第26章 TENDRIL-BEARERS(3)

They continue growing for some time, even after they have clasped an object.When fully grown, though borne by a young plant, they are 9inches in length.The three divergent toes are shorter relatively to the tarsus than in the former species; they are blunt at their tips and but slightly hooked; they are not quite equal in length, the middle one being rather longer than the others.Their outer surfaces are highly sensitive; for when lightly rubbed with a twig, they became perceptibly curved in 4 m.and greatly curved in 7 m.In 7hrs.they became straight again and were ready to re-act.The tarsus, for the space of one inch close to the toes, is sensitive, but in a rather less degree than the toes; for the latter after a slight rubbing, became curved in about half the time.Even the middle part of the tarsus is sensitive to prolonged contact, as soon as the tendril has arrived at maturity.After it has grown old, the sensitiveness is confined to the toes, and these are only able to curl very slowly round a stick.A tendril is perfectly ready to act, as soon as the three toes have diverged, and at this period their outer surfaces first become irritable.The irritability spreads but little from one part when excited to another: thus, when a stick was caught by the part immediately beneath the three toes, these seldom clasped it, but remained sticking straight out.

The tendrils revolve spontaneously.The movement begins before the tendril is converted into a three-pronged grapnel by the divergence of the toes, and before any part has become sensitive; so that the revolving movement is useless at this early period.The movement is, also, now slow, two ellipses being completed conjointly in 24 hrs.18m.A mature tendril made an ellipse in 6 hrs.; so that it moved much more slowly than the internodes.The ellipses which were swept, both in a vertical and horizontal plane, were of large size.The petioles are not in the least sensitive, but revolve like the tendrils.We thus see that the young internodes, the petioles, and the tendrils all continue revolving together, but at different rates.The movements of the tendrils which rise opposite one another are quite independent.Hence, when the whole shoot is allowed freely to revolve, nothing can be more intricate than the course followed by the extremity of each tendril.A wide space is thus irregularly searched for some object to be grasped.

One other curious point remains to be mentioned.In the course of a few days after the toes have closely clasped a stick, their blunt extremities become developed, though not invariably, into irregular disc-like balls which have the power of adhering firmly to the wood.

As similar cellular outgrowths will be fully described under B.

capreolata, I will here say nothing more about them.

Bignonia aequinoctialis, var.Chamberlaynii.--The internodes, the elongated non-sensitive petioles, and the tendrils all revolve.The stem does not twine, but ascends a vertical stick in the same manner as the last species.The tendrils also resemble those of the last species, but are shorter; the three toes are more unequal in length, the two outer ones being about one-third shorter and rather thinner than the middle toe; but they vary in this respect.They terminate in small hard points; and what is important, cellular adhesive discs are not developed.The reduced size of two of the toes as well as their lessened sensitiveness, seem to indicate a tendency to abortion; and on one of my plants the first-formed tendrils were sometimes simple, that is, were not divided into three toes.We are thus naturally led to the three following species with undivided tendrilsBignonia speciosa.--The young shoots revolve irregularly, making narrow ellipses, spires or circles, at rates varying from 3 hrs.30m.to 4 hrs.40 m.; but they show no tendency to twine.Whilst the plant is young and does not require a support, tendrils are not developed.Those borne by a moderately young plant were five inches in length.They revolve spontaneously, as do the short and non-sensitive petioles.When rubbed, they slowly bend to the rubbed side and subsequently straighten themselves; but they are not highly sensitive.There is something strange in their behaviour: Irepeatedly placed close to them, thick and thin, rough and smooth sticks and posts, as well as string suspended vertically, but none of these objects were well seized.After clasping an upright stick, they repeatedly loosed it again, and often would not seize it at all, or their extremities did not coil closely round.I have observed hundreds of tendrils belonging to various Cucurbitaceous, Passifloraceous, and Leguminous plants, and never saw one behave in this manner.When, however, my plant had grown to a height of eight or nine feet, the tendrils acted much better.They now seized a thin, upright stick horizontally, that is, at a point on their own level, and not some way up the stick as in the case of all the previous species.Nevertheless, the non-twining stem was enabled by this means to ascend the stick.

The extremity of the tendril is almost straight and sharp.The whole terminal portion exhibits a singular habit, which in an animal would be called an instinct; for it continually searches for any little crevice or hole into which to insert itself.I had two young plants;and, after having observed this habit, I placed near them posts, which had been bored by beetles, or had become fissured by drying.