638 THE POPULAR SCIENCE MONTHLY. 



One may find a further illustration of the action of twining in the 

 swinging-rope model. It is a peculiarity of twining plants that they 

 can only ascend moderately thin supports. A scarlet-runner can climb 

 up a bit of string, or a thin stick, an inch or two in diameter, but when 

 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 be 

 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 rod 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 toward 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 backward and forward from right to left. But now 

 imagine that the growth of the rod on the sides nearest to and farthest 

 from us enters 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 toward 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 continuation 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 com- 

 pletely solved problem, and in a certain sense it is so. I think that the 

 explanation which I have given will remain as the fundamental state- 

 ment 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 



