These Algæ are coloured green by chlorophyll, seldom in combination
with other colouring matter, and then especially with red. The product
of assimilation is frequently starch, which generally accumulates round
certain specially formed portions of protoplasm termed pyrenoids.
The thallus is uni- or multicellular; in the higher forms (certain
Siphoneæ) the organs of vegetation attain differentiation into stem
and leaf. The asexual reproduction takes place in various ways; the
sexual reproduction is effected by conjugation of motile gametes, or
by oogamous fertilisation. The swarm-cells (zoospores, gametes, and
spermatozoids) are constructed symetrically, and have true protoplasmic
cilia, these generally being attached to the front end of the
swarm-cells. Most of these Algæ live in water (fresh or salt); some are
found upon damp soil, stones, or tree-stems, and some live enclosed in
other plants.