Fig. 2. A, Species of Gleocapsa, one of the Palmelleæ, in various stages. A becomes B, C, D, and E by repeated division. Magnified 300 diameters.
Following the classification adopted by Professor Harvey, which is that generally employed in English systematic manuals, we divide the order into three sub-orders, named from the prevailing colour of their spores. 1. Chlorospermeæ, with green spores; 2. Rhodospermeæ, with red spores; and 3. Melanospermeæ, with olive-coloured spores. The entire plant in the first group is usually grass-green, but occasionally olive, purple, blue, and sometimes almost black; in the second it is some shade or other of red, very seldom green; and in the third, while generally olive green, it is occasionally brown olive or yellow.
The Chlorospermeæ are extremely varied in form, often threadlike, and are propagated either by the simple division of the contents of their cells (endochrome), by the transformation of particular joints, or by the change of the contents of the cells into zoospores, which are cells moving freely in water by means of hairlike appendages. In their lower forms they are among the most rudimentary of all plants, and thus of special interest physiologically, as representing the component parts of which higher plants are formed. They are subdivided into twelve groups, as follows:
The first group, Palmelleæ, are unicellular plants, the cells of which are either free or surrounded by a gelatinous mass, and they are propagated by the division of the endochrome. One of the most remarkable of the species of this family is Protococcus cruentus, which is found at the foot of walls having a northern aspect, looking as if blood had been poured out on the ground or on stones. Protococcus nivalis, again, is the cause of the red snow, of which early arctic navigators used to give such marvellous accounts. ([Fig. 2.])
Fig. 3. A, Fragment of a Filament of Zygnema, one of the Conjugateæ;
B, Closterium; C, Euastrium; two desmids.
The Desmideaceæ, together with the plants of the next succeeding group, are favourite subjects of investigation or observation by the possessors of microscopes, an attention they merit from the beauty and variety of their forms. They are minute plants of a green colour, consisting of cells generally independent of each other, but sometimes forming brittle threads or minute fronds, and are reproduced by spores generated by the conjugation of two distinct individuals. The process of conjugation in Desmids and Diatoms consists in the union of the endochrome of two individuals, each of which in these families is composed of a single cell. This ultimately forms a rounded body or resting spore, which afterwards germinates, the resulting plant not however acquiring the normal form until the third generation. ([Fig. 3.])