of movement by which its shoot may sometimes point in one direction, sometimes in another. But this is only half the phenomenon, and, if we examine closely, we shall find that the movement is constant and regular, the stem first pointing north, then east, then south, then west, in regular succession, so that its tip is constantly traveling round and round like the hand of a watch, making on an average, in warm August weather, one revolution in two hours. Here, then, is a most curious power possessed by the shoots of twining plants, which is worth inquiring further into, both as regards the way in which the movement is produced, and as to how it can be of any service to the plant. Questions are often asked in gardening periodicals as to how hops or other climbing plants always manage to grow precisely in the direction in which they will find a support. This fact has surprised many observers, who have supposed that climbing plants have some occult sense by which they discover the whereabouts of the stick up which they subsequently climb. But there is in reality no kind of mystery in the matter the growing shoot simply goes swinging round till it meets with a stick, and then it climbs up it. Now, a revolving shoot may be more than two feet long, so that it might be detained in its swinging-round movements by a stick fixed into the ground at a distance of nearly two feet. There would then be a straight bit of stem leading from the roots of the plant, in a straight line to the stick up which it twines, so that an observer who knew nothing of the swinging-round movement might be pardoned for supposing that the plant had in some way perceived the stick and grown straight at it. This same power of swinging round slowly comes into play in the very act of climbing up a stick.

Suppose I take a rope and swing it round my head: that may be taken to represent the revolving of the young hop-shoot. If, now, I allow it to strike against a rod, the end of the rope which projects beyond the rod curls freely round it in a spiral. And this may be taken as a rough representation of what a climbing plant does when it meets a stick placed in its way. That is to say, the part of the shoot which projects beyond the stick continues to curl inward till it comes. against the stick; and, as growth goes on, the piece of stem which is projecting is, of course, all the while getting longer and longer; and, as it is continually trying to keep up the swinging-round movement, it manages to curl round the stick. But there is a difference between the rope and the plant in this-that the rope curls round the stick at the same level as that at which it is swung, so that, if it moves round in an horizontal plane at a uniform height above ground, it will curl round the stick at that level, and thus will not climb up the stick it strikes against. But the climbing plant, although it may swing round when searching for a stick, at a fairly uniform level, yet, when it curls round a stick, does not retain a uniform distance from the ground, but by winding round like a corkscrew it gets higher and higher at each turn.

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 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, while every single plant in a row of beans twines the other way; nor why in some rare instances a species is divided, like the human race, into right- and left-handed individuals, some twining like a left-handed, others like a right-handed screw. Or, again, why some very few plants will twine half-way up a stick in one direction, and then reverse the spiral and wind the other way. Nor, though we know that in all these plants the twining is caused by the change in the region of quickest growth, have we any idea what causes this change of growth. There is still much to work at, and it is to be hoped that there are still plenty of workers to solve the problems. It is by looking to exceptions that the key to a problem is often found. It is the exceptions to general rules that often lead us to understand the meaning and origin of the rules themselves; and it is to such exceptions that any one who wants to work at climbing plants should turn. Now, it is a general rule that a climbing plant twines in the same way that it revolves. It seems an obvious thing that in the case of the rope model, if we swing the rope round our head in the direction of the hands of a watch, it must twine round the stick against which it strikes in the same direction. But in plants it is not always so. In the large majority of cases it is so, for, if this were not the case, the illustration of the rope would not have been applicable; but it is not universally the rule. Every individual of the plant Hibbertia always twines round its stick in the same direction, but, when it is performing the swinging movement in search of a support, it is found that some plants travel round with the sun, others in the opposite direction. This fact forms an exception of a striking kind-and such exceptions are worthy of close study.

There are other facts of a different nature, which seem to show how difficult the problem is, and how delicately balanced is that part of the organization of the plant which is connected with the power of climbing. For instance, if we cut a branch of most shrubs, and put it in water, it goes on growing, apparently as healthily as ever. Indeed, the practice of making cuttings-where a cut-off branch or shoot develops roots and turns into a new plant-shows us that no serious injury is thus caused. But the twining organization is sensitive to such treatment. A cut branch of hop placed in water was observed to make its revolutions in about twenty hours, whereas in its natural condition-growing on the plant-it makes a complete turn in two or three hours. Again, if a plant growing in a pot is moved from one greenhouse to another, the slight shaking thus caused is sufficient to stop the revolving movement for a time-another proof of the delicacy of the internal machinery of the plant.

Some of the problems, as, for instance, why twining plants can not as a rule climb thick stems, may be looked at from the natural-history point of view. Most of our climbing plants die down in the winter, so that, if they were able to climb round big tree-trunks, they would

waste all the precious summer weather in climbing a few feet, whereas the same amount of longitudinal growth devoted to twining up a thin stick would have raised them up to the light after which they are striving. And as a plant exercises no choice, but merely swings round till it hits against an object, up which it will then try to twine, it seems as if the inability to climb thick stems might be a positive advantage to a plant, by forcing it to twine up such objects as would best repay the trouble.

In the classification of climbing plants, proposed by my father in his book, he makes a subdivision of "hook-climbers." These may be taken as the simplest representatives of that class of climbers which are not twining plants. The common bramble climbs or scrambles up through thick underwood, being assisted by the recurved spines which allow the rapidly growing shoot to creep upward as it lengthens, but prevent it from slipping backward again; the common goose-grass (Galium) also climbs in this way, sticking like a burr to the side of a hedge-row up which it climbs. Most country boys will remember having taken advantage of this burr-like quality of Galium in making sham birds'-nests, the prickly stems adhering together in the desired form. Such plants as the bramble or Galium exhibit none * of the swinging-round movement which I have described in twiners: they simply grow straight on, trusting to their hooks to retain the position gained.

In some species of clematis we find a mechanism which reminds one of a simple hook-climber, but is in reality a much better arrangement. The young leaves projecting outward and slightly backward from the stem may remind us of the hooked spines of a bramble, and like them easily catch on neighboring objects, and support the trailing stem. Or the leaf of the species of clematis given in Fig. 1 may serve as an example of a leaf acting like a hook. The main stalk of the leaf is seen to be bent angularly downward at the points where each successive pair of leaflets is attached, and the leaflet at the end of the leaf is bent down at right angles, and thus forms a grappling apparatus. The clematis does not, like the bramble, trust to mere growth, to thrust itself among tangled bushes, but possesses the same powers of revolving in search of a support which simple or true twining plants possess. Indeed, many species of clematis are actually twining plants, and can wind spirally up a stick placed in their way. And the same revolving movement which enables them thus to wind spirally also helps them to search for some holding-place for their hook- or grapple-like leaves, and in many species the search is carried on by the leaves swinging round, quite independently of the revolving movement of the stem on which they are borne.

* That is to say, the revolving movement is not sufficiently developed to be of practical importance. The same remark is applicable to the other cases in which I have spoken of the absence of revolving movement in the growing parts of plants.

If a leaf of a clematis succeed by any means in hooking on to a neighboring object, the special characteristic of leaf-climbing plants comes into play. The stalk of the leaf curls strongly over toward the object touching it, and clasps it firmly. It is obvious how great is the advantage thus gained over a mere hook. A leaf such as that shown

in Fig. 2 might be made to catch on to a neighboring twig by its bent stalk, in such a way that, although it managed to stay where it was, it could bear none of the weight of the plant, and would be liable to be displaced by a strong wind or other disturbance. But, when the stalk of the leaf had curled close round the twig, nothing could displace it, and it could take its share in the work of supporting the plant.

The extreme sensitiveness of the leaf-stalk to slight and gentle touches gives a curious idea of the alertness of the plant in its search for supporting objects. A leaf may be excited to bend by a loop of string weighing only one-sixteenth of a grain. It is an interesting fact that, in such a hook-like leaf as that of Clematis viticella (Fig. 1), the hooked end of the leaf, which has the best chance of coming into contact with obstacles, is the most sensitive part. This has been made out by hanging small weights on different parts of the leaf, and it is found that the terminal leaflet bends in a few hours after a loop of string weighing less than a grain is hung on it, and which produced no effect in twentyfour hours on the other petioles. One may see proof of the sensitiveness of the leaf-stalks of the wild English clematis, which sometimes catches withered leaves or delicate stalks of the quaking-grass. The same thing is shown by a leaf after having been touched with a little water-color, the delicate crust of dry paint being mistaken for something touching the plant. In such cases, or when the leaf has been merely rubbed with a twig, which is taken away before the leaf seizes.

*For the loan of this and the other woodcuts illustrating this article, we are indebted to the kindness of Mr. Charles Darwin and Mr. Murray.

VOL. XVII.-41