The plants in this class are multicellular, and brown, with apical
growth, new cells being derived either from a flat apical cell, or from
a border of apical cells. The thallus is flat, leaf- or strap-shaped,
attached by haptera, which are either found only at the base, or
on the whole of the lower expansion of the thallus. The cells are
differentiated into the following systems of tissues: an external,
small-celled layer of assimilating cells, generally one cell in
thickness, and an internal, large-celled layer of one or only a few
cells in thickness, forming the mechanical and conducting tissues.
All the reproductive cells are motionless. Asexual reproduction by
naked, motionless spores (tetraspores) which are formed 1–4 in each
tetrasporangium, the latter being outgrowths from the surface cells
of special, sexless individuals. Zoospores are wanting. The sexual
organs are of two kinds, oogonia and antheridia, which are formed from
the surface cells, either on the same or different individuals. The
oogonia are spherical or oval, and are generally placed close together;
each contains one oosphere, which on maturity is ejected into the
surrounding water, and is then naked and motionless. The antheridia
are formed of longitudinal cells, united in groups, whose contents
by repeated divisions--transverse and longitudinal--are divided into
a large number of small, colourless, motionless spermatia--round or
elongated--which are set free by the dissolution of the wall of the
antheridium. The process of fertilisation has not yet been observed.
The Dictyotales, in having tetraspores and spermatia, deviate
considerably from the Phæophyceæ, but may be classed near to the
Tilopteridæ, in which there are asexual spores with 4 cell-nuclei,
which may be considered as an indication of the formation of
tetraspores.
Order 1. =Dictyotaceæ.= _Dictyota dichotoma_ which has a thin,
regularly dichotomously divided thallus, occurs on the coasts of
the British Isles. _Padina_ is found on the south coast.