The lowest of the _Cyanophyceæ_ are strictly single-celled, separating as soon as formed, but cohering usually in masses or colonies by means of a thick mucilaginous substance that surrounds them (Fig. 7, _D_). The higher ones are filaments, in which there may be considerable differentiation. These often occur in masses of considerable size, forming jelly-like lumps, which may be soft or quite firm (Fig. 7, _A_, _B_). They are sometimes found on damp ground, but more commonly attached to plants, stones, etc., in water. The masses vary in color from light brown to deep blackish green, and in size from that of a pin head to several centimetres in diameter. [Illustration: FIG. 7.--Forms of _Cyanophyceæ_. _A_, _Nostoc_. _B_, _Glœotrichia_, × 1. _C_, individual of _Glœotrichia_. _D_, Chroöcoccus. _E_, _Nostoc_. _F_, Oscillaria. _G_, _H_, _Tolypothrix_. All × 300. _y_, heterocyst. _sp._ spore.] In the higher forms special cells called heterocysts are found. They are colorless, or light yellowish, regularly disposed; but their function is not known. Besides these, certain cells become thick-walled, and form resting cells (spores) for the propagation of the plant (Fig. 7, C. _sp._). In species where the sheath of the filament is well marked (Fig. 7, _H_), groups of cells slip out of the sheath, and develop a new one, thus giving rise to a new plant. The bacteria (_Schizomycetes_), although among the commonest of organisms, owing to their excessive minuteness, are difficult to study, especially for the beginner. They resemble, in their general structure and methods of reproduction, the blue-green slimes, but are, with very few exceptions, destitute of chlorophyll, although often possessing bright pigments,--blue, violet, red, etc. It is one of these that sometimes forms blood-red spots in flour paste or bits of bread that have been kept very moist and warm. They are universally present where decomposition is going on, and are themselves the principal agents of decay, which is the result of their feeding upon the substance, as, like all plants without chlorophyll, they require organic matter for food. Most of the species are very tenacious of life, and may be completely dried up for a long time without dying, and on being placed in water will quickly revive. Being so extremely small, they are readily carried about in the air in their dried-up condition, and thus fall upon exposed bodies, setting up decomposition if the conditions are favorable.