Fifthly, 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 towards 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.
Sixthly, and lastly, 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 cannot 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, Cobaea, 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.They belong to many and widely different orders.To gain some rude idea of their distribution in the vegetable series, I marked, from the lists given by Mohl and Palm (adding a few myself, and a competent botanist, no doubt, could have added many more), all those families in Lindley's 'Vegetable Kingdom'
which include twiners, leaf-climbers, or tendril-bearers.Lindley divides Phanerogamic plants into fifty-nine Alliances; of these, no less than thirty-five include climbing plants of the above kinds, hook and root-climbers being excluded.To these a few Cryptogamic plants must be added.When we reflect on the wide separation of these plants in the series, and when we know that in some of the largest, well-defined orders, such as the Compositae, Rubiaceae, Scrophulariaceae, Liliaceae, &c., species in only two or three genera have the power of climbing, the conclusion is forced on our minds that the capacity of revolving, on which most climbers depend, is inherent, though undeveloped, in almost every plant in the vegetable kingdom.
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 it self.It is acted on by the light, and bends towards or from it, or disregards it, whichever may be 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.
End
