MOTION AND SENSITIVENESS IN CLIMBING PLANTS. 57 
Light has a remarkable power in hastening the revolutions. 
Thus : — 
Ipomcea jucunda performed its first circle in 5h. 30m. ; the 
semicircle from light in 4h. 30m., and to light in lh. 30m. ; 
the difference being 3h. 30m. It must be observed, however, 
that the rate of revolution in all plants was nearly uniform 
during night as well as day ; hence Mr. Darwin infers the 
action of the light to be confined to retarding one semicircle 
and accelerating the other, so that the whole rate is not 
greatly modified. 
Heat likewise affects the rapidity of revolution, by increasing 
it; thus, e. g. } of Loasa aurantiaca , one plant which moved 
against the sun, completed its first circle in 2h. 37 m. (June 
30). Another, which followed the sun, completed its circle 
in lh. 51m. (July 11), and its 4th circle in lh. 48m., that 
being a very hot day ; whereas its 5th circle, on the cool 
morning of July 12th, was finished in 2h. 35m. 
Mr. Darwin describes a peculiar instance of a natural reversal 
of movement in Hibbertia dentata. He found that, although 
its long flexible shoots were evidently well fitted for twining, 
yet they would make a whole, or half, or quarter circle in one 
direction, and then in the opposite one. He could not at 
first discover for what purpose was this adaptation, until after 
offering the plant various arrangements of sticks and twigs, 
&c., he surrounded it with several thin upright sticks; and 
“ now the Hibbertia had got what it liked, for it twined up the 
parallel sticks, sometimes winding round one and sometimes 
round several. . . . Though the revolving movement was 
sometimes in one direction and sometimes in another, the 
twining was invariably from left to right. ... It would 
appear that this Hibbertm is adapted to ascend by twining, 
and to ramble laterally over the thick Australian scrub.” 
Mr. Darwin concludes the first part with recording several 
miscellaneous and curious cases. For example, he observes 
that “ the main stem of Tamus Elephantipes does not twine : 
only the branches.” In a species of Asparagus , the leading 
shoot, and not the branches, twine. Gombretum argenteum 
produces two kinds of shoots, several of the first formed 
showed no tendency to climb until (C one appeared from the 
lower part of one of its main branches, five or six feet in length, 
differing greatly in appearance from its leaves being little de- 
veloped. It revolved vigorously, and twined.” Lastly, a still 
more remarkable instance occurs in Ipomcea argyrceoides , which, 
in S. Africa, almost always grows erect and compact, from twelve 
to eighteen inches ; whereas seedlings raised at Dublin twined 
up sticks eight feet high ! “ These facts,” says Mr. Darwin, 
“ are highly remarkable, for there can hardly be a doubt that 
