Page:Darwin - On the movements and habits of climbing plants.djvu/107

 helix), Ficus repens, and F. barbatus, have no power of movement, not even from the light to the dark. As previously stated, the Hoya carnosa (Asclepiadaceæ) is a spiral twiner, and can likewise adhere by rootlets even to a flat wall; the tendril-bearing Bignonia Tweedyana emits roots, which curve half round and adhere to thin sticks. The Tecoma radicans (Bignoniaceæ), which is closely allied to many spontaneously revolving species, climbs by rootlets; but its young shoots apparently move about rather more than can be accounted for by the varying action of the light.

I have not closely observed many root-climbers, but can give one curious little fact. Ficus repens climbs up walls just like Ivy; when the young rootlets were made to press lightly on slips of glass, they emitted (and I observed this several times), after about a week's interval, minute drops of clear fluid, not in the least milky like that exuded from a wound. This fluid was slightly viscid, but could not be drawn out into threads; it had the remarkable property of not drying. One drop, about the size of half a pin's head, I slightly spread out, and scattered on it some minute grains of sand. The slip of glass was left exposed in a drawer during hot and dry weather, and, if the fluid had been water, it would certainly have dried in one or two minutes; but it remained fluid, closely surrounding each grain of sand, during 128 days: how much longer it would have remained I cannot say. Some other rootlets were left in contact with the glass for about ten days or a fortnight, and the drops of fluid secreted by them were rather larger, and so viscid that they could be drawn out into threads. Some other rootlets were left in contact during twenty-three days, and these were firmly cemented to the glass. Hence we may conclude that the rootlets first secrete a slightly viscid fluid, and that they subsequently absorb (for we have seen that it will not dry by itself) the watery parts, and ultimately leave a cement. When the rootlets were torn from the glass, atoms of yellowish matter were left on it, which were partly dissolved by a drop of bisulphide of carbon; and this extremely volatile fluid was rendered, by what it had dissolved, very much less volatile.

As the bisulphide of carbon has so strong a power of softening indurated caoutchouc, I soaked in it during a short time many