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two in diameter, but wben it comes to anything thicker than 
this, it fails to do so. Just as when the swinging-rope strikes 
against a large trunk of a tree, it would he unable to take a 
turn round it, and would fall to the ground instead of gripping 
it with a single turn, as it does a thin stick. The difficulty 
which a climbing plant has in ascending a thick stick will be 
better understood by going back to the original swinging round 
movement which the plant makes in search of a stick, and 
considering how the movement is produced. 
As plants have no muscles, all their movements are produced 
by unequal growth ; that is, by one-half of an organ growing 
in length quicker than the' opposite half. Now the difference 
between the growth of a twining plant which bends over to 
one side, and an ordinary plant which grows straight up in the 
air, lies in this, that in the upright shoot the growth is nearly 
equal on all sides at once, whereas the twining plant is always 
growing much quicker on one side than the other. 
It may be shown by means of a simple model, how unequal 
growth can be converted into revolving movement. The stem 
of a young hop is represented by a flexible rod, of which the 
lower end is fixed, the upper one being free to move. At first 
the ro:l is supposed to be growing vertically upward, but when 
it begins to twine, one side begins to grow quicker than any 
of the others ; suppose the right side to do so, the result will 
be that the rod will bend over towards the left side. Now let 
the region of quickest growth change, and let the left side 
begin to grow quicker than all the others, then the rod will 
be forced to bend back over to the other side. Thus, by an 
alteration of growth, the rod will bend backwards and for- 
wards from right to left. But now imagine that the growth 
of the rod on the sides nearest to and furthest from us enter 
into the combination, and that after the right side has been 
growing quickest for a time, the far side takes it up, then the 
rod will not bend straight back towards the right, as it did 
before, but will bend to the near side. Now the old movement 
caused by the left side growing quickest, will come in again, to 
be followed by the near side growing quickest. Thus by a 
regular succession of growth on all the sides, one after another, 
the swinging-round movement is produced, and by a continua- 
tion of this action, as I have explained, the twining movement 
is produced. 
I have spoken as if the question of how plants twine were a 
completely solved problem, and in a certain sense it is so. I 
think that the explanation which I have given will remain as 
the fundamental statement of the case. But there is still much 
to be made out. We do not in the least know why every 
single hop-plant in a field twines like a left-handed screw, 
