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. [Illustration: 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.]