Addenda

The following additions will render more complete the directions for mounting Algae which have been given in the Introduction.

The rarer species of Desmidieae are frequently scattered amongst decayed vegetable matter, so that it is difficult to procure good spe-

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cimens for mounting. In such cases a small portion of the mass should be mixed with a little of the creosote fluid, and stirred briskly with a needle. After this has been done the Desmidieae will sink to the bottom, when the refuse should be carefully removed. Successive portions having been thus treated, specimens will at length be procured sufficiently free from foreign matter. Even in ordinary circumstances, if a small extra quantity of fluid be placed in the cell, and the slide gently inclined, most of the dirt can be removed by a needle before the cell is closed, which process will materially increase, the beauty of the preparation.

If the cells are insufficiently baked, the japan occasionally peels off the glass after the specimen has been mounted for some time. To obviate this inconvenience, Mr. Jenner previously heats the cell, with much caution, over a rushlight, until the japan becomes of a dark colour, and vapour ceases to arise from it.

When gold-size is used for closing the cell, the intrusion of some of it frequently destroys valuable specimen, whatever care may be taken: Mr. Jenner has therefore relinquished it, and now employs a varnish made of coarsely comminuted purified shell-lac or translucent sealing-wax, to which is added rectified spirits of wine, in sufficient quantity to cover it. This varnish will be ready for use in about twelve hours; when it is too thick a little more spirit should be added. Mr. Jenner applies three coats of this varnish, and about a week afterwards a fourth composed of japan varnish or gold-size.

I have tried this method extensively, and have never found my specimens spoiled by the varnish insinuating itself into the cell. This process requires less time, and herein it possesses another great advantage over the gold-size method, for the second coat being applied within half an hour, the risk of admitting air into the cell is much diminished.

To preserve the brush in a fit state it should always be cleaned with spirits of wine whenever it has been used.

At page 16 of the Introduction I have stated that I had never witnessed the circulation in the Closteria; but since the printing of that part Mr. Bowerbank has shown me the circulation in Closterium Lunula and in Penium Digitus. It seems, at least in the Closterium, to

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be restricted to the space between the mass of endochrome and the integument; for neither Mr. Bowerbank nor myself could detect it in the internal parts of the endochrome. I at first supposed that the circulation was confined to the margins; nor, did I perceive it elsewhere until Mr. Bowerbank adjusted the microscope and showed me that the motion extended over the whole surface of the endochrome.

The circulation being, as I have just, stated, carried on between the integument and the mass of endochrome, which is usually brought fully into view by the observer, explains the difficulty experienced in detecting the circulation, except at the margins. The motion was very irregular: the fluid flowed at one time towards the extremities, and at an other in the opposite direction, and the intervals between these changes were of uncertain duration. Streams also, though apparently not separated by any partitions, flowed side by side in contrary directions. The currents evidently consisted of an homogeneous fluid; but from time to time minute granules were detached from the internal mass of endochrome and carried along in the stream for short periods, after which they either returned to the quiescent portion or passed into other currents. It seems to deserve particular notice that the circulation was not interrupted at the suture. The process in Penium Digitus was somewhat different from that just described. The flow was continuous from the middle to the extremities, whence the returning streams apparently poured back through the centre of' the mass of endochrome. I ought however to add that, as regards the Penium, I witnessed the movement only in one specimen, and in that but for a short period. I have subsequently succeeded now and then in obtaining with the triplet some obscure indications of the existence of a circulation; but for the exhibition of it with any accuracy or certainty the simple microscope seems altogether inadequate.

I have mentioned in the Introduction many points of similarity between Desmidieae and acknowledged Algae, and as every additional fact illustrating the resemblance tends to confirm the opinion advocated in this work, that the former belong to the vegetable kingdom, I gladly seize this opportunity of stating that Mr. Jenner has recently shown me in a species of Tyndaridea evident, though faint, longitudinal striae, similar to those which are present in many Closteria, and that Mr. Bowerbank has pointed out to me the same appearance in a species of Tiresias.

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Professor Bailey has recently presented me with a collection of American Desmidieae, preserved in Goadby's solution, in small sealed bottles; one of these, filled with specimens gathered in a pond near Princeton, New Jersey, is so rich in species, that I feel assured a list of them will be very acceptable to my readers, since it adds so many species to the great number which have been recorded in this work as common to both countries.