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All these observations are liable to several
causes of error, but we believe, from what will hereafter be shown with
respect to the movements of the radicles of other plants, that they may be
largely trusted.
Hypocotyl.--The hypocotyl protrudes through the seed-coats as a rectangular
projection, which grows rapidly into an arch like the letter U turned
upside down; the cotyledons being still enclosed within the seed. In
whatever position the seed may be embedded in the earth or otherwise fixed,
both legs of the arch bend upwards through apogeotropism, and thus rise
vertically above the ground. As soon as this has taken place, or even
earlier, the inner or concave surface of the arch grows more quickly than
the upper or convex surface; and this tends to separate the two legs and
aids in drawing the cotyledons out of the buried seed-coats. By the growth
of the whole arch the cotyledons are ultimately dragged from beneath the
ground, even from a considerable depth; and now the hypocotyl quickly
straightens itself by the increased growth of the concave side.


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