Читать книгу An Introduction to Nature-study - E. Stenhouse - Страница 21

1. Hooking stems.—Examine a bramble or a wild rose plant in a hedge. Why does it need to climb? How does it climb? Pull a branch and notice by what means it resists the pull. What is the shape of the prickles? Do they point upwards or downwards? On what parts of the plant are the prickles found? Notice how easily a prickle may be pushed off, sideways. Is it as easy to tear it off lengthways? Is the prickle a little branch, or merely an extension of the rind?

Contrast a prickle with the thorn of the hawthorn. The thorn does not come off easily, and it contains a woody core which is continuous with the wood of the branch. Cut lengthwise through the thorn and the branch which bears it to see this. The thorn is a short pointed branch; it arises in the axil of a leaf, and sometimes bears leaves itself.

2. The ivy.—Observe how the climbing stem of the ivy is attached to a wall or tree. It puts out a line of roots on its shaded side. These roots give out a sticky fluid, which, on hardening, fixes them to the wall.

3. Twining stems.—Watch the growth of a convolvulus seedling. At first the young stem grows straight up, but soon the tip begins to move round and round. Try to find out how long it takes to describe one revolution. Put a long stick in the ground near the plant and notice how, when the revolving stem touches the stick, the spiral is henceforth described round the support and the stem consequently clings to it. Lay your watch face-upward, and notice whether the stem moves in the same direction as the hands (the “clockwise” direction), or in the opposite (“counter-clockwise”) direction.

Make similar observations on the hop, honeysuckle, and scarlet runner, and note the results.

4. Leaf climbers.—Examine a climbing tropœolum (often, though wrongly, called “nasturtium”). Which parts of the plant clasp the support? Watch a young plant coming up. Does the stem revolve before the leaf stalks come in contact with the support? Compare the clematis.

5. Tendril climbers.—Examine plants of sweet pea, bryony, vine, passion flower, and cucumber. Try to find out in each case which part has been modified to form the tendrils. Watch a plant day by day until a free tendril grasps a support, and notice how it becomes spirally coiled. Can you straighten the tendril by pulling, without leaving any kinks? Why? Is the spiral of the tendril continuous, or does it change its direction in the middle? Make a continuous spiral with wire, and notice the kinks formed when the wire is straightened by pulling the ends. Which is better for the plant in a gale of wind, a continuous or a reversed spiral? Why?

Notice the sucker-like tendrils of the Virginian creeper, and the way in which they fix the plant to the wall.

Plants whose stems are not strong enough to stand erect without support must adopt some special means of spreading out their leaves to the light and air. One of the commonest devices of such plants is that of climbing up other and stronger plants, walls, trellis-work, etc.

Scramblers and climbers.—In the simplest cases the plant simply scrambles over other plants. Many brambles and roses are merely scramblers, but more often they are true climbers, weaving themselves among their neighbours by the help of hooked prickles (Fig. 76). The prickles point backwards, and therefore anchor the twigs firmly, as is very evident on trying to pull a branch out of the hedge. A prickle may easily be broken off by a side push, for it is merely an outgrowth of the rind, and does not contain any woody core. It is so attached, however, that it resists much greater force in the lengthwise direction—a manifest advantage to the plant.

Fig. 45.—Ivy climbing up a wall. R, aërial roots. (×⅓.)

The differences between such a prickle and a thorn like that of the hawthorn should be carefully noticed. A thorn is really a little twig which has remained short and become pointed at the end. It has a core of wood which is continuous with the wood of the branch bearing it. That a thorn is really a little branch is shown by its origin in the axil of a leaf, and by its often giving rise to leaves and buds.

Fig. 46.—Climbing stem Fig. 47.—Climbing stem

of Honeysuckle. (×¼.)of Convolvulus. (×¼.)

The stem of the ivy climbs by means of little roots, which it puts out on the side furthest from the light (Fig. 45). These give out a sticky liquid which, on drying, cements them to the wall or tree.

Twining stems are much in advance of these. There seems to be something approaching intelligence in the manner in which the young stem of a hop, honeysuckle, or convolvulus, which at first grows straight up, begins to wander round and round, tracing a spiral path in the air until it touches a support. Then, however, as if the plant could feel, the movement below the point of contact stops; but the upper part of the stem still revolves and therefore twines round the support. The stems of the honeysuckle (Fig. 46) and the hop turn in the same direction as the hands of a clock. This is called the “clockwise” direction. On the other hand the convolvulus (Fig. 47) and most other twining stems are “counter-clockwise” climbers. The stem of the bittersweet revolves indifferently in either direction.

Sensitive clasping organs in their simplest form are seen in the twining leaf stalk of the clematis and tropœolum; the stem itself revolves as if to give its leaf stalks every opportunity of finding suitable supports. The leaf stalks seize these and twine round them.

Most wonderful of all climbing organs are the tendrils. They are well seen in Fig. 48. A part of the plant—sometimes a leaflet, as in the pea (Fig. 28); sometimes a branch, as in the passion flower; or a flower stalk, as in the vine—becomes modified into a thread, slight but strong. When the end of the thread touches and then twines round a support, the whole tendril forms itself into a spiral which, like a wire spring, draws the plant up to the support, and can yet lengthen and yield to the wind when necessary. In the middle of the tendril the direction of the spiral is reversed, so that the tendril can be straightened without being twisted.

The tendrils of the Virginian creeper do not twine, but on meeting a wall they form round red suckers at the end, and attach the plant (Fig. 49).

Fig. 48.—How a tendril grasps a support. Fig. 49.—Virginian Creeper.

The spiral is reversed at x.R, R, stem tendrils. (×½.)