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But the whole subject of the spiral contraction of tendrils will be discussed after all the tendril-bearing species have been described. Bignonia littoralis. The young internodes revolve in large ellipses. An internode bearing immature tendrils made two revolutions, each in 3 hrs. 50 m.; but when grown older with the tendrils mature, it made two ellipses, each at the rate of 2 hrs. 44 m.
On two occasions the terminal branches became slightly curved in 10 m. after they had touched a stick; and in 30 m. the tips were curled quite round it. The basal part is less sensitive. The tendrils revolved in an apparently capricious manner, sometimes very slightly or not at all; at other times they described large regular ellipses.
The way in which the planetary orbits are tilted is slight in comparison with that of the orbits of comets, for these are at all sorts of angles some turned almost sideways, and others slanting, and all of them are ellipses long drawn out and much more irregular than the planetary orbits; but erratic as they are, in every case a line drawn through the sun and extended both ways would touch each side of the orbits.
The allegorical figures, stretched on segments of ellipses beneath the pedestals of the two Dukes, indicate phases of darkness and of light, of death and life. They are two women and two men; tradition names them Night and Day, Twilight and Dawning.
"Nor uglier follow the night-hag, when, call'd In secret, riding through the air she comes, Lured by the smell of infant blood, to dance With Lapland witches, while the laboring moon Ellipses at their charms." Paradise Lost, II. 662. Again, in Comus:
What model has the Megachile when cutting her neat ellipses out of the delicate material for her wallets, the robinia-leaves? What mental pattern guides her scissors? What system of measurement tells her the dimensions?
We say still that the orbits of the planets are ellipses, because, for all ordinary purposes, that is a sufficiently near approximation to the truth; but, as a matter of fact, the centre of gravity of a planet describes neither an ellipse or any other simple curve, but an immensely complicated undulating line.
The force of gravitation, even admitting that to be, as it were, a condition of the creation of matter, would have made those bodies revolve in ellipses of any degree of eccentricity just as well, provided the angle and the force of projection had been varied. Then, why was this form rather, than any other chosen? No one knew; yet no one doubted that there was ample reason for it.
The apex of the tendril describes elongated ellipses, sometimes narrow and sometimes broad, with their longer axes inclined in slightly different directions. The plant can ascend a thin upright stick by the aid of its tendrils; but the stem is too stiff for it to twine spirally round it, even when not interfered with by the tendrils, these having been successively pinched off at an early age.
The great peculiarity of cometic paths, as compared with the planetary ones, is, that they consist of ellipses of very much more eccentric proportions; and that, therefore, the bodies moving in them, go alternately to much greater and less distances from the sun than the planets do. It must not be imagined, however, that all comets revolve about the sun even in the most lengthened ellipses.
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