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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,^[1] and is at all times a