836 
SPONTANEOUS MOVEMENTS IN PLANTS. 
impossible, except as room is made by a new growth. It is 
in this w ? ay that we have a rational and adequate explanation 
of the selective power of the plant.” The “rational and 
adequate explanation” seems to me, on the contrary, to be 
merely a restatement of this selective power of the tissues in 
other terms. Because the tissues want the silica, is no 
explanation of how they get it. 
The curious and interesting movements of climbing plants 
have been investigated by Palm, Mohl, and Asa Gray, and 
form the subject of one of the most charming of Mr. Darwin’s 
works. It is well known that climbing plants, such as the 
hop, honeysuckle, 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 con¬ 
trives to change its position from one side to the other of the 
stem. If the extremity 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 horizontal 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 revolution 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 excite 
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 
