Midland Naturalist/Volume 01/Freshwater Algæ (2)

by Arthur Winkler Wills

Freshwater Algæ.^[1]

 

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By A. W. Wills, F.C.S.

 

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(Continued from page 117.)

We must now turn to the very important question of the modes of multiplication and reproduction of these singular plants.

In the simplest forms of unicellular Algæ, (Palmellaceæ and Volvocineæ,) the only mode by which most of the genera seem to be perpetuated consists of the conversion or breaking up of the contents of certain cells into small oval bodies, (zoospores,) which move freely through the water for a While by means of delicate lashes or cilia, and then, after lasing these motile organs, undergo cell-division into fresh individuals, identical with the parents, [Plate I., Fig. 2.)

In Desmidiaceæ, to this process is superadded that of reproduction by the formation of large "resting-spores" resulting from the commingling of the contents of two separate individuals in the space between their adjacent sides. [Plate II, Fig. 7.] Such spores ore capable of retaining their vitality for a long period, and then producing from their contents a fresh generation of plants of their specific kind; while, on the other hand, zoospores undoubtedly perish, unless placed in conditions favourable to their immediate germination.

In many of the higher order of Algæ (freshwater and marine) both these phenomena can be traced, and if appears probable that in all cases the development of spores alone constitutes a tine sexual reproduction of the species, comparable to that which prevails among Phanerogams, whereby its indefinite perpetuation is secured; while the so-called "reproduction" by zoospores is rather to be compared to the increase of flowering plants by buds or offsets, or artificially by cuttings, a process by which, as is believed by most florists, while the multiplication of individuals is accomplished, the continuation of the species is not secured beyond certain limits.

It is probable that even in those genera in which no such sexuality or polarisation of cell-contents has been observed, it has yet to be discovered, much as the fine researches of Messrs. Dallinger and Drysdale "On the Life History of Monads" have proved, that after the succession of many generations of asexual individuals among the lowest Infusoria, a sexual process suddenly intervenes.

The production of spores in Desmidiaceæ is a most interesting phenomenon, and will be best understood by those to whom it is not familiar by reference to our sketch. (Plate II., Fig. 7]^[2] Two individuals approach one another, and a communication being established between them through the bases of their separated halves, the cell-contents are poured out into the intervening space and there form a spore, generally spherical and at first smooth, but in the mature state often ornamented with tubercles or spines. In this condition the structure and markings of the empty cellulose cases are beautifully displayed.

What is the exact series of changes by which the original form is developed from the spore requires further investigation, in spite of Berkeley's statement that it is propagated in some species by division, exactly after the fashion of the ordinary cells, and in the third generation acquires its normal form.

Other points in the physiology of these plants also await further research, as, for instance, the nature of the curious circular hyaline spaces filled with moving granules, which are seen in the ends of the frustules of Closterium and Penium; the circulation of similar particles between the cell-walls and the endochrome; and the remarkable "swarming" of the entire cell-contents, which occurs in some genera, probably before conjugation, when every grain of endochrome seems to be rushing wildly in and out among its fellows in a sort of giddy dance.

Passing from the unicellular to the filamentous Algæ, which consist for the most part of elongated cylindrical cells placed in juxtaposition, and to end, in various arrangements, we are compelled by want of space only to glance at many large and interesting orders. Among these are the Batrachospermeæ, represented by the exquisitely graceful genus Batrachospermum, whose gelatinous tresses of tender grey or olive green whorled branchlets are not uncommon in clear streams in this district; the almost equally beautiful Chætaphoraceæ, of which abundant examples are to be found in the deep green gelatinous masses of Chætophora endiciæfolia and C. elegans, in the tender preen filaments of Draparnaldia plumosa waving gracefully in clear running waters, and in the singular disk-shaped fronds of Coleochæte scutata, found adherent lo leaves of submerged aquatic plants; the Oscillatoriaceæ, which comprise many singular forms, inhabitants of pools, wet rocks, damp ground, &e., and well represented in our district; and finally the Nostochaceæ, of which the typical genus Nostoe contains many species, consisting of long beaded filaments. twisted like intricate coils of rope, and agglomerated into gelatinous fronds ranging in size from that of a pea to that of a walnut, most cosmopolitan in their habitats. rejoicing in situations so various as clear streams, exposed moorlands, and thatched roofs.

In what remains of this paper we purpose to refer in some detail to three of the remaining groups of Confervoid Algæ, each of which presents interesting points of structure and remarkable aspects of the phenomena of reproduction.

First among those is the large family of Siphovaceæ, of which the commonest examples are the various species of Vaucheria, some of which form a close felt-like coating at the bottom of almost every ditch. They consist of long, more or less branching filaments, not divided into a number of cells, but open from end to end and filled with very dark green endochrome. The cell contents are frequently poured forth from the ends of the filaments in the form of large ciliated zoospores, of which operation a good description will be found in Hassall's "Freshwater Algæ." But there is, in addition to this, a true process of sexual reproduction, as singular as if is interesting,

At various points on the filaments are formed two very distinct kinds of projections—the first consisting of one or more narrow "horns," as Unger appropriately terms them, forming short branches from the parent filament, sometimes straight and at others curiously curved; the second, in close proximity, assuming the form of one, two, or occasionally more, flask-shaped cells, open at their apex. [Plate II., Fig. 8.] In one species, F. racemosa, both kinds of organ are carried upon a common shaft or pedicel. [Plate II., Fig. 9.]

The contents of the flask cells gradually assume an irregular spherical shape, while these of the "horns" are converted into active Spermatozoids, which issue into the surrounding water, and thence swarm through the narrow neck of the flask-shaped spore-cells, and there fertilise their contents. These thereafter assume a true spherical form and a distinct cell-covering, while between them and the filaments which bear them dissepiments are formed, at which the heads finally break off, and by decay of their outer walls at length liberate the spores.

The singular plant Hydrodictyon utriculatum, remarkable for its growth in a network of hexagons, and for the marvellous rapidity of its increase, is referred to the same family (Siphonaceæ.) A good summary of its life-history is to be found in the Micrographic Dictionary, and a detailed investigation is recorded in the researches of A. Braun, (Ray Society, 1853.) Some years ago it suddenly appeared in Blackroot Pool, Sutton Park, in enormous quantity; in a few weeks it wholly disappeared, and we have never seen a trace of it since in that habitat.

The Œdogoniaceæ comprise two genera, Bulbochæte and Œdogonium; the former containing only one species, the elegant little B. setigera, a plant of great beauty by reason of the long slender bristle-shaped cells which form the ends of its lateral branches—a lovely object, especially by dark back-ground illumination.

It is said to be reproduced by zoospores, and also by a process very similar to that described above as occurring in Vancheria, each resting-spore being ultimately resolved into four smaller ones, which develop into four new plants. We have not been fortunate enough to witness these phenomena, although we have frequently gathered the plant, both in our own district and elsewhere. The genus Œdogonium includes several common species, some characterised by a very curious annular structure at the base of many of the cylindrical cells of which the plant is made up, which becomes especially manifest when the filaments break up into separate joints for the emission of zoospores. [Plate II, Fig, 10.) We ware ounce fortunate enough to see this curious process taking place under the microscope in a specimen which had been just mounted, and think that a brief description of one of the most remarkable sights we have ever been privileged to witness, taken from notes made at the moment, and illustrated by sketches made from the specimen in question, will prove more than usually interesting,

“At 12 20 this afternoon, on placing the slide under the microscope, I observed that one of the long filaments was breaking up rapidly into a zig-zag of separate joints, each of which remained attached only at a single point to its neighbour, Placing one cell in the middle of the field, I watched it closely, and observed that the somewhat oval muss of endochrome was gradually leaving the cell-case, though clinging to its base us if reluctant to leave it, At the same time there appeared a very faint transparent membrane across the mouth of the open cell, exactly like a thin soap bubble being blown out of it. This quickly increased in size pari passu with the extrusion of the endochrome, At 12 25 this had completely left the cell and formed a spherical green rags inside the "bubble," in fact an incipient zoospore, This now began to move very slightly, and at one side of it appeared faint traces of cilia waving a very little, suggestive of a piece of machinery just getting started, At 12 30 the "bubble" was munch blown out, and the cilia active, the zoospore twirling round for put of a revolution and then stopping, and so on. At 12 35 the membrane was more enlarged, and only faintly discernible by very careful illumination, At 12 46 the zoospore was liberated; the ciliated end slightly protruded and transparent; its motion regular and swift, At 12 47 this motion was much diminished, the zoospore slowly creeping round with an irregular motion from left to right. At 12 50 the ciliary action had become sluggish, and the endochrome was receding from the cell wall. The vibration of the cilia gradually diminished till 1, when it quite suddenly ceased altogether; at the same moment the whole mass of endochrome was violently convulsed with a sort of shuddering movement, and the transparent point was much protruded. By 1 3 the cilia had disappeared, the projection of the point was largely increased, and there is no doubt that if the germination had not been arrested by enclosure in the glass cell, the next stage would have been the formation of a distinct nucleus, and the separation through this into two cells, in fact the first process in the development of a filament like the parent one.”

How enormous is the rate of increase in this plant may be inferred from the fact that in one filament in this specimen, both ends of which were broken, the number of cells still remaining was 683.

It will be seen from Plate II, Fig. 10, that each zoospore is the product of the endochrome of one sell only.

The formation of true spores in Œdogonium is the result of the fertilisation of the contents of certain cells by the entrance through small slits in their margins of spermatozoids; these being produced either from other cells in the ordinary filaments or otherwise in small "dwarf male plants," which appear to be developed from the zoospores above described, and of which numbers may be often seem parasitic upon the larger plants, [Plate II. Fig. 11.]

"To be continued."

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1. Description or Figures in Plate II.

   • Fig. 7.—Formation of resting-spore of Micrasterias rotata, from contents of two plants.
   • Fig. 8.—Vaucheria ornithocephala.
   • Fig. 9.—Vaucheria racemosa, showing "horns" and flask-shaped cells, the latter containing spores.
   • Fig. 10.—Formation of zoospores from contents of single cells inŒdogonium.
   • Fig. 11.—Formation of spores in Œdogonium, the contents of the enlarged cell being fertilised by spermatozoids developed in dwarf parasitic male, or antheridial plants.
2. The figures given in these papers are all drawn under the microscope, from natural specimens. We do not hesitate to insert them, because those given in text books are often only diagrammatical, and convey but an imperfect idea of the actual object.