The individuals are 1--many celled; the thallus consists in many of a
single cell, in others of chains of cells, the cells dividing in only
one definite direction (Figs. 18, 21). In certain Fission-Algæ the
cell-chain branches (Fig. 30) and a difference between the anterior
and the posterior ends of the chain is marked; in some, the cells may
be united into the form of flat plates by the cell-division taking
place in two directions; and in others into somewhat cubical masses, or
rounded lumps of a less decided form, by the divisions taking place in
three directions; or less defined masses may be formed by the divisions
taking place in all possible directions.
The cell-walls rarely contain cellulose, they often swell considerably
(Figs. 20, 22), and show distinct stratifications, or they are almost
completely changed into a mucilaginous mass in which the protoplasts
are embedded, _e.g._ in _Nostoc_ (Fig. 22), and in the “Zooglœa” stage
of the Bacteria (Fig. 27). Sexual reproduction is wanting. Vegetative
reproduction by division and the separation of the divisional products
by the splitting of the cell-wall or its becoming mucilaginous; among
the Nostocaceæ, Lyngbyaceæ, Scytonemaceæ, etc., “Hormogonia” are found;
in _Chamæsiphon_ and others single reproductive akinetes are formed.
Many Fission-Algæ conclude the growing period by the formation of
resting akinetes or aplanospores.
The Schizophyta may be divided into 2 families: