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.

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    after the secretion has been all washed away by heavy rain; and this often occurs, though the secretion is so viscid that it can be removed with difficulty merely by waving the leaves in water. If the falling drops of water are small, they adhere to the secretion, the weight of which must be increased in a much greater degree, as before remarked, than by the addition of minute particles of solid matter; yet the drops never cause the tentacles to become inflected. It would obviously have been a great evil to the plant (as in the case of occasional touches) if the tentacles were excited to bend by every shower of rain; but this evil has been avoided by the glands either having become through habit insensible to the blows and prolonged pressure of drops of water, or to their having been originally rendered sensitive solely to the contact of solid bodies.* We shall hereafter see that the filaments on the leaves of Diontea are likewise insensible to the impact of fluids, though exquisitely sensitive to momen- tary touches from any solid body. When the pedicel of a tentacle is cut off by a sharp pair of scissors quite close beneath the gland, the tentacle generally becomes inflected. I tried this experiment repeatedly, as I was much surprised at the fact, for all other parts of the pedicels are insensible to any stimulus. These headless tentacles after a time re-expand; but I shall return to this subject. On the other hand, I occasionally succeeded in crushing a gland between a pair of pincers, but this caused no inflection. In this latter case the tentacles seem paralysed, as likewise follows from the action of too strong solutions of certain salts, and of too great heat, whilst weaker solutions of the same salts and a more gentle heat cause movement. We shall also see in future chapters that various other fluids, some vapours, and oxygen (after the plant has been for some timo excluded from its action), all induce inflection, and this likewise results from an induced galvanic current.-)- * [Pfeifer's experiments, given above (p. 22), explain the failure of rain to cause movement.—F. D.] t My son Francis, guided by the observations of Dr. Burdon Sanderson on Dionsea, finds that, if two needles are inserted into the blade of a leaf of Drosera, the tentacles do not move ; but that, if similar needles in connection with the secondary coil of a Du Bois induction apparatus are inserted, the tentacles curve inwards in the course of a few minutes. My son hopes soon to publish an account of his observations.
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