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are often barren, while the later ones, which have no corolla, are fertile. Von Mohl has seen the pollen escape from the anthers on to the stigmas and give out abundance of pollen-tubes. Monnier says that the ordinary spring flowers of Viola. M11?!» and odwata never produce seed ; but this statement is disputed by others. The “cleistogamous” flowers of the violet appear long after those that are so familiar in the spring, and may be found in abundance about July and August, very small, but still not difficult to make out. Fig. 7, a, shows the appearance of one of these unopened flowers, the only visible part being the five sepals, of nearly the same form, though much smaller, than the ordinary ones. On opening the appearance is presented shown in b; no trace of petals ; there are five stamens, two of them represented in the figure, with long filaments and very small anthers, ofi'ering scarcely any resemblance to those of the open flowers, which have very large anthers and no filaments. The pollen, again very small in quantity, is contained in two almost transparent bags at the base of the anther, shown at n in c, and is discharged directly on to the stigma. The pistil, d, consists of a conical ovary, and a very large stigma curved completely over in a semi-circle so as to bring the papillose receptive surface (st) into a horizontal position in which it will most readily receive the pollen. A most instructive contrast is afforded between the arrangements of the reproductive organs in these two kinds of flowers on the same plant. In the showy spring flowers the stigma projects horizontally in the form of a beak above and quite clear of the stamens, the arrangement of which is such that it is scarcely possible for any of the pollen to reach the stigma without the intervention of insect agency. In these closed summer flowers it will be seen that the arrangements have evidently an exactly opposite purpose. They produce abundance of seed. Another section of the genus Viola, of which the wild pansy ( Viola tricolor) may be taken as the type, produce no cleistogamous flowers; and the contrivances for fertilisation are, as has already been mentioned, quite different from those in the true violets.

In two Indian species of Campanula, the closed flowers are described by Professor Oliver as being altogether different in shape to the conspicuous ones. They are covered by a completely closed membrane, the rudiment of the corolla; the stamens are extended horizontally, and the anthers are quite connate, and together adnate to the stigma. As the flowers have only at present been observed in dried herbarium specimens, the mode in which the pollen-grains reach the stigma is still uncertain. In Juncus bufonius it is said that the pollentubes are emitted while still within the anther, the wall of which they pierce. In the wood-sorrel, Owalts acetosella, the closed flowers, which appear towards the end of the summer, resemble much more closely the well-known spring flowers, which are in this case certainly fertile.

In accordance with the ordinary practice of economy by nature, the amount of pollen in the “ cleistogamous ” is generally very much less than in the open flowers, since it has very little chance of being wasted. In the small flowers of Malpighiaceae, J ussieu states that there are only a very few grains of pollen ; in those of the wood-sorrel, where twenty to thirty ovules have to be fertilised, Von Mohl gives the quantity as from one to two dozen grains in each anther-cell; in Impatiens it is considerably larger, while in Viola the number of grains is very small.

More detailed examination of these closed flowers in different plants will doubtless yield interesting and important results.

EXPLANATION OF PLATE CII.

FIG. 1. (a-d) Different stages in the development of the pistil and stamens of Malva sylvestn'a, (n) nectar-glands; (e) mature pistil and stamens of Malva rotundifolia.

,, 2. Dianthus deltoides; (a) open flower, the first row of five stamens developed; (6, c) pistil, with the two stigmas closely rolled together, (n) nectar glands; (d) pistil at a later stage, with the stigmas unrolled and receptive.

,, 3. Foot of a butterfly with pollen-masses of Asclepiaa curassam'ca attached to it.

,, 4. (a) Right hind-leg of Prosqn's variegata; (6) right hind—leg of

Bomlnw Scrimsh'iranus.

5. End of the proboscis of Enhtalzls tenax; (a) closed, (b) open, to

show the cross—bars (r) with which it is furnished.

,, 6. Drosera rotundifolia; (a) unopened flower, (b) pistil and one stamen from the same. v

,, 7. Oleistogamous flower of Viola cam'na; (a) external view of flower; (b) the same opened, showing pistil and two stamens; (c) anther, with the pollen-bags (n); (d) pistil; (st) the receptive stigmatic surface.

(Figs. 1-5 after H. Miiller.)

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HE planet Jupiter has passed during the last year through a singular process Of change. The planet has not, indeed, assumed a new appearance, but has gradually resumed its normal aspect after three or four years, during which the mid zone of Jupiter has been aglow with a peculiar ruddy light. The zone is now of a creamy-white colour, its ordinary hue. We have, in fact, reached the close of a period of disturbance, and have received a definite answer to questions which had arisen as to the reality of the change described by observers. Many astronomers of repute were disposed to believe that the peculiarities recently observed were merely due to the instruments with which the planet has been Observed—not, indeed, to any fault in those instruments, but, in fact, to their good qualities in showing colour. A considerable number of the earlier accounts of Jupiter’s change of aspect came from Observers who used the comparatively modern form of telescope known as the silvered-glass reflectors, and it is well known that these instruments are particularly well suited for the study of colour-changes. Nevertheless, observations made with the ordinary refracting telescope were not wanting; and it had begun to be recognised that Jupiter really had altered remarkably in appearance, even before that gradual process Of change which, by restoring his usual aspect, enabled every telescopist to assure himself that there had been no illusion in the earlier observations.

I propose now to discuss certain considerations which appear to me to indicate the nature and probable meaning of the phenomena which have recently been ObseIVed in Jupiter. It seems to me that these phenomena are full of interest, whether considered in themselves or in connection with those circumstances on which I had been led to base the theory that Jupiter is a planet altogether unlike our earth in condition, and certainly unfit to be the abode of living creatures.

I would first direct special attention to the facts which have been ascertained respecting the atmosphere of Jupiter.

It does not appear to have been noticed, as a remarkable circumstance, that Jupiter should have an atmosphere recognisable from our distant station. Yet, in reality, this circumstance is not only most remarkable, but is positively inexplicable on any theory by which Jupiter is regarded as a world resembling our own. It is certain that, except by the effects produced when clouds form and dissipate, our terrestrial atmosphere could not be recognised at Jupiter’s distance with any telescopic power yet applied. But no one who has studied Jupiter with adequate means can for a moment fail to recognise the fact that the signs of an atmosphere indicate much more than the mere formation and dissipation of clouds. I speak here after a careful study of the planet during the late opposition, with a very fine reflecting telescope by Browning, very generously placed at my disposal by Lord Lindsay; and I feel satisfied that no one can study Jupiter for many hours (on a single night) without becoming convinced that the cloudmasses seen on his disc have a depth comparable with their length and breadth. Now the depth of terrestrial cloud-masses would at Jupiter’s distance be an absolutely evanescent quantity. The span of his disc represents about 84,000 miles, and his satellites, which look little more than points in ordinary telescopes, are all more than 2,000 miles in diameter. I am satisfied that anyone who has carefully studied the behaviour of Jupiter’s cloud-belts will find it difficult to believe that their depth is less than the twentieth part of the diameter of the least satellite. Conceive, however, what the depth of an atmosphere would be in which cloud-masses a hundred miles deep were floating! o

It may be asked, however, in what sense such an atmosphere would be inexplicable, or, at least, irreconcilable with the theory that Jupiter is a world like our earth. Such an atmosphere would be in strict proportion, it might be urged, to the giant bulk of the planet, and such relative agreement seems more natural than would be a perfect correspondence between the depth of the atmosphere on Jupiter and the depth of our earth’s atmosphere.

But it must not be forgotten that the atmosphere of Jupiter is attracted by the mass of the planet ; and some rather remarkable consequences follow when we pay attention to this consideration. Of course a great deal must be assumed in an inquiry of the sort. Since, however, we are discussing the question whether there can be any resemblance between Jupiter and our earth, we may safely (so far as our inquiry is concerned) proceed on the assumption that the atmosphere of Jupiter does not differ greatly in constitution from that of our earth. We may further assume that at the upper part of the cloud-layers we see, the atmospheric pressure is not inferior to that of our atmosphere at a height of seven miles above the sea-level, or one-fourth of the pressure at our sea-level. Combining these assumptions with the conclusion just mentioned, that the cloudlayers are at least 100 miles in depth, we are led to the following singular result as to the pressure of the Jovian atmosphere at the bottom of the cloud-layer :—The atmosphere of any planet doubles in pressure with descent through equal distances, these distances depending on the power of gravity at the planet’s surface. In the case of our earth, the pressure is doubled with descent through about 3% miles; but gravity on Jupiter is more than 2% times as great as gravity on our earth, and descent through 1% mile would double the pressure in the case of a Jovian atmosphere. Now 100 miles contain this distance (1% mile) more than seventy-one times; and we must therefore double the pressure at the upper part of the cloud-layer seventyone successive times to obtain the pressure at the lower part. Two doublings raise the pressure to that at our sea-level ; and the remaining sixty-nine doublings would result in a pressure exceeding that at our sea-level so many times that the number representing the proportion contains twenty-one figures.* I say would result in such a pressure, because in reality there are limits beyond which atmospheric pressure cannot be increased without changing the compressed air into the liquid form. What those limits are we do not know, for no pressure yet applied has changed common air, or either of its chief constituent gases, into the .liquid form, or even produced any trace of a tendency to assume that form. But it is easily shown that there must be a limit to the increase of pressure which air will sustain‘without liquefying. For the density of

“ The problem is like the well-lmown one relating to the price of a horse, where one farthing was to be paid for the first nail of 24 in the shoes, a halfpenny for the next, a penny for the third, two pence for the fourth, and so on. It may be interesting to some of my readers to learn, that if we want to know roughly the proportion in which the first number is increased by any given number of doublings, we have only to multiply the number of doublings by lsfiths, and add 1 to the integral part of the result, to give the number of digits in the number representing the required proportions. Thus multiplying 24 by f‘aths gives 7 (neglecting fractions) ; and therefore the number of farthings in the horse problem is represented by an array of 8 digits.

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