinguished as being bisected by the lunar equator, and as being one of the few spots to which both Sun and Earth may be vertical; all these being, of course, comprised in an elliptical area, whose centre is that of the Moon, and its boundary the extreme amount of libration (which is greater in longitude than latitude), as referred to the centre of the Earth (not to the observer's position, in which it may be increased by parallax). Very strange, certainly, would be the aspect of the sky to any one of ourselves, if we could conceive ourselves transported there; the Sun describing a slow but cloudless course from rising to setting, through the vertical region of the sky, and often through the zenith itself; and the Earth oscillating around that point for a short distance successively in every direction-an enormous globe, waxing and waning with all the features of the Moon, and turning every part in comparatively rapid rotation to the eye of the spectator. The Rhaticus of

*It should have been stated in our last number that the minute prolongation of this cleft, noticed in p. 97 as having been discovered by Gruithuisen, has been seen on several occasions by Messrs. Birt and Freeman.-I may be permitted also to take this opportunity of rectifying two former mistakes, which have been obligingly pointed out to me by Messrs. Knott and Proctor. The first occurs in INT. OBS. vii. 134, where the R.A. of the Great Star of 1572 has been given at 4h. 19m. 577s, instead of 4° 19′ 57 7′′ (= Oh. 17m. 19.8s.): the difference also from Hind should have been, not 3m. 10s.. but 3' 10".-The other is in INT. OBS. x. 148, where, instead of density of the ring of Saturn, it should have been mase.

Riccioli, it should be observed, lies further E., but its name was transferred to this spot by B. and M., in despair of its identification. This, no doubt, may have been impracticable, so far as it depended upon the relief of the surface, and an apology is thus obtained in this case for a change, generally speaking to be avoided; but the original Rhaeticus, which has only recently been recovered by Knott, is a grey opening among the luminous rays issuing from the S.W. side of Copernicus (30), and consequently only to be recognized, where B. and M., no doubt, did not think of looking for it, under high illumination. Three contiguous dark spaces of a circular form were figured here by Hevel, and denominated Lacus Herculei from Riccioli they received the separate names of Rhaeticus, Stadius, and Dominicus Maria, occupying respectively the E., S.W., and N.W. angles of the triangular area in which they are grouped. They are not difficult objects in the Full Moon, Rhæticus, in particular, which is the darkest, and is divided centrally by a more luminous ray; yet still they are now, especially the other two, so unimportant in character, being merely duller patches in a labyrinth of bright streaks, and so much less conspicuous than very many other anonymous objects, that a suspicion may reasonably arise, whether they were not, at the date of those early observations, of a more decidedly contrasted grey hue than at present. Should there be anything in this, it would of course involve a consequence of some interest-that the streak-system of Copernicus is, in this place at least, on the increase; and when we bear in mind the very small amount of our actual knowledge as to the local colouring of the Moon, we shall feel that attention may be suitably directed to this spot, where identification and comparison are proportionally easy. Instances may be given in which variations of brightness in high illumination are probable -Linné, and a bright spot in Werner, may be specified; and it is time that observers should take this curious point in hand. On the earth, analogous changes, no doubt, may be perceived, but they would result from that cultivation of which we have no suspicion in our satellite.

It is in this ancient Rhæticus that we are to look for one of the curious "rampart-works" discovered by Gruithuisen. His sketch, in the "Astronomisches Jahrbuch" for 1828, represents a comparatively regular white figure in a longish grey area, consisting of one vertical stripe, bent to the left at the top, where it terminates in a small hill casting a shadow; ending in something like a little crater, with internal and external shade, at the bottom; and crossed at an angle of about 60° by four similar bright streaks: the figure might have been worth copying, but that he complains, in the next volume, of its

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