This fact illustrates in a striking manner that their movements
are not governed by the actual amount, but by a change in the intensity or
degree of the light. A similar experiment was tried with two sets of
seedlings, both exposed to a dull light, but different in degree, and the
result was the same. The movements of the cotyledons of this Cassia are,
however, determined (as in many other cases) largely by habit or
inheritance, independently of light; for seedlings which had been
moderately illuminated during the day, were kept all night and on the
following morning in complete darkness; yet the cotyledons were partially
open in the morning and remained open in the dark for about 6 h. The
cotyledons in another pot, similarly treated on another occasion, were open
at 7 A.M. and remained open in the dark for 4 h. 30 m., after which time
they began to close. Yet these same seedlings, when brought in the middle
of the day from a moderately bright into only a moderately dull light
raised, as we have seen, their cotyledons high above the horizon.
Sensitiveness of Cotyledons to contact.
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