The free stratified bundles contain the simplest form of the Oscillatoriæ. Each filament is a straight or slightly curved chain of cells, full of coloured matter, and enclosed in a common transparent colourless tube. Multiplication takes place in these by division; when about to multiply, two adjacent coloured cells, or the two halves of a divided cell, recede from one another, and the outer tube contracts at the point of division, and separates them into two distinctly new filaments. Sometimes the transparent outer tube does not yield, so that the divided parts retain their places in the tube, which dilates when these new parts are again divided. The manner of division varies with the species, and the generic characters of the Oscillatoriæ depend upon the different conditions of the external tube, and the form and arrangement of the coloured cells within it. The tube often contracts to the finest point during division, and frequently consists of distinct coats, the number of which increases upwards, sometimes with such regularity as to produce a beautiful streaked effect. Like their allies, the Oscillatoriæ are reproduced by zoospores. While these parts are growing, but especially during their dissolution, the endochrome undergoes various changes of colour, staining the water they die in, and rendering it putrid; some of the common kinds emit a strong odour of sulphuretted hydrogen. In the compound gelatinous Oscillatoriæ, the jelly is of very different degrees of tenacity. The mass of the Dasyglœa is so slippery that it can scarcely be taken hold of; Rivularia nitida (fig. 19) is equally so, its tubes being so thick and tender. Many species of the genus Rivularia have a peculiar mode of oblique alternate branching; species of that genus grow on the stems of aquatic plants, on rocks in rapid streams, on cliffs when washed by cataracts, or sometimes in calcareous water, in consequence of which crystals of carbonate of lime are deposited on their substance. The Rivularia nitida occurs among Algæ exposed at low tides, and a species of another genus floats on fresh-water lakes like green stars. [Illustration: Fig. 19. Threads of Rivularia nitida.] The Oscillatoriæ are found in every part of the world, most abundantly in the temperate zones. They chiefly inhabit fresh water, but these minute plants attain their greatest size in the sea. Numerous species grow in warm springs, and one species, Trichodesmium erythræum (fig. 20), spreads for many square miles over the surface of the Indian seas in faggots of red-brown threads, like fragments of chopped hay; the same species is said to abound in the Red Sea also.[36] The Conjugatæ are fresh-water plants of numerous species, which have almost the same structure as the Confervæ, but the green endochrome within the cells of their articulated threads is more highly organized, and the manner of reproduction is altogether different and very peculiar.