The Phæophyceæ are Algæ, with chromatophores in which the chlorophyll
is masked by a brown colour (phycophæin). The product of assimilation
is a carbohydrate (fucosan), _never true starch_. In the highest
forms (_Fucaceæ_), the thallus presents differentiation into stem,
leaf, and root-like structures. The asexual reproduction takes place
by means of zoospores. The sexual reproduction is effected by the
coalescence of motile gametes, or by oogamous fertilisation. The
swarm-cells are _monosymmetric_, each moved by two cilia which are true
protoplasmic structures, and generally _attached laterally_ (Fig. 65).
The Phæophyceæ are almost entirely saltwater forms; a few species of
_Lithoderma_ live in fresh water.