Insectivorous plants / by Charles Darwin ; revised by Francis Darwin.

  • Charles Darwin
Date:
1908
Catalogue details

Licence: In copyright

Credit: Insectivorous plants / by Charles Darwin ; revised by Francis Darwin. Source: Wellcome Collection.

    30/404 page 12
    the other hand, tartrate of antimony produces no such effect. Immersion in many acids (of the strength of one part to 437 of water) likewise causes a wonderful amount of secretion, so that, when the leaves are lifted out, long ropes of extremely viscid fluid hang from them. Some acids, on the other hand, do not act in this manner. Increased secretion is not necessarily dependent on the inflection of the tentacle, for particles of sugar and of sulphate of zinc cause no movement. It is a much more remarkable fact, that when an object, such as a bit of meat or an insect, is placed on the disc of a leaf, as soon as the surrounding tentacles become considerably inflected, their glands pour forth an increased amount of secretion. I ascertained this by selecting leaves with equal- sized drops on the two sides, and by placing bits of meat on one side of the disc ; and as soon as the tentacles on this side became much inflected, but before the glands touched the meat, the drops of secretion became larger. This was repeatedly observed, but a record was kept of only thirteen cases, in nine of which increased secretion was plainly observed; the four failures being due either to the leaves being rather torpid, or to the bits of meat being too small to cause much inflection. We must therefore conclude that the central glands, when strongly excited, transmit some in- fluence to the glands of the circumferential tentacles, causing them to secrete more copiously. It is a still more important fact (as we shall see more fully when we treat of the digestive power of the secretion), that when the tentacles become inflected, owing to the central glands having been stimulated mechanically, or by contact with animal matter, the secretion not only increases in quantity, but changes its nature and becomes acid; and this occurs before the glands have touched the object on the centre of the leaf. This acid is of a different nature from that contained in the tissue of the leaves. As long as the tentacles remain closely inflected, the glands continue to secrete, and the secretion is acid; so that, if neutralised by carbonate of soda, it again becomes acid after a few hours. I have observed the same leaf with the tentacles closely in- flected over rather indigestible substances, such as chemi- cally prepared casein,* pouring forth acid secretion for eight * [These observations are not trustworthy, owing to the mode of preparation of the casein. See p. 95.—F. D.]
    digitised image 1
    digitised image 2
    digitised image 3