Substances which are not Digested by the Secretion.

All the substances hitherto mentioned cause prolonged inflection of the tentacles, and are either completely or at least partially dissolved by the secretion.  But there are many other substances, some of them containing nitrogen, which are not in the least acted on by the secretion, and do not induce inflection for a longer time than do inorganic and insoluble objects.  These unexciting and indigestible substances are, as far as I have observed, epidermic productions (such as bits of human nails, balls of hair, the quills of feathers), fibro-elastic tissue, mucin, pepsin, urea, chitine, chlorophyll, cellulose, gun-cotton, fat, oil, and starch. [page 122]

To these may be added dissolved sugar and gum, diluted alcohol, and vegetable infusions not containing albumen, for none of these, as shown in the last chapter, excite inflection.  Now, it is a remarkable fact, which affords additional and important evidence, that the ferment of Drosera is closely similar to or identical with pepsin, that none of these same substances are, as far as it is known, digested by the gastric juice of animals, though some of them are acted on by the other secretions of the alimentary canal.  Nothing more need be said about some of the above enumerated substances, excepting that they were repeatedly tried on the leaves of Drosera, and were not in the least affected by the secretion.  About the others it will be advisable to give my experiments.

[Fibro-elastic Tissue.—­We have already seen that when little cubes of meat, &c., were placed on leaves, the muscles, areolar tissue, and cartilage were completely dissolved, but the fibro-elastic tissue, even the most delicate threads, were left without the least signs of having been attacked.  And it is well known that this tissue cannot be digested by the gastric juice of animals.*

Mucin.—­As this substance contains about 7 per cent. of nitrogen, I expected that it would have excited the leaves greatly and been digested by the secretion, but in this I was mistaken.  From what is stated in chemical works, it appears extremely doubtful whether mucin can be prepared as a pure principle.  That which I used (prepared by Dr. Moore) was dry and hard.  Particles moistened with water were placed on four leaves, but after two days there was only a trace of inflection in the immediately adjoining tentacles.  These leaves were then tried with bits of meat, and all four soon became strongly inflected.  Some of the dried mucin was then soaked in water for two days, and little cubes of the proper size were placed on three leaves.  After four days the tentacles

* See, for instance, Schiff, ‘Phys. de la Digestion,’ 1867, tom. ii., p. 38. [page 123]