Page:Darwin - On the movements and habits of climbing plants.djvu/86

 little disks or cushions, which adhere firmly to the surface. In one case these tips became slightly swollen in 38 h. after coming into contact with a brick; in another case they were considerably swollen in 48 h., and in an additional 24 h. they were firmly attached to a smooth board; and lastly, the tips of a younger tendril not only swelled but became attached to a stuccoed wall in 42 h. These adhesive disks resemble, except in colour and in being larger, those of Bignonia capreolata. When they were developed in contact with a ball of tow, fibres were separately enveloped, but not in so effective a manner as with B. capreolata. Disks are never developed, as far as I have seen, without the stimulus of at least temporary contact with some object. They are generally first formed on one side of the curved tip, the whole of which often becomes so much changed, that a line of green unaltered tissue can be traced only along the concave surface. When, however, a tendril has clasped a cylindrical stick, an irregular rim or disk is formed along the inner surface at some little distance from the curved tip; this was also observed (S. 71) by Mohl. The disks consist of enlarged cells, with smooth projecting hemispherical surfaces, coloured red, and at first gorged with fluid (see section given by Mohl, S. 70), but they ultimately become woody.

As the disks can almost immediately adhere firmly to such smooth surfaces as planed and painted wood, or to the polished leaf of the ivy, this alone would render it probable that some cement is secreted, as has been asserted to be the case (quoted by Mohl, S. 71) by Malpighi. I removed a number of disks formed during the previous year from a stuccoed wall, and placed them in warm water, diluted acetic acid and alcohol during many hours; but the attached grains of silex were not loosened: immersion in sulphuric ether for 24 h. much loosened them; but warmed essential oils (I tried oil of thyme and peppermint) in the course of a few hours completely released every atom of stone. This seems to prove that some resinous cement is secreted; the quantity secreted, however, must be small; for when a plant ascended a thinly whitewashed wall, the disks adhered firmly to the whitewash; but as the cement never penetrated the thin layer, they were easily withdrawn, together with little scales of the whitewash. It must not be supposed that the attachment is by any means exclusively effected by the cement; for the cellular outgrowth completely envelopes every minute and irregular projection, and insinuates itself into every crevice.

A tendril which has not become attached to any body, does not