Insectivorous Plants · Charles Darwin
ACIDS, MUCH DILUTED, WHICH CAUSE INFLECTION.
To this view it may with truth be objected that when leaves are immersed in various strong solutions, or are subjected to a heat of above 130° Fahr. (54°.4 Cent.), aggregation ensues, but there is no movement. Again, various acids and some other fluids cause rapid movement, but no aggregation, or only of an abnormal nature, or only after a long interval of time; but as most of these fluids are more or less injurious, they may check or prevent the aggregating process by injuring or killing the protoplasm. There is another and more important difference in the two processes: when the glands on the disc are excited, they transmit some influence up the surrounding [page 260] tentacles, which acts on the cells at the bending place, but does not induce aggregation until it has reached the glands; these then send back some other influence, causing the protoplasm to aggregate, first in the upper and then in the lower cells.
The Re-expansion of the Tentacles.—This movement is always slow and gradual. When the centre of the leaf is excited, or a leaf is immersed in a proper solution, all the tentacles bend directly towards the centre, and afterwards directly back from it. But when the point of excitement is on one side of the disc, the surrounding tentacles bend towards it, and therefore obliquely with respect to their normal direction; when they afterwards re-expand, they bend obliquely back, so as to recover their original positions. The tentacles farthest from an excited point, wherever that may be, are the last and the least affected, and probably in consequence of this they are the first to re-expand. The bent portion of a closely inflected tentacle is in a state of active contraction, as shown by the following experiment. Meat was placed on a leaf, and after the tentacles were closely inflected and had quite ceased to move, narrow strips of the disc, with a few of the outer tentacles attached to it, were cut off and laid on one side under the microscope. After several failures, I succeeded in cutting off the convex surface of the bent portion of a tentacle. Movement immediately recommenced, and the already greatly bent portion went on bending until it formed a perfect circle; the straight distal portion of the tentacle passing on one side of the strip. The convex surface must therefore have previously been in a state of tension, sufficient to counter-balance that of the concave surface, which, when free, curled into a complete ring.
Text from Project Gutenberg, public domain in the United States.