Before proceeding to trace the structure and development of the Algæ, it may be desirable to indicate something of the classification of this curious group of plants. As already stated, they are without exception aquatic plants. They comprise three distinct orders, the Chlorospermeæ, having green spores; the Rhodospermeæ, having red spores; and the Melanospermeæ, having olive-coloured spores. These groups embrace all the varied plants known as sea-weeds, as well as the cellular plants which are developed in fresh water.
The Chlorospermeæ are separable into three groups, namely, those which are simply cellular, including the Palmelleæ, the green Desmidiaceæ, and the yellow-brown silicious-coated Diatomaceæ; those which are filamentous, called generally confervas, and including the true Confervaceæ, in which the threads have no compound axis, the Batrachospermeæ, in which the threads are partially incorporated with an axis, the Nostochineæ, in which the slender moniliform threads are invested with a mucous or gelatinous mass, the Oscillatoriæ, and some others; and those which are foliaceous, comprising the Ulvaceæ. All these are monœcious plants, whose reproductive bodies are zoospores provided with ciliary appendages, or motionless cysts filled with endochrome, true spermatozoids being rarely present.
The Rhodospermeæ divide primarily into two groups defined by the nature and position of their spores: one having the spores indefinite, produced within mother cells; the other having the spores single in the upper joints of the threads of the nucleus. The first group includes the Ceramiaceæ, which are filiform articulate plants, with the nucleus naked, and the Rhodymeniaceæ, which are compound inarticulate plants, with the spores generated within the cells of moniliform threads. The second group includes, amongst others, the Rhodomeliaceæ and the Laurenciaceæ, the former articulate, the latter inarticulate, and both bearing terminal spores, and having the nucleus conceptacular. To this group also belong the calcareous Corallinaceæ and the cartilaginous or membranaceous Sphærococcoideæ. The plants of this group are diœcious, with two kinds of fruit, spores and tetraspores, and they bear antheridia filled with active spermatozoids.
The Melanospermeæ divide into two series, the articulate and inarticulate. The former comprise the Ectocarpeæ, which are filiform plants with external cysts, and the Chordariæ, which are interlaced cylindrical plants with immersed cysts. The latter include the Laminariæ, flat, often strap-shaped, sometimes gigantic plants, having the spores superficial and indefinite, and the Fucaceæ, which constitute a large proportion of the shore-weeds of our seas and estuaries, and which bear their spores in elliptic or spherical conceptacles sunk in the frond. The Melanospermeæ are either monœcious or diœcious, and spermatozoids are general amongst them, though occasionally propagation is effected by means of zoospores resembling the spermatozoids.
Having thus indicated the several groups of the great Algal family, their structure and development will now be traced, commencing with the most simple forms, which occur among the Chlorospermeæ.
Spring water absorbs oxygen, nitrogen, and a large proportion of carbonic acid gas from the earth and the atmosphere, without losing its limpidity, but notwithstanding this apparent purity, if exposed for a time to the sun, green slime appears, and this the microscope shows to be full of globules or vesicles filled with green matter—the primordial cell in its earliest form. No green slime is formed in spring water if kept in darkness, so solar light is the principal agent in this growth, which is by no means a spontaneous birth; it is merely the development of one or more of the many kinds of germs, invisible to the naked eye, that exist in the earth, air, and water in myriads, waiting till favourable circumstances enable them to germinate.
Fig. 6. Palmoglœa macrococca:—A, full grown cell; B-E, successive stages of binary division; F, row of cells produced by a succession of subdivisions; G-I, cells treated by iodine; K-M, cells in conjugation.
The slime that covers damp walls or stones, and moist cliffs or rocks in the sea, also the slime or mucus that sometimes swims on the surface of water, are said by M. Bory de St. Vincent to be provisional creations waiting to be organized. Of this the conferva, Palmoglœa macrococca ([fig. 6]), is an example. It is a green slime covering damp places, consisting of microscopic primordial cells, each of which is surrounded by a gelatinous envelope, and filled with green granular matter occasionally concentrated into a nucleus. This singular plant is propagated in two different ways. The endochrome or green matter within the cell spontaneously divides into two equal parts, the thin coat of the cell bends round the two ends, separates them, then each half takes a globular or ovoid form, and secretes a gelatinous substance round itself which completes the separation, so that they form two distinct and independent plants, in every respect similar to that from whence they were derived. After a little time, each of these plants undergoes a similar bisection, so that four new plants are formed with their gelatinous envelopes; by the same process eight are produced and so on indefinitely, the organ of nutrition being the same with that of reproduction. Again, the membrane or film that covers each of these primordial cells is so thin and soft, that occasionally two adjacent cells of the series unite into one mass by a fusion of their sides and internal matter, which is then coated by a membrane, and after various internal changes becomes a spore which terminates a generation. By and by the spore germinates, produces a green primordial cell which secretes a gelatinous coat, and becomes by the process of bisection the parent of a new generation, which terminates by the union of two adjacent cells to produce a spore, a cycle of alternate modes of reproduction that may be continued till ended by some external circumstance, as the cold of winter.
When the matter in two adjacent cells joins to form a spore, it becomes granular, and mixed with minute particles of oil, which unite in a drop; and the spore, which is at first green, gradually assumes a yellow brown colour; conversely when the spore begins to grow, the oil disappears, and the green matter takes its place. This is a frequent occurrence during the formation of spores in this class of plants, for the endochrome or internal matter,—which consists of a small variety of elements probably in a state of unstable equilibrium or change,—is easily decomposed and recombined into new substances by chemical action, but the bisection of the cells of the Palmoglœa so as to form new individuals is probably owing to heat alone. There is no apparent difference between the cells selected to produce spores by their union, and the others. It seems that in every plant certain cells are reserved for certain purposes. Professor Karsten conceives the nucleated cells to be reserved for reproduction, while those destitute of nuclei are designed for secretion.