It has already been stated that the
cotyledons of Phalaris and Avena, the plumules of Asparagus and the
hypocotyls of Brassica, were likewise able to displace the same kind of
sand, either whilst simply circumnutating or whilst bending towards a
lateral light.
As long as an arched hypocotyl or epicotyl remains buried beneath the
ground, the two legs cannot separate from one another, except to a slight
extent from the yielding of the soil; but as soon as the arch rises above
the ground, or at an earlier period if
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the pressure of the surrounding earth be artificially removed, the arch
immediately begins to straighten itself. This no doubt is due to growth
along the whole inner surface of both legs of the arch; such growth being
checked or prevented, as long as the two legs of the arch are firmly
pressed together. When the earth is removed all round an arch and the two
legs are tied together at their bases, the growth on the under side of the
crown causes it after a time to become much flatter and broader than
naturally occurs. The straightening process consists of a modified form of
circumnutation, for the lines described during this process (as with the
hypocotyl of Brassica, and the epicotyls of Vicia and Corylus) were often
plainly zigzag and sometimes looped.
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