Darwinism stated by Darwin himself · Charles Darwin
THE MOVEMENTS AND HABITS OF PLANTS.
2. If a twining plant or a tendril gets by any accident into an inclined position, it soon bends upward, though secluded from the light. The guiding stimulus no doubt is the attraction of gravity, as Andrew Knight showed to be the case with germinating plants. If a shoot of any ordinary plant be placed in an inclined position in a glass of water in the dark, the extremity will, in a few hours, bend upward; and, if the position of the shoot be then reversed, the downward-bent shoot reverses its curvature; but if the stolon of a strawberry, which has no tendency to grow upward, be thus treated, it will curve downward in the direction of, instead of in opposition to, the force of gravity. As with the strawberry, so it is generally with the twining shoots of the _Hibbertia dentata_, which climbs laterally from bush to bush; for these shoots, if placed in a position inclined downward, show little and sometimes no tendency to curve upward. 3. Climbing plants, like other plants, bend toward the light by a movement closely analogous to the incurvation which causes them to revolve, so that their revolving movement is often accelerated or retarded in traveling to or from the light. On the other hand, in a few instances tendrils bend toward the dark.
4. We have the spontaneous revolving movement which is independent of any outward stimulus, but is contingent on the youth of the part, and on vigorous health; and this again, of course, depends on a proper temperature and other favorable conditions of life. 5. Tendrils, whatever their homological nature may be, and the petioles or tips of the leaves of leaf-climbers, and apparently certain roots, all have the power of movement when touched, and bend quickly toward the touched side. Extremely slight pressure often suffices. If the pressure be not permanent, the part in question straightens itself and is again ready to bend on being touched. 6. Tendrils, soon after clasping a support, but not after a mere temporary curvature, contract spirally. If they have not come into contact with any object, they ultimately contract spirally, after ceasing to revolve; but in this case the movement is useless, and occurs only after a considerable lapse of time. With respect to the means by which these various movements are effected, there can be little doubt, from the researches of Sachs and H. de Vries, that they are due to unequal growth; but, from the reasons already assigned, I can not believe that this explanation applies to the rapid movements from a delicate touch. Finally, climbing plants are sufficiently numerous to form a conspicuous feature in the vegetable kingdom, more especially in tropical forests. America, which so abounds with arboreal animals, as Mr. Bates remarks, likewise abounds, according to Mohl and Palm, with climbing plants; and, of the tendril-bearing plants examined by me, the highest developed kinds are natives of this grand continent, namely, the several species of _Bignonia_, _Eccremocarpus_, _Cobæa_, and _Ampelopsis_. But even in the thickets of our temperate regions the number of climbing species and individuals is considerable, as will be found by counting them.
THE POWER OF MOVEMENT IN ANIMAL AND PLANT COMPARED. [Page 206.] It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement. It should rather be said that plants acquire and display this power only when it is of some advantage to them; this being of comparatively rare occurrence, as they are affixed to the ground, and food is brought to them by the air and rain. We see how high in the scale of organization a plant may rise, when we look at one of the more perfect tendril-bearers. It first places its tendrils ready for action, as a polypus places its tentacula. If the tendril be displaced, it is acted on by the force of gravity and rights itself. It is acted on by the light, and bends toward or from it, or disregards it, whichever maybe most advantageous. During several days the tendrils or internodes, or both, spontaneously revolve with a steady motion. The tendril strikes some object, and quickly curls round and firmly grasps it. In the course of some hours it contracts into a spire, dragging up the stem, and forming an excellent spring. All movements now cease. By growth the tissues soon become wonderfully strong and durable. The tendril has done its work, and has done it in an admirable manner.
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