Billeder på siden
PDF
ePub

surfaces. It appears therefore that the cells must contract. The chief seat of the movement is evidently in the thick mass of cells which overlies the central bundle of vessels in the midrib. To ascertain whether this part contracts, a leaf was fastened on the stage of the microscope in such a manner that the two lobes could not become quite shut, and having made two minute black dots on the midrib, in a transverse line and a little towards one side, they were found by the micrometer to be of an inch apart. One of the filaments was then touched and the lobes closed; but as they were prevented from meeting, I could still see the two dots, which now were of an inch apart, so that a small portion of the upper surface of the midrib had contracted in a transverse line of an inch (0508 mm.).

We know that the lobes, whilst closing, become slightly incurved throughout their whole breadth. This movement appears to be due to the contraction of the superficial layers of cells over the whole upper surface. In order to observe their contraction, a narrow strip was cut out of one lobe at right angles to the midrib, so that the surface of the opposite lobe could be seen in this part when the leaf was shut. After the leaf had recovered from the operation and had re-expanded, three minute black dots were made. on the surface opposite to the slit or window, in a line at right angles to the midrib. The distance between the dots was found to be of an inch, so that the two extreme dots were 8% of an inch apart. One of the filaments was now touched and the leaf closed. On again measuring the distances between the dots, the two next to the midrib were nearer together by of an inch, and the two further dots by to of an inch, than they were before; so that the two extreme

to 2

000

80

3 4

00

000

dots now stood about T of an inch (127 mm.) nearer together than before. If we suppose the whole upper surface of the lobe, which was of an inch in breadth, to have contracted in the same proportion, the total contraction will have amounted to about Tor of an inch (635 mm.); but whether this is sufficient to account for the slight inward curvature of the whole lobe, I am unable to say.

25

Finally, with respect to the movement of the leaves, the wonderful discovery made by Dr. Burdon Sanderson* is now universally known; namely that there exists a normal electrical current in the blade and footstalk; and that when the leaves are irritated, the current is disturbed in the same manner as takes place during the contraction of the muscle of an animal.

The Re-expansion of the Leaves.—This is effected at an insensibly slow rate, whether or not any object is enclosed.† One lobe can re-expand by itself, as occurred with the torpid leaf of which one lobe alone had closed. We have also seen in the experiments with cheese and albumen that the two ends of the same lobe can re-expand to a certain extent independently of each other. But in all ordinary, cases both lobes open at the same time. The re-expansion is not determined by the sensitive filaments; all three filaments on one lobe were cut off close to their bases; and the three

[ocr errors][merged small][merged small]

leaves thus treated re-expanded,-one to a partial extent in 24 hrs., —a second to the same extent in 48 hrs., and the third, which had been previously injured, not until the sixth day. These leaves after their re-expansion closed quickly when the filaments on the other lobe were irritated. These were then cut off one of the leaves, so that none were left. This mutilated leaf, notwithstanding the loss of all its filaments, re-expanded in two days in the usual manner. When the filaments have been excited by immersion in a solution of sugar, the lobes do not expand so soon as when the filaments have been merely touched; and this, I presume, is due to their having been strongly affected through éxosmose, so that they continue for some time to transmit a motor impulse to the upper surface of the leaf.

The following facts make me believe that the several layers of cells forming the lower surface of the leaf are always in a state of tension; and that it is owing to this mechanical state, aided probably by fresh fluid being attracted into the cells, that the lobes begin to separate or expand as soon as the contraction of the upper surface diminishes. A leaf was cut off and suddenly plunged perpendicularly into boiling water: I expected that the lobes would have closed, but instead of doing so, they diverged a little. I then took another fine leaf, with the lobes standing at an angle of nearly 80° to each other; and on immersing it as before, the angle suddenly increased to 90°. A third leaf was torpid from having recently re-expanded after having caught a fly, so that repeated touches of the filaments caused not the least movement; nevertheless, when similarly immersed, the lobes separated a little. As these leaves were inserted perpendicularly into the boiling water, both surfaces and the filaments

must have been equally affected; and I can understand the divergence of the lobes only by supposing that the cells on the lower side, owing to their state of tension, acted mechanically and thus suddenly drew the lobes a little apart, as soon as the cells on the upper surface were killed and lost their contractile power. We have seen that boiling water in like manner causes the tentacles of Drosera to curve backwards; and this is an analogous movement to the divergence of the lobes of Dionæa.

In some concluding remarks in the fifteenth chapter on the Droseraceæ, the different kinds of irritability possessed by the several genera, and the different manner in which they capture insects, will be compared.

CHAPTER XIV.

ALDROVANDA VESICULOSA.

Captures crustaceans Structure of the leaves in comparison with those of Dionæa-Absorption by the glands, by the quadrifid processes, and points on the infolded margins - Aldrovanda vesiculosa, var. australis-Captures prey-Absorption of animal matterAldrovanda vesiculosa, var. verticillata — Concluding remarks.

THIS plant may be called a miniature aquatic Dionæa. Stein discovered in 1873 that the bilobed leaves, which are generally found closed in Europe, open under a sufficiently high temperature, and, when touched, suddenly close.* They re-expand in from 24 to 36 hrs., but only, as it appears, when inorganic objects are enclosed. The leaves sometimes contain bubbles of air, and were formerly supposed to be bladders; hence the specific name of vesiculosa. Stein observed that water-insects were sometimes caught, and Prof. Cohn has recently found within the leaves of naturally growing plants many kinds of crustaceans and larvæ.† Plants which had been kept in filtered water were placed by him in a vessel con

*Since his original publication, Stein has found out that the irritability of the leaves was observed by De Sassus, as recorded in 'Bull. Bot. Soc. de France,' in 1861. Delpino states in a paper published in 1871 (Nuovo Giornale Bot. Ital.' vol. iii. p. 174)

that " una quantità di chioccio

line e di altri animalcoli acquatici" are caught and suffocated by the leaves. I presume that

chioccioline are fresh-water molluscs. It would be interesting to know whether their shells are at all corroded by the acid of the digestive secretion.

I am greatly indebted to this distinguished naturalist for having sent me a copy of his memoir on Aldrovanda, before its publication in his 'Beiträge zur Biologie der Pflanzen,' drittes Heft, 1875, p. 71.

« ForrigeFortsæt »