And different sides of the surrounding tentacles must be acted on, in order that all should bend with precision to the point of excitement.
The motor impulse, as it spreads from one or more glands across the disc, enters the bases of the surrounding tentacles, and immediately acts on the bending place. It does not in the first place proceed up the tentacles to the glands, exciting them to reflect back an impulse to their bases. Nevertheless, some influence is sent up to the glands, as their secretion is soon increased and rendered acid; and then the glands, being thus excited, send back some other influence (not dependent on increased secretion, nor on the inflection of the tentacles), causing the protoplasm to aggregate in cell beneath cell. This may [page 277] be called a reflex action, though probably very different from that proceeding from the nerve-ganglion of an animal; and it is the only known case of reflex action in the vegetable kingdom.
About the mechanism of the movements and the nature of the motor impulse we know very little. During the act of inflection fluid certainly travels from one part to another of the tentacles. But the hypothesis which agrees best with the observed facts is that the motor impulse is allied in nature to the aggregating process; and that this causes the molecules of the cell-walls to approach each other, in the same manner as do the molecules of the protoplasm within the cells; so that the cell-walls contract. But some strong objections may be urged against this view. The re-expansion of the tentacles is largely due to the elasticity of their outer cells, which comes into play as soon as those on the inner side cease contracting with prepotent force; but we have reason to suspect that fluid is continually and slowly attracted into the outer cells during the act of re-expansion, thus increasing their tension.
I have now given a brief recapitulation of the chief points observed by me, with respect to the structure, movements, constitution, and habits of Drosera rotundifolia; and we see how little has been made out in comparison with what remains unexplained and unknown. [page 278]
CHAPTER XII.
ON THE STRUCTURE AND MOVEMENTS OF SOME OTHER SPECIES OF DROSERA.
Drosera anglica--Drosera intermedia--Drosera capensis--Drosera spathulata--Drosera filiformis--Drosera binata--Concluding remarks.
I EXAMINED six other species of Drosera, some of them inhabitants of distant countries, chiefly for the sake of ascertaining whether they caught insects. This seemed the more necessary as the leaves of some of the species differ to an extraordinary degree in shape from the rounded ones of Drosera rotundifolia. In functional powers, however, they differ very little.
[Drosera anglica (Hudson).*--The leaves of this species, which was sent to me from Ireland, are much elongated, and gradually widen from the footstalk to the bluntly pointed apex. They stand almost erect, and their blades sometimes exceed 1 inch in length, whilst their breadth is only the 1/5 of an inch. The glands of all the tentacles have the same structure, so that the extreme marginal ones do not differ from the others, as in the case of Drosera rotundifolia. When they are irritated by being roughly touched, or by the pressure of minute inorganic particles, or by contact with animal matter, or by the absorption of carbonate of ammonia, the tentacles become inflected; the basal portion being the chief seat of movement. Cutting or pricking the blade of the leaf did not excite any movement. They frequently capture insects, and the glands of the inflected tentacles pour forth much acid secretion. Bits of roast meat were placed on some glands, and the tentacles began to move in 1 m. or
* Mrs. Treat has given an excellent account in 'The American Naturalist,' December 1873, p. 705, of Drosera longifolia (which is a synonym in part of Drosera anglica), of Drosera rotundifolia and filiformis. [page 279]
1 m. 30 s.; and in 1 hr. 10 m.