A decrease in temperature always caused a considerable
retardation in the rate of revolution; but Dutrochet (tom. xvii. pp.
994, 996) has given such precise observations on this head with
respect to the common pea that I need say nothing more. When twining
plants are placed near a window in a room, the light in some cases
has a remarkable power (as was likewise observed by Dutrochet, p.
998, with the pea) on the revolving movement, but this differs in
degree with different plants; thus Ipomoea jucunda made a complete
circle in 5 hrs. 30 m.; the semicircle from the light taking 4 hrs.
80 m., and that towards the light only 1 hr. Lonicera brachypoda
revolved, in a reversed direction to the Ipomoea, in 8 hrs.; the
semicircle from the light taking 5 hrs. 23 m., and that to the light
only 2 hrs. 37 m. From the rate of revolution in all the plants
observed by me, being nearly the same during the night and the day, I
infer that the action of the light is confined to retarding one
semicircle and accelerating the other, so as not to modify greatly
the rate of the whole revolution. This action of the light is
remarkable, when we reflect how little the leaves are developed on
the young and thin revolving internodes.
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