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On comparing a thin transverse slice of such a petiole with one from an older leaf growing close beneath, which had not clasped anything, its diameter was found to be fully doubled, and its structure greatly changed. In two other petioles similarly compared, and here represented, the increase in diameter was not quite so great.
When a petiole has clasped a twig, it undergoes some remarkable changes, which may be observed with the other species, but in a less strongly marked manner, and will here be described once for all. The clasped petiole in the course of two or three days swells greatly, and ultimately becomes nearly twice as thick as the opposite one which has clasped nothing.
The honeysuckle genus is, as a rule, devoid of the stipules at the base of the petiole, but Lonicera etrusca has been observed to develop such organs, which were seen to be free in some, but in other specimens were adnate to the base of the leaf, and even connate with those of the opposite leaf.
The leaves of O. gigas are broader, of a deeper green, the blade more sharply set off against the stalk, all the rosettes becoming stout and crowded with leaves. Those of O. rubrinervis on the contrary are thin, of a paler green and with a silvery white surface; the blades are elliptic, often being only 2 cm. or less in width. They are acute at the apex and gradually narrowed into the petiole.
The flexible petiole of a half or a quarter grown leaf which has clasped an object for three or four days increases much in thickness, and after several weeks becomes so wonderfully hard and rigid that it can hardly be removed from its support.
On this under side a system of branching vessels carrying a pale milky fluid were seen coming from the extremities of the leaf, and covering the whole under side of it, and joining two large veins, one on each side of the red artery in the middle rib of the leaf, and along with it descending to the foot-stalk or petiole.
The upper part of this ring of woody vessels, formed by the prolongation of the horns of the original semilunar band, is narrower than the lower part, and slightly less compact. This petiole after clasping the stick had actually become thicker than the stem from which it arose; and this was chiefly due to the increased thickness of the ring of wood.
In each leaf, at the base of the laminae, the petiole, or stalk, is furnished with a couple of pouches, divided from each other by the mid-rib, as shown in the figure. Into each of these pouches there is an entrance from the lower side of the leaf. I noticed them first in Northern Brazil, in the province of Maranham; and afterwards at Para.
The main purpose of the revolving movement in all the species of Tropaeolum manifestly is to bring the petioles into contact with some supporting object. The petiole of a young leaf, after a slight rub, became curved in 6 m.; another, on a cold day, in 20 m., and others in from 8 m. to 10 m.
A loop of thread weighing 0.125th of a grain caused a petiole to curve; but the stimulus was not sufficient, the loop remaining suspended, to cause a permanent flexure. If a much heavier loop be placed in the angle between the petiole and the stem, it produces no effect; whereas we have seen with Clematis montana that the angle between the stem and petiole is sensitive. Tropaeolum peregrinum.
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