Peridinium, by reflecting much more light than it did at first, comes rapidly into notice, and by its numbers gives a general red colour to that part of the sea in which it may be present. The same is frequently-indeed, commonly-the course with Euglena in fresh-water ponds. The little Protococcus which colours the salt red in the salt-pans of the Island of Bombay, is green in the active period of its existence, but becomes red, and settles down into the "still form" of the same colour; while the common green Protococcus of the freshwater tanks loses its red spot in the still form, and gains it again in the active or reproducing period of its existence. So red Euglena often becomes green; but the usual course appears to be for the green to appear first.

The red colour also appears to herald the termination of some period in the existence of the species. Thus the Peridinium above mentioned, after becoming red, loses its cilia, assumes the still form, and sinks to the bottom. The same is the case with the Protococcus of the salt-pans of Bombay; but instead of adhering to the salt, it seeks out and settles upon the crystals of carbonate of lime that are among those of the salt. The chlorophyll changes from green to red also in some of the resting spores of the confervoid Algæ, as in Sphæroplea* and in Protococcus pluvialis, where also in both it becomes green again on germination, which led Cohn to state that the green colour is connected with "vegetation" or the early part of the existence of the individual, and the red with "fructification" or the terminationt. So that, altogether, the passage of the colour from green to red in the filament seems to be more likely than the opposite.

Thus, as the evidence regarding Trochodesmium in the seas above mentioned is more, if anything, in favour of its yellow than its red colour, and that it is also sometimes green, while, in the common course, where algae present red and green colours in their respective cycles of existence, the latter appears first, and the Peridinium above mentioned passes from green to yellow and then to red, &c., it seems not unreasonable to infer that Trichodesmium Ehrenbergii does the same, and that, therefore, so much of Montagne's generic characters of Trichodesmium Ehrenbergii as relate to its colour (viz., that it is "at first red and at length green") should be reversed.

If it were desirable to adduce evidence of the faint green colour which Trichodesmium probably presents in the first stage of its existence, from the observation, too, of probably Colin, Ann. des Sc. Nat.,' 4 sér., t. v, p. 187. Ray Soc. vol. for 1853, p. 519.

the same organism in other parts of the world, one might cite those of Péron, who likens it to "poussière grisâtre," and of Darwin, who compares it to "cut hay," &c. (op. cit.); but it seems better for this argument not to go beyond the seas washing the shores of Arabia.

To what the "intense green," under which this organism sometimes presents itself in the Red Sea, owes its production I am ignorant, unless it be indicative of sporidification, which, from what I think that I have seen in Oscillatoria princeps, seems to take place in this family, not from the conjugation of its cells, but from the division of their contents into zoospores. Much, therefore, remains to complete the history of this little plant; and this, unfortunately, can only be obtained by watching it long and narrowly in its proper habitat.

TRANSLATIONS.

On the CONTRACTILE FILAMENTS of the CYNAREA (Thistle Tribe). By Dr. F. COHN.

(From the Zeitsch. f. Wissensch. Zool.,' xii, p. 366.)

THE following observations are contained in a letter in the above Periodical addressed to Professor Von Siebold.

After referring to the circumstance that he had already on a previous occasion noticed, in a communication to the same correspondent, the most important facts relating to the contractile filaments in plants belonging to the thistle tribe, Professor Cohn proceeds to remark that in the Cynareæ the five filaments are inserted into the tube of the corolla, and support at their extremities the anthers which, as in all the Compositæ, are conjoined into a complete tube.

At the time of flowering, this anther-tube is closed at the end, and envelopes the pistil which arises at the base of the corolla from the inferior ovary.

At this period the anther-tube rises about 4 mm. above the summit of the corolla. When touched, pollen-masses are extruded from its apex, and at the same time the tube exhibits a peculiar twisting movement.

After about five minutes the experiment can be repeated; the pollen is again forced out of the tube, and the twisting movement will be again witnessed.

Gradually, however, the pistil rises above the summit of the anther-tube, and in proportion as it does so the irritability diminishes, until at length, when the stigma projects 4-5 mm. beyond the anther-tube, that property ceases to be manifested at all.

But it is not till this time, when its lobes begin to divaricate, that the stigma becomes capable of impregnation.

In general, not more than twenty-four hours at most elapse from the beginning to the cessation of the irritability, and frequently the space of time during which it exists is still shorter.

COHN, ON THE CONTRACTILE FILAMENTS OF THE CYNAREÆ. 187

In many Cynarea, when the irritability is not manifested, this will be found to arise from the circumstance that the flowers have been examined at too late a period. As a rule, it may be said to be too late when the stigma is visible above the anther-tube.

As is well known, the cause of these phenomena resides wholly and solely in the filaments, which each time they are touched instantly contract, and after a while extend themselves to their original length. The expulsion of the pollen from the anther-tube depends upon the circumstance that the tube, as the filaments shorten, is drawn downwards on the pistil about 1-2 mm., and is afterwards pushed upwards again. The contractility of the filaments is shown in the most interesting manner in preparations in which nothing but the anther-tube is left, and in which the five filaments have been cut away from the corolla, and thus rendered free to move independently. Under these circumstances they exhibit the liveliest irritability whenever they are touched; retracting themselves, bending and twisting out, and again becoming extended, and then bending over on the opposite side, twining themselves together, &c., so that it is hardly possible to escape the impression that we are witnessing the movements of a Hydra, and not those of any part of a plant.

Professor Cohn has, on a former occasion,* pointed out the laws by which these motions are regulated, and the conclusions he then arrived at have since been confirmed by the further observations of Kabscht and of Unger.‡

He has shown that the contractile filaments were energetically affected by the electric current; contracting instantly under a feeble current, but again extending themselves after a time, and then again manifesting irritability.

A powerful current kills the filaments instantly; the consequence of which is that the contractile filaments do not again extend themselves, but, on the contrary, continue to contract more and more, until at the end of about an hour they are not more than half their original length.

When killed by other means, as, for instance, by immersion in alcohol, glycerine, or water, a similar shortening of the filaments to less than half their original length is observed; it is clear, therefore, that this contraction cannot be due simply to a shrinking, from desiccation. It may also be

* In a paper in the Abhandl. d. Schlesisehen Gesellschaft. f. Vaterl. Cultur,' 1861. (An abstract of this valuable paper, by Dr. Arlidge, will be found in the Annals of Nat. History' for March, 1863.)

+ Botanisch. Zeitung,' 1861.

Ibid., 1862.

remarked that after spontaneous or natural death the filaments contract to the utmost.

Although, eventually, the pistil may project about 5 mm. beyond the anther-tube, this arises in the smallest possible degree from the growth of the pistil itself, after the flower has burst. The true cause of the apparent elongation is the retraction of the anther-tube by the shortening of the filaments after their death, and in consequence of which the tube will at length be found to 1 mm. below the summit of the corolla, above which, a few hours before, it had proprojected 3-4 mm.

Having a short time since obtained a new microscope by Hartneck, Professor Cohn determined to investigate the anatomical changes undergone by the contractile filaments in their contraction.

In order to examine the filaments in the elongated irritable condition, it is necessary, first of all, to remove the air with which certain passages in the internal tissue are partially filled, and owing to which the transparency of the tissue is much impaired.

The air may be removed by placing upon a filament surrounded with water, a covering-glass, upon which the objective is screwed down with moderate force, and so as to subject the filament to a slight compression. The object is then to be pushed for its entire length under the objective. By this means the whole of the air is, as it were, squeezed out, and its place supplied with the water or glycerine, as the case may be; and the internal tissue covering the epidermis readily brought in view.

O

O

FIG. a.

O

FIG. V.

The tissue of the filament consists of a central vascular bundle, containing principally annular and closely wound spiral vessels, and surrounded by rows of elongated cylindrical cells, placed one above another, and separated by straight, transverse dissepiments. (Fig. a.)

Externally the filament is covered with an epidermis composed of similar cells, which, on the upper side, are thicker and convex, so that the filament appears, as it were, to be grooved.