Order 4. Nostocaceæ. The individuals are formed of multicellular, unbranched filaments, without differentiation into apex and base; heterocysts present. Reproduction by synakinetes and resting akinetes.
Fig. 22.—Nostoc verrucosum. A The plant in its natural size; an irregularly folded jelly-like mass. B One of the cell-chains enlarged, with its heterocysts (h), embedded in its mucilaginous sheath.
Some genera are not mucilaginous, e.g. Cylindrospermum (Fig. [19]). The cell-chains in others, e.g. Nostoc, wind in between one another and are embedded in large structureless jelly-like masses, which may attain the size of a plum or even larger (Fig. [22]); sometimes they are found floating in the water, sometimes attached to other bodies. Other genera as follows: Aphanizomenon and Anabæna (in lakes and smaller pieces of water); Nodularia is partly pelagic. Some occur in the intercellular spaces of higher plants, thus Nostoc-forms are found in Anthoceros, Blasia, Sphagnum, Lemna, and in the roots of Cycas and Gunnera; Anabæna in Azolla.
Order 5. Rivulariaceæ. The individuals are multicellular filaments, with differentiation into apex and base; spurious branching, and a heterocyst at the base of each filament, reproduction by synakinetes and resting akinetes, rarely by simple reproductive akinetes. Rivularia, Glœotrichia, Isactis, Calothrix.
Order 6. Scytonemaceæ. The individuals are formed of multicellular filaments with no longitudinal division; differentiation into apex and base very slight or altogether absent; branching spurious; heterocysts present. Reproduction by synakinetes, rarely by resting akinetes and ordinary reproductive akinetes. Tolypothrix, Scytonema, Hassalia, Microchæte.
Order 7. Sirosiphoniaceæ. The individuals are formed of multicellular threads with longitudinal divisions; true branching and heterocysts, and often distinct differentiation into apex and base. Reproduction by synakinetes, rarely by resting akinetes and ordinary reproductive akinetes. Hapalosiphon, Stigonema, Capsosira, Nostocopsis, Mastigocoleus.
Family 2. Bacteria.[6]
The Bacteria (also known as Schizomycetes, and Fission-Fungi) are the smallest known organisms, and form a parallel group to the Blue-green Algæ, but separated from these Algæ by the absence of their colouring material; chlorophyll is only found in a few Bacteria.
The various forms under which the vegetative condition of the Bacteria appear, are termed as follows: