Of the Volvocineæ, by some regarded as fresh-water microscopic plants, the Stephanosphæra pluvialis may be taken as a type. This plant consists of a colourless transparent globe not more than the 48/1000th of an inch in diameter, containing eight green primordial cells arranged in a circle in its equator. Each primordial cell is furnished with a pair of cilia; these 16 cilia pierce through the hyaline globe, and by their vibrations, they make it rotate about an axis perpendicular to the plane of its equator, and move actively through the water. Each of the primordial cells, which are green with a spot of red in the centre, secretes a cellular covering, and they swim about in the interior of the globe as free cells. Eventually they escape either by fissure of the globe, or by its gradual dissolution. After swimming about for a short time they become motionless, lose their cilia, and sink to the bottom as green still spores. If, after being dried, water be poured on one of these green still spores, it takes up the water, its contents become closely granular, and fill the whole membrane of the spore. Then it divides, first into halves, then into quadrants or heart-shaped segments, meeting in a point in the centre of the membrane. These quadrants are ultimately divided into 8 wedge-shaped segments, whose contour lines, like the spokes of a wheel, meet in the centre, and each gets a pair of cilia. The coloured matter is driven back in each individual towards the thick end of the wedge as if by centrifugal force, and a colourless plasm remains in the points or beak. These disappear, a cavity is formed in the centre of the disc, the eight bodies assume the form of a wreath in close contact, and the original cilia, which continue to vibrate, cause the rotatory and progressive motion of the whole organism. Sometimes the eight globular bodies have been seen to divide into a number of extremely minute motile cells, while yet within the parent globe. These gonidia, as they are called, are, with a few exceptions which may reproduce the plant, believed to perish when they come into the water. The division of the primordial cell of this plant is confined to a certain time of day; it begins towards evening, and is completed the following morning, and according to Mr. F. Currey, the exact time is the same in Lapland, where there is no night, and at Berlin in spring when the day and night are almost equal. The fertility is enormous. It is calculated that in eight days, under favourable circumstances, 16,777,216 families of the Stephanosphæra pluvialis may be formed from one resting spore. The transmutation of chlorophyll in the Protococcus and Volvocineæ, from green to red and vice versâ, which so frequently occurs in the lowest class of plants, shows that its molecules must be united by very feeble affinities, and easily converted into new combinations either by direct chemical action, or by other substances also in a state of change.