SPONTANEOUS MOVEMENTS IN PLANTS. 
373 
suckle, or major convolvulus, always twine round the stem or 
other object which supports them in one direction, that is, 
always either from right to left or from left to right ; but few 
probably have reflected, and fewer still attempted to observe, by 
what process the end of the growing shoot contrives to change 
its position from one side to the other of the stem. If the ex- 
tremity of a living stem, say of convolvulus, growing perfectly 
free and in a normal position, is observed, it is seen to hang 
over from its support in a horizontal direction ; and this hori- 
zontal portion is found, if observed at intervals of some hours, 
to point in different directions. The end of the growing shoot 
has, in fact, the property of revolving in a large circle round the 
support, always with the same species in the same direction, 
either with the sun or opposed to the sun. The rate of revo- 
lution varies with different plants, and with the same plant at 
different periods of its growth ; it is much quicker in warmer 
than in cooler weather. With the hop Darwin found it to 
vary from two and-a-half hours to nine hours. The object of 
the climbing power of plants is no doubt to reach the light and 
to expose a large surface of leaves to its action and to that of 
the free air ; but the mode by which this power of motion is 
gained is by no means clear. The late eminent physiologist 
Mohl supposed that it was caused by a dull kind of irritability 
in the stem, which caused it to bend towards the support when 
in contact with it. Mr. Darwin has, however, carefully tested 
this theory experimentally, and always with negative results. 
He rubbed many shoots much harder than was necessary to ex- 
cite movement in any tendril or in any foot-stalk of a leaf- 
climber, but without result. This view seems also entirely 
negatived by the fact that not only do the stems of climbing- 
plants revolve when they are not in contact with any support, 
but even more freely under such circumstances than when 
climbing. When a climbing plant first springs from the ground, 
the extremity of the shoot performs slow gyrations in the air, 
as if, as Darwin expresses it, it were searching for a support. I 
do not here discuss the question whether this habit may be the 
result of a tendency transmitted and enhanced through thou- 
sands of generations ; the movement itself is, in the indi- 
vidual plant, entirely 66 spontaneous ” in every sense of the term ; 
that is, is not the necessary result of known physical laws acting 
upon the individual. Darwin’s paper “ On the Movements 
and Habits of Climbing Plants ” published in the Journal of the 
Linnean Society, contains a number of the most interesting ob- 
servations on this class of plants ; and the language employed 
is everywhere suggestive of some hidden sentient controlling- 
power in the plant itself. 
The same purpose as that served by a climbing stem is 
