Midland Naturalist/Volume 01/Freshwater Algæ (3)

by Arthur Winkler Wills

Note: Continues from Midland Naturalist/Volume 01/Freshwater Algæ (2) - page 189 is missing from the scan.

Conjugation, as the term implies, consists of the yoking together of two contiguous filaments which, by some mysterious means, approach one another and assume a position of strict parallelism. Projections are then thrown out between opposite pairs of cells, and gradually increase till they finally meet and form connecting tubes. At the same time the endochrome loses its spiral arrangement, and becomes an irregular, confused mass, [Plate III, Fig 14.] It then passes, as in Zygqæma [Plate III, Fig. 12] and Spirogyra, [Figs. 13, 14] into the opposite cell and there, mingling with the contents of the ladler, forms a round or oval spore With distinct cellulose coatings; or, as in Mesocarpus [Fig. 15] and Stauroearpus, [Fig. 16], meets the contents of the opposite cell, which move forward to join it, in the connecting tube, and there forms a spherical or cruciate spore.

A curious modification of this process occurs in some species of Spirogyra, where the spares are formed not from the contents of two opposite cells of different filaments, but by the union of those two contiguous cells of the same filament, the mingling of which is effected through a little tube bridging over, as it were, the septum between them, [Plate III, Figs. 17 and 18,] It is asserted by some writers that this phenomenon is abnormal, and occurs in species which usually conjugate in the ordinary way; but the writer has only once seen the two processes occurring simultaneously in the same plant, and has always observed this form of conjugation in specimens the proportions of which stamp them as distinct species.

The most striking point about the operation just described is the assumption by the contents of the cells of different plants, or by those of special cells in the same individual, of the opposite properties upon which depend respectively the powers of imparting and receiving fertilisation, although the most careful scrutiny under the highest powers of the microscope fails to reveal the least difference in their condition. It has been stated that this polarisation, as it may fitly be termed, in the ordinary form of conjugation, is capricious, the cells of the two filaments assuming indiscriminately these converse functions, but in the many hundreds of specimens which we have examined and mounted, we only remember finding one exception to the rule that all the cells of one conjugating filament assume "male" and those of the other "female" sexual functions; this exception occurred in the specimen already referred to, in which conjugation of contiguous cells of the same individual also took place, and in this ease the spores formed in one filament were large, while those in the other and alongside of the cells which had discharged their contents were much smaller, and apparently imperfectly developed.

It now remains to answer the two last questions which we proposed in the outset, viz.:—How are the Algæ best collected? and how should observations on their structure, &c., be recorded?

The larger filamentous Algæ are best brought home in small glass tubes of thick glass well annealed.

A compact form of collecting apparatus consists of a number of pieces of strong class a couple of inches square. t» each of which is cemented with gold size or marine glue an indiarubber ring about one-eighth of an inch thick. These, when piled on one another, and held together by indiarubber rings, take up but little space. Lastly, for these coarser plants nothing answers much better than to screw them up in bits of strong paper and bring them home in a wide-mouthed bottle, tin box, or even loose in the pocket. A specimen need, at any rate, never be left behind for want of a more elaborate vasculum.

The Desmidinceæ require more care, and the gathering should be transferred with as little shaking as possible to one of the glass tubes which should he filled with water.

It is a useful plan, when out for a long walk, to number the specimens. and note down their exact habitat in a pocket book. Some years ago the writer returned from a five and twenty mile walk across the Welsh Mountains, with some fifty "dips" of all sorts, Next day was devoted to their examination, and in one tube, among a quantity of common species, were found two frustules of Dogidium rodgsum, a Desmid hitherto recognised as exclusively an American species, but which has been since found, we believe, by Mr. Archer, in Ireland. Unfortunately. no such record as we suggest had been kept; and, although the writer stated off next morning at daybreak, took exactly his previous route, and searched sedulously till nightfall in every tiny pool in which he remembered dipping in his previous ramble, not a trace of the new plant was found.

The specimens being brought home, each should be transferred in turn to a small saucer, or water-glass, and portions of it examined under a convenient power, generally about half an inch. If any new species is spotted, it should be set aside for mounting, duly labelled temporarily; but, if the gathering seem to contain nothing but old friends, it is a useful plan to give it a parting squeeze between the fingers, and catch the dippings in a watch-glass, Small Desmids and Diatoms, previously entangled, are pressed out in this way, and new species often reward the examination. The squeezed mass should not be thrown away till the washings have been searched over. It often pays to repeat the process.

A specimen should never be thrown away because it is a poor one, or consists of one individual. where a dozen would be acceptable. The rarest plants are naturally often met with singly, Once, in examining a mass of very dirty stuff from a Welsh bog, the writer pounced in his first dip on the rare Desmid, Micrasterias radiosa. Rashly concluding that there were sure to he plenty more, he swilled back the contents of the slide into the mess. But dip after dip, and washing after washing, were examined in vain; end, as the species was too rare to be lightly lost, it cost the work of two long nights to haunt over the entire mass, drop by drop, till the individual plant, whose diameter was about 1-140th of an inch, was recovered.

We say, therefore, mount a poor specimen rather than none. If you find a better you can throw it away or give it to a friend, who will value it.

It is worth adding that the Desmids are easily separated from other plants by shaking the gathering up in a tube and pouring the whole into a watch glass, when, from their higher specific gravity, they sink to the bottom, and, by a little careful manipulation, may generally be recovered quite lean and free from dirt.

The methods of mounting the various classes of Algæ we reserve for discussion in a subsequent paper. For the present let it suffice to state that, by attention to certain indispensable details of manipulation, these plants may be preserved for indefinite periods.

A few words, in conclusion, on the subject of recording observations made under the microscope. All Algæ, however carefully mounted, do, in course of time, more or less lose their exact form and natural appearance. Hence it is most desirable to supplement their collection by sketches in pencil, pen and ink, or other material, made from the living plant. Various ways of doing this are recommended, but we have found the use of what is known as a neutral-tint reflector, to be by far the easiest and most effectual. This apparatus consists simply of e piece of the thinnest possible microscopic glass, fitted into a cap, which replaces that of the ordinary eye-piece, and holds the glass-plate at an angle of 45° to the axis of the instrument. It is, in fact, a miniature Pepper's ghost arrangement; and the microscope being clamped in a horizontal position and focused, upon looking down upon the glass-slip the observer sees the image of the object reflected to the eye, but apparently at a distance below the reflector equal to that between it and the object itself. By placing a sheet of paper on the table underneath, and adjusting the relative illumination of the object and the paper, so that the point of a pencil is clearly seen on the latter, it becomes easy, with a few hours' practice, to trace the smallest details, In practice, it is best, however, to obtain an accurate outline in this way, and the spirit of the object is better given by filling in details from direct vision in the microscope.

This method gives a drawing perfectly accurate, and to a scale easily ascertained.

Many of the descriptions given in Hassall's "Freshwater Algæ," the only systematic work upon this subject in the English language, are wholly useless from the absence of all discussions, and it is impossible to decide whether the species described are really distinct ones or are needlessly multiplied, by reason of this vital defect. It is of primary importance that dimensions should be in all cases recorded in fractions of an inch or in millimetres; and the following method of ascertaining than at a glance will be found simple and satisfactory.

A plain circle of strong glass may be obtained from any optician, of size to drop into the focal point of the eye piece, and ruled into squares, conveniently of 1-50th of an inch. A glass slip ruled into thousandths of an inch is placed on the stage, and each of the objectives in ordinary use, say from ¼ to 2 inch, being screwed into its place in turn, it is only necessary to observe which divisions of the ruled eye piece correspond with the actual thousandth-lines in the slip to obtain a gauge of the dimensions of an object corresponding to each division of the eye pieces.

For example: the slip being so arranged that the left hand 1-1000th line corresponds accurately with one of those on the eyepiece, suppose that the 5th line of the eye-piece corresponds with the 9th 1-1000th line from the left on the slip. Hence five divisions of the eye-piece = 9-1000ths or 1 division = 9-500ths = 1-555th of an inch. In this manner the actual value of one division of the eye-piece for each power should he registered once for all: it is then the work of a moment to record the actual dimensions of any object under examination.

If such records were kept systematically by observers, a comparison of their observations would go far to facilitate a harmonious classification of a huge and beautiful tribe of plants, the bibliography of which is at present most unsatisfactory, and has suffered grievously from a needless multiplication of supposed species, consequent upon imperfect observations still more imperfectly recorded.

The object of this paper will he amply fulfilled if it induces some of the Naturalists of the Midland counties to engage in the study of these minute organisms, and to do their share towards elucidating the many points in their physiology which are still obscure.

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