Page:Darwin - On the movements and habits of climbing plants.djvu/36

 , T. peregrinum, T. elegans, T. tuberosum, and a dwarf variety of, as I believe, T. minus.

Tropæolum tricolorum, var. grandiflorum.—The flexible shoot, which first rises from the tuber, is as thin as thin twine. One such shoot revolved in a course opposed to the sun, at an average rate, judging from three revolutions, of 1 h. 23 m.; but no doubt the direction of the revolving movement is variable. When the plant had grown tall and much branched, all the many lateral shoots continued to revolve. The stem, whilst young, twined regularly round a thin vertical stick; in one case I counted eight spiral turns: but when grown older, the stem often runs straight up for a space, and, being arrested by the clasping petioles, makes one or two spires in a reversed direction. Until the plant has grown to a height of two or three feet, about a month after the first shoot has appeared above ground, no true leaves, but in their place little filaments, coloured like the stem, are produced. The extremities of these filaments are pointed, a little flattened, and furrowed on the upper surface. They never become developed into leaves. As the plant grows in height new filaments are produced with slightly enlarged tips; then others, bearing on each side of the enlarged medial tip a rudimentary segment of a leaf; and soon other segments appear, until a perfect leaf is formed with seven deep segments. So that on the same plant we may see every step from tendril-like filaments to perfect leaves. Hence this plant, whilst young, might be classed with tendril-bearers. After the plant has grown to a considerable height, and is secured to its support by the clasping petioles of the true leaves, the clasping filaments on the lower part of the stem wither and drop off; so that they perform only a temporary service.

These filaments, as well as the petioles of the perfect leaves, whilst young, are highly sensitive on all sides to a touch. The slightest rub causes them to curve towards the rubbed side in about three minutes: one bent itself into a ring in six minutes; they subsequently became straight again: if, however, they have once completely clasped a stick, when this is removed, they do not recover themselves. The most remarkable fact, and which I have observed in no other species of the genus, is that the filaments and petioles of the young leaves, if they catch no object, after standing in their original position for some days spontaneously and slowly move, oscillating a little from side to side, towards the stem of the plant. Hence all the petioles and filaments, though arising on different sides of the axis, ultimately bend towards and