The flexibility of the tendrils is of service in allowing them to be blown about by a breath of wind, and they can thus be made to seize hold of objects which are out of reach of the ordinary revolving movements. Many tendrils can only seize a stick by curling round it, and this even in the most sensitive tendril must take a minute or two; but with Cobœa the sharp hooks catch hold of little irregularities on the bark the moment the tendril comes into contact with it, and afterward the tendril can curl round and make the attachment permanent. The importance of this power of temporary attachment is shown by placing a glass rod near a cobæa-plant. Under these conditions the tendrils always fail to get hold of the glass, on which its grapple-like hooks can not seize.
The movement of the little hook-bearing branches is very remarkable in this species. If a tendril catches an object with one or two hooks, it is not contented, but tries to attach the rest of them in the same way. Now, many of the branches will chance to be so placed that their hooks do not naturally catch, either because they come laterally, or with their blunt backs against the wood, but after a short time, by a process of twisting and adjusting, each little hook becomes turned, so that its sharp point can get a hold on the wood.
The sharp hook on the tendrils of Cobœa is only a very perfect form of the bluntly curved tip which many tendrils possess, and which serves the same purpose of temporarily holding the object caught until the tendril can curl over and make it secure. There is a curious proof of the usefulness of even this blunt hook in the fact that the tendril is only sensitive to a touch on the inside of the hook. The tendril, when it comes against a twig, always slips up it till the hook catches on it, so that it would be of no use to be sensitive on the convex side. Some
Fig. 4.—A caught tendril of Bryonia dioica, spirally contracted in reversed directions.
tendrils, on the other hand, have no hook at the end, and here the tendrils are sensitive to a touch on any side. These tendrils led my father at first into a curious mistake, which he mentions in his book. He pinched a tendril gently in his fingers, and, finding that it did not move, concluded it was not sensitive. But the fact was that the tendril, being touched on two sides at once, did not know which stimulus
