11. _Palmellaceae_.--Plants forming gelatinous or pulverulent crusts on
damp surfaces of stone, wood, earth, mud, swampy districts, or more
or less regular masses of gelatinous substance or delicate
pseudo-membranous expansion or fronds, of flat, globular, or tubular
form, in fresh water or on damp ground; composed of one or many,
sometimes innumerable, cells, with green, red, or yellowish contents,
spherical or elliptical form, the simplest being isolated cells found in
groups of two, four, eight, etc., in course of multiplication. Others
permanently formed of some multiple of four; the highest forms made up
of compact, numerous, more or less closely joined cells. Reproduction:
by cell division, by the conversion of the cell contents into zoospores,
and by resting spores, formed sometimes after conjugation; in other
cases, probably, by fecundation by spermatozoids. All the unicellular
algae are included under this head.

12. _Desmidiaceae_.--Microscopic gelatinous plants, of a screen color,
growing in fresh water, composed of cells devoid of a silicious coat,
of peculiar forms such as oval, crescentic, shortly cylindrical,
cylindrical, oblong, etc., with variously formed rays or lobes, giving
a more or less stellate form, presenting a bilateral symmetry, the
junction of the halves being marked by a division of the green contents;
the individual cells being free, or arranged in linear series, collected
into fagot-like bundles or in elegant star like groups which are
embedded in a common gelatinous coat. Reproduced by division and by
resting spores produced in sporangia formed after the conjugation of
two cells and union of their contents, and by zoospores formed in the
vegetative cells or in the germinating resting spores.

13. _Diatomaceae_.--Microscopic cellular bodies, growing in fresh,