Four of these had caught rather small insects, viz. three of them ants, and the fourth a rather small fly, but the other ten had all caught large insects, namely, five elaters, two chrysomelas, a curculio, a thick and broad spider, and a scolopendra. Out of these ten insects, no less than eight [page 313] were beetles,* and out of the whole fourteen there was only one, viz. a dipterous insect, which could readily take flight. Drosera, on the other hand, lives chiefly on insects which are good flyers, especially Diptera, caught by the aid of its viscid secretion. But what most concerns us is the size of the ten larger insects. Their average length from head to tail was .256 of an inch, the lobes of the leaves being on an average .53 of an inch in length, so that the insects were very nearly half as long as the leaves within which they were enclosed. Only a few of these leaves, therefore, had wasted their powers by capturing small prey, though it is probable that many small insects had crawled over them and been caught, but had then escaped through the bars.
The Transmission of the Motor Impulse, and Means of Movement.--It is sufficient to touch any one of the six filaments to cause both lobes to close, these becoming at the same time incurved throughout their whole breadth. The stimulus must therefore radiate in all directions from any one filament. It must also be transmitted with much rapidity across the leaf, for in all ordinary cases both lobes close simultaneously, as far as the eye can judge. Most physiologists believe that in irritable plants the excitement is transmitted along, or in close connection with, the fibro-vascular bundles. In Dionaea, the course of these vessels (composed of spiral and ordinary vascular
* Dr. Canby remarks ('Gardener's Monthly,' August 1868), "as a general thing beetles and insects of that kind, though always killed, seem to be too hard-shelled to serve as food, and after a short time are rejected." I am surprised at this statement, at least with respect to such beetles as elaters, for the five which I examined were in an extremely fragile and empty condition, as if all their internal parts had been partially digested. Mrs. Treat informs me that the plants which she cultivated in New Jersey chiefly caught Diptera. [page 314]
tissue) seems at first sight to favour this belief; for they run up the midrib in a great bundle, sending off small bundles almost at right angles on each side. These bifurcate occasionally as they extend towards the margin, and close to the margin small branches from adjoining vessels unite and enter the marginal spikes. At some of these points of union the vessels form curious loops, like those described under Drosera. A continuous zigzag line of vessels thus runs round the whole circumference of the leaf, and in the midrib all the vessels are in close contact; so that all parts of the leaf seem to be brought into some degree of communication. Nevertheless, the presence of vessels is not necessary for the transmission of the motor impulse, for it is transmitted from the tips of the sensitive filaments (these being about the 1/20 of an inch in length), into which no vessels enter; and these could not have been overlooked, as I made thin vertical sections of the leaf at the bases of the filaments.
On several occasions, slits about the 1/10 of an inch in length were made with a lancet, close to the bases of the filaments, parallel to the midrib, and, therefore, directly across the course of the vessels. These were made sometimes on the inner and sometimes on the outer sides of the filaments; and after several days, when the leaves had reopened, these filaments were touched roughly (for they were always rendered in some degree torpid by the operation), and the lobes then closed in the ordinary manner, though slowly, and sometimes not until after a considerable interval of time. These cases show that the motor impulse is not transmitted along the vessels, and they further show that there is no necessity for a direct line of communication from the filament which is [page 315] touched towards the midrib and opposite lobe, or towards the outer parts of the same lobe.