These plants consist of strings of cylindrical cells joined end to end by their flat ends, and generally float freely on or near the surface of still water, especially when buoyed up by the bubbles of gas which are liberated from them by the heat and light of the sun. In the early stage of their life, while as yet the cells are undergoing multiplication by self-division, the endochrome is diffused pretty uniformly in each cell; but as the plant approaches towards maturity, it undergoes various modifications, according to the species. In some it consists of large granules disposed in rows; in others it is formed into broad spiral bands with large granules in binary or stellar groups placed at intervals on it; and, in the œdogonium capillare and others, the granules are united in spiral lines which cross one another and form a network.[37] [Illustration: Fig. 20. Trichodesmium erythræum.] The act of conjugation by which spores are formed, usually takes place between the cells of two distinct parallel filaments which happen to be adjacent to each other, and all the cells of the two filaments generally take part in it at once. The cells that are opposite to one another put out little protuberances, which come into contact with each other; the intervening partitions disappear, so that a tube is formed which establishes a free communication or passage between the cavities of the conjugating cells. In the genus Mesocarpus and others, the conjugating cells pour their endochromes into a dilatation of the passage that has been established between them, and it is there that the matter mingles to form a spore or embryo cell. But in the Zygnema (fig. 21), which is the commonest form of these plants, the endochrome of one cell passes entirely over into the cavity of the other, and within the latter the two endochromes coalesce into a single mass, round which a firm coat is developed, and it becomes a spore. All the cells of one filament are thus left empty, while spores are formed in all the cells of the other.[38] Sometimes cells in the same filament conjugate, and occasionally the endochrome in a cell divides into two parts, each of which becomes a spore. [Illustration: Fig. 21. Conjugation of Zygnema quininum:—A. two filaments in the first stage of conjugation; B, completion of the act of conjugation.] Some of the spores are quiescent, others have cilia and are motile, but both after a time become attached at one end by two or three root-like fibres, and grow into filaments by repeated bisection. According to the observations of M. Itzigsohn, the endochrome in certain filaments of Spirogyra breaks up before conjugation into little spherical aggregations, which are gradually converted into nearly colourless spiral filaments, having an active spontaneous motion, and therefore corresponding precisely to antherozoids. With the exception of South America, the Conjugatæ are widely dispersed in warm and temperate climates.