Therefore their weight, which is considerable
when almost fully developed, probably came into play in originally
determining the downward movement. The periodicity of this movement is much
influenced by the degree of light to which the seedlings have been exposed
during the day; for three kept in an obscure place began to sink about
noon, instead of late in the evening; and those of another seedling were
almost paralysed by having been similarly kept during two whole days. The
cotyledons of several other species of Ipomoea likewise sink downwards late
in the evening.
Cerinthe major (Boragineae).--The circumnutation of the hypocotyl of a
young seedling with the cotyledons hardly
Fig. 36. Cerinthe major: circumnutation of hypocotyl, with filament fixed
across its summit, illuminated from above, traced on horizontal glass, from
9.26 A.M. to 9.53 P.M. on Oct. 25th. Movement of the bead magnified 30
times, here reduced to one-third of original scale.
expanded, is shown in the annexed figure (Fig. 36), which apparently
represents four or five irregular ellipses, described in the course of a
little over 12 hours.
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