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. The Protococcus pluvialis (fig. 7), one of the unicellular Confervæ, is frequently met with in rain-water cisterns. The spore of the plant (fig. 7 A) is a globular primordial cell invested with a double coat of cellulose, sometimes separated by an aqueous fluid, sometimes not. The cell is filled with protoplasm, a colourless watery liquid in which red and green particles are scattered. When this spore begins to grow, the endochrome, or solid matter in the primordial cell, divides spontaneously into two similar and equal parts, round at one end, and tapering to a point or beak at the other, each being coated by a very thin film of the transparent colourless protoplasm. [Illustration: Fig. 7. Protococcus pluvialis:—A, encysted cell; B, C, cells divided into two; D, cell divided into four; E, cell divided into eight; F, cell divided into thirty-two; G, escaped motile gonidia; H-L, primordial utricles furnished with cilia.]