Botany; Geotropism; Plants -- Irritability and movements
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. We 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 free, he would raise the upper part of his body,
whilst still on
his knees and still wriggling; and this may represent the bowing
backwards of the basal leg of the arch, which in most cases aids in the
withdrawal of the cotyledons from the buried and ruptured seed-coats,
and the subsequent straightening of the whole hypocotyl or
epicotyl—circumnutation still continuing.
Circumnutation 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 every few
minutes the lines would have been more or less curvilinear, and
irregular ellipses or ovals, or perhaps occasionally circles, would
have been formed. The 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, Cerinthe major, and
Cucurbita ovifera about four such figures were completed in 12 h.;
whereas with Solanum palinacanthum and Opuntia basilaris, scarcely more
than one. The figures likewise differ greatly in size; thus they were
very small and in some degree doubtful in Stapelia, and large in
Brassica, etc. The ellipses described by Lathyrus nissolia and Brassica
were narrow, whilst those made by the Oak were broad. The figures are
often complicated by small loops and zigzag lines.
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