Midland Naturalist/Volume 01/Freshwater Algæ
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Freshwater Algæ.[1]
By A. W. Wills, F.C.S.
The objects of the "Midland Naturalist" are, like those of the several societies in the Union which it represents, at least threefold, viz.:—1st, to record such original researches and observations as their members may be fortunate enough to make; 2nd, to facilitate a complete a record of the flora and faunas of the several districts of the Midland Counties; and lastly, to enable lovers of Natural History to hold out a helping hand to one another by pointing out how others may pursue most profitably such branches of study as they have themselves specially affected, and by communications to one another such hints as are derived from their individual experience, whereby success may be ensured, time saved, or gaps in special lines of investigation filled up.
The present paper fails within the scope of the last division; it records little or nothing which is original; its object is to invite attention to a branch of microscopical study in which there is ample scope for observation, and to point out in what fashion the examination of the great group of plants known as the Freshwater Algæ may be successfully begun and carried on.
In doing this it will be convenient to ask and briefly to answer the following questions:—What are Freshwater Algæ, and why will they repay patient investigation? Where must they be sought? What are the general features of their stricture, modes of reproduction and morphology, and the characteristics of the principal groups into which they are divided? How are they best collected and preserve? And how should observations be recorded?
1st—Freshwater Algæ are minute plants, mostly requiring considerable magnifying power to reveal their structure, belonging to that great group of waterweeds included in the somewhat loose term of Confervoidoæ or Chloraspermeæ, of which the coarser green weeds of the sea shore are the most familiar examples.
The imperfection of a classification by reference to the colony of these plants or of their spores will become evident from the outset; it must be accepted only as a rough division by which the majority only of the genera are covered.
The freshwater species are far more delicate in texture and various in form than the marine ones, and comprise, in addition to those larger filamentous kinds which everyone would recognise as first cousins of the familiar green seaweeds, an immense number of minuter forms, the relationship of which to their more robust kindred is not so evident, and at least two vast groups, totally different in external aspect, rich in the tenderest colours and midst exquisite shapes.
But the whole order is especially attractive to the Botanist, not only by reason of its singular gracefulness and beauty, but because in no other can he watch so easily the mysterious fundamental processes of cell-division and of reproduction. "To penetrate everywhere to these first rudiments of structure, to follow out from them the course of the development of the tissues of all parts, and to make out the laws according to which the ¢ell-formation progresses to produce the various arrangements on which the structure of the plant essentially depends, is one of the most different, but at the same time most profitable tasks."[2]
The facility with which these plants can be kept alive for a length of time—often long enough to enable the observer to trace in one individual its entire life-history—the translucency of their coll walls, which lays open to his observation under the microscope the active processes going on within; the many points in their morphology still awaiting solution, and the comparative ease with which they may be preserved for an indefinite period with little loss of their natural form; all these are grounds upon which they possess a high degree of interest, and challenge a more extensive study than they generally receive.
2nd.—Where are the Freshwater Algæ to be sought?
One is almost tempted, from their universal diffusion, to reply, "everywhere." It is, at any rate, safe to answer, "wherever moisture oy fresh water is to be found—on the pots and walls in a greenhouse, on the shady sides of tree-trunks, on damp banks, on the moist faces of old walls, in the dripping from water-taps, in every ditch, in the hoof-holes where cattle have trodden tu marshy ground, on thatched roofs, in hogs, on moist moorlands; above all, in every clear pool, lake, and mountain tarn, in cold springs and hot springs, flouting on the surface of water wherever it is found, clinging as parasites to submerged roots, sticks, or larger water-plants, or entangled among bag mosses and the like.”
In describing the principal families we shall revert to their habitats and give some hints as to the signs by which their presence may be recognised.
3rd.—Let us now proceed to consider briefly the principal orders into which they naturally fall. Omitting, however, for convenience, that vast group of minute brittle siliceous organisms, the Diatomaceæ, whose vegetable character was so long disputed, and is not even now universally admitted, with which every Microscopist is familiar, inasmuch as their amazing variety of form and the great beauty of their sculptured markings lave long caused them to stand foremost among the preparations of dealers and the objects of popular exhibition.
We begin, therefore, with the large and universally distributed division of Unicellular Algæ, which form (with the exception of the Desmidiaceæ) the least conspicuous, but by no means the interesting member of the family, by reason of the facility which they afford for studying the phenomena of indefinite increase by cell-division.
The obscurity of the characters of these minute forms has led to a reckless multiplication of supposed genera and species; indeed there is little doubt that some of these really represent different stages in the life of the same species or even early stages in the development of higher cryptogams.
The simplest, and at the same time the most widely distributed plant in this order, probably the most universally distributed of all plants, is Chlorecoccum vulgare, the humble organism to which the green colour of the bark of most trees, of old palings, gates, &c., is due. It consists of myriads of minute spherical cells, from 1-1500th to 1-2000th of an inch in diameter, [Plate I., Fig. 1,] in which the process of division of each original cell into two, four, or eight secondary ones, is admirably exhibited. This appears to be the only process by which this species is multiplied, but in that which we shall next consider we shall trace the addition of a further mode of increase which prevails, with some modifications, through many of the more highly organised families of Algæ. This plant, probably allied to the Protococcus pluviatis of some writers, appeared some time ago in a freshwater aquarium, and consists of innumerable spherical cells, from 1-1250th to 1-2000th of an inch in diameter, the outer coating of which takes the characteristic blue tint of cellulose when treated with iodine. The inner granular contents are sometimes bright red and at others green. Both forms exhibit the phenomenon of cell-division, giving rise sometimes to a still or motionless progeny, and at others to active zoospores, which move through the water by means of pairs of cilia, or by a single cilium. The relations of these two forms are somewhat obscure, and we have not been able altogether to reconcile our own observations with the statements to be found in books upon this subject. The red cells, at any rate, are capable of lying dormant in a dry state for long periods, the active processes of subdivision being re-established when water is added. Plate I., Fig. 2, represents some of the conditions of this plant. Protococcus pluvialis has been elaborately investigated by F. Cohn, whose research is translated in the Ray Society's volume for 1853.
We also have sketches of two kindred species, belonging respectively to the genus Urococcus, Huss., [Plate I., Fig. 3] and Protococcus [Plate I., Fig. 4,] From these simplest forms there is an easy transition to such genera as Tetrespora, in which numerous green cells are arranged in a continuous gelatinous frond, in groups which show very beautifully their repeated sub-division into two and again into four, whence the generic use. Tetraspora lubrica [Plate I, Fig. 5] is abundant in summer in little streamlets among boggy ground in Sutton Park,[3] and is at all times a pleasing object, especially when viewed by dark back-ground illumination.
A farther step towards mere complex structure is traced in the nearly allied Enteromorpha intestinalis, which, in late summer, forms pale green tubular fronds, often a foot long, in similar habitats, and is found in great abundance in warm pools in the "Black Country." Like Tetraspora, it is a charming object, especially in a young state, and it possesses further interest, as connecting the marine and freshwater floras, being an inhabitant of salt marshes also, and loading up to several species which are exclusively marine.
To the same order as Protococcus belongs that most strange and beautiful of all the lower vegetable organisms, Volvox globator, to which the limits of this paper do not allow more than a passing reference. It must suffice to remark that the analogy between Volvox and Protococcus will be more easily realised by conceiving a number of the green cells of the latter in that motile condition thickly and symmetrically embedded in a larger glassy sphere, with the cilia only protruded.
Volvox should be examined in cells shallow enough to prevent its movement, and by transmitted light, in order to observe its structure; but its most lovely aspect is when it is viewed by powerful dark back-ground illumination in a cell sufficiently deep to allow its exquisite pale green crystal spheres, will their miniature Volvoces within, to maintain their swift and graceful rolling motion across the field of the microscope under low powers, when it becomes an object of matchless beauty.
It is not an abundant plant in this neighbourhood. The ruthless and wanton invasion of Sutton Park by a railway, and consequent filling-up of the well-known "Webb's Stows," have destroyed a constant habitat of this as of several other choice plants, as well as the first known locality for the beautiful Rotifer Melicerta Tyro, discovered there last year by Dr. Hudson.
The distribution of Volvox seems to be capricious, for it often appears in sufficient quantity to render the water pale green, then disappears as suddenly, and perhaps only reappears months or years afterwards,
There are, however, situations in which it appears every year, and where good specimens may be obtained by straining a few pints of the water through muslin, and so, as it were, concentrating the spheres of Volvox, which may then be washed off into a small bottle. If it is desired to keep this plant in cultivation, all Entomostraca must be carefully removed with the pipette, as they devour it greedily.
For details of its structure and mode of reproduction the student should refer to the researches of Busk, Colin, and Williamson, of which a slight summary is given it the "Micrographic Dictionary."
Before passing from the Unicellular Algæ, we must glance at one other large order, which surpasses all others in variety and singularity of form, in perfection of symmetry, and in brilliance of colour—the Desmidiaceæ.
Fortunately they are widely distributed; it is rather the exception than the rule to take up a dip of water from a pool or marsh in which some of the brilliant crescents of Closterium or the sculptured discs of Micrasterias do not delight the eye. The most distinctive feature in their appearance is the perfect bilateral symmetry of the two halves into which each plant is generally divided.
The fronds of Closterium are more or less crescent-shaped, from the slightly curved form of the Tartar bow to the complete crescent form of the young moon; in Cosmarium, Enastrum, and Micrasterias they consist of thin discs of a more or less oval or oblong shape, deeply constricted in the middle, and with their edges cut, crenated, or sinuated into forms of exquisite beauty and endless variety; while in Staurastrum and Nanthidium they assume a triangular aspect or have their edges adorned With spines or other appendages.
A glance at the figures in Ralfs' "Desmidieæ," at Plate X. in the "Micrographic Dictionary," or better still, at a few specimens in a friend's microscope, will give the beginner a better idea of their characteristic appearance than any description, and enable him at all times to recognise them among his own gatherings.
These plants rejoice in peaty bogs, where they occur either scattered here and there among larger plants, or in thin film encrusting their submerged stems; or floating in delicate clouds in the recesses of shallow pools, where the eye only detects them when it has become accustomed to the dim light by steadily gazing info the water for some minutes.
From such positions Desmids are best removed by carefully passing a watch glass under them, and raising the contents with slow and steady motion to the surface. Many fine specimens may be obtained also by squeezing out the water from handfuls of clean Sphagnum moss into a shallow basin, allowing a few moments for the plants to settle to the bottom, and then pouring off the surplus water, and transferring the greenish residue to a tube.
Some of the best habitats in Sutton Park have been destroyed by the railway and by drainage, but some of the commoner species, as Micrasterias denticulata, Penium digitus, Closterium acerosum, C. Diane, &c., and the filamentous species, Hyalotheca dissiliens, are sometimes to be found in tolerable abundance in boggy ground by the side of the streams. One morning's search in any bog on a Welsh moorland will, however, yield more and rarer species than any amount of hunting in this district.
It is worth adding that Desmids flourish for years in cultivation in small bottles, exposed to the light of a north window, and their growth can thus be watched de die in diem.
The singular modification of cell-division by which they increase will be at once understood from the accompanying figures of Microsterias rotata and Cosmarium colatum, sketched from specimens in the cabinet of the writer. [Plate I., Fig. 6.]
This work was published before January 1, 1930, and is in the public domain worldwide because the author died at least 100 years ago.
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- ↑
Description of Figures.—Plate I.
Fig. 1.—Chlorococcum vulgare, from the bark of an ash tree, showing subdivision of cells info groups of two, four, &c.
Fig. 2.—Protococcus—Species allied to P. pluvialis, from a freshwater aquarium, showing subdivision of cells and formation of motile forms.
Fig. 3.—Urococcus sp? Hass., from damp wall of a greenhouse.
Fig. 4.—Protococcus sp?, from wet rocks at Church Stretton.
Fig. 5.—Tetraspora tubrica, from bogs in Sutton Park.
Fig. 6.—Micrasterias rotula and Cosmarium cœlatum, from bogs in Sutton Park, showing increase by cell-division.
- ↑ A. Braun. "The Phenomenon of Rejuvenescence in Nature," p. 123, (Ray Society, 1853.)
- ↑ Sutton Park is a property of some 2,500 acres, held under an ancient Charter by the Corporation of Sutton Coldfield, for the benefit of the inhabitants. It comprises woods, moor land, large sheets of water, and clear streams, and, being only seven miles from Birmingham by rail, is the “happy hunting-ground" of the Naturalists of the town and district. Many of its rarer plants and animals have gradually disappeared before the vast numbers of visitors who now frequent it, before recent "improvements,” and still more before the railway works which have sorely mutilated the beauty of some of its most charming parts.