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Show 106 HUPTURE OF 'fHE SEED-COATS. CuAr. II. allowed to germinate in damp air, and now a thin flat disc was developed all round. the base of the hypocotyl and grew to an extraordinary breadth, like the frill described under Mimosa, but somewhat broader. Flahault says that with Mirabilis, a member of the same family with Abronia, a heel or collar is developed all round the base of the hypocotyl, but more on one side than on the other; and that it frees the cotyledons from their seed-coats. We observed only old seeds, and these were ruptured by the absorption of moisture, independently of any aid from the heel and before the protrusion of the radicle; but it does not follow from our experience that fresh and tough fruits would behave in a like manner. In concluding this section of the present chapter it may be convenient to summarise, under the form of an illustration, the usual movements of the hypocotyls and epicotyls of seedlings, whilst breaking through the ground and immediately afterwards. vV e may suppose a man to be thrown down on his hands and knees, and at the same time to one side, by a load of hay falling on him. He would :first endeavour to get his arched back upright, wriggling at the same time in all directions to free himself a little from the surrounding pressure; and this may represent the combined effects of apogeotropism and circumnutation, when a seed is so buried that the arched hypocotyl or epicotyl protrudes at first in a horizontal or inclined plane. The man, still wriggling, would then raise his arched back as high as he could; and this may represent the growth and continued circumnutation of an arched hypocotyl or epicotyl, before it has reached the surface of the ground. As soon as the man felt himself at all fr~e, he would raise the upper part of his body, whilst st1ll on CHAP. II. CIRCUMNUTATION OF HYPOCOTYLS, E'fC. 107 his knees and still wriggling; and this may represent the bowing backwards of the basn.l log of the arch, which in most cases aids in the withdrawal of the cotyledons from the buried and rupturou seed-coats, and the subsequent straightening of tho whole hypocotyl or epicotyl-circumnutation still continuing. O£rcumnutation of Hypocotyls and Epicotyls, when erect.-The hypocotyls, epicotyls, and first shoots of the many seedlings observed by us, after they had become straight and erect, circumnutated continuously. The diversified figures described by them, often during two successive days, have been shown in the woodcuts in the last chapter. It should be recollected that the dots were joined by straight lines, so that the figures are angular; but if the observations had been made e~ery few minutes the lines would have been more or less curvilinear, and irregular ellipses or ovals, or perhaps occasionally circles, would have been formed.. T,he direction of the longer axes of the ellipses made during the same day or on successive days generally changed completely, so as to stand at right angles to one another. The number of irregular ellipses or circles made within a given time differs much with different species. Thus with Brassica oleracea, Oerinihe rna;'or, and Oucurbita ovifera about four such figures were completed in 12 h.; whereas with Solanum pali'nacanthum and Opuntia basilaris, scar eel y more than one. 'fhe figures likewise differ greatly in size; thus they were very small and in some degree doubtful in Stapelia, and large in Brassica, &c. The ellipses described by Lathyrus nissolia and. Brassica wore narrow, whilst those made by the Oak were broad. 'l1he figures are often complicated by small loops and zigzag lines. As most seedling plants before the development of true leaves n.re of low, sometimes very low stature, |
