The family consists of multicellular plants, whose cells are firmly
united together to form a thallus; this, in the simplest cases, may be
a branched filament of cells (_Ectocarpus_), or, in the highest, may
resemble a stem with leaves (_Laminariaceæ_), while all transitional
forms may be found between these two. The thallus grows by intercalary
divisions (_e.g. Ectocarpus_), or by an apical cell (_e.g._
_Sphacelaria_); pseudo-parenchymatous tissue may sometimes be formed by
cells, which were originally distinct, becoming united together. The
size of the thallus varies; in some species it is quite small--almost
microscopical,--while in the largest it is many metres in length.
The vegetative cells in the lower forms are nearly uniform, but in
those which are more highly developed (_Laminariaceæ_ and _Fucaceæ_),
they are sometimes so highly differentiated that mechanical,
assimilating, storing and conducting systems may be found; the last
named systems are formed of long cells with perforated, transverse
walls, which bear a strong resemblance to the sieve-tubes in the higher
plants.
[Illustration: FIG. 65.--Swarmspore of _Cutleria multifida_.]
The colouring matter in the living cells (“phæophyl”) contains
chlorophyll; but this is concealed by a brown (“phycophæin”), and a
yellow (“phycoxanthin”) colouring material, and hence all these Algæ
are a lighter or darker _yellow-brown_. Starch is not formed. Asexual
reproduction takes place, (1) by zoospores which arise in unilocular
zoosporangia, and are monosymmetric, with two cilia attached laterally
at the base of the colourless anterior end (Fig. 65), the longer
one being directed forwards and the shorter backwards; or (2) by
aplanospores (?).
[Illustration: FIG. 66.--_Ectocarpus siliculosus_. _I a-f_ A
female gamete in the various stages of coming to rest. _II_ A
motionless female gamete surrounded by male gametes. _III a-e_
Stages in the coalescence of male and female gametes.]
[Illustration: FIG. 67.--_Zanardinia collaris_. _A_ Male
gametangia (the smaller celled) and female gametangia (the
larger celled). _C_ Female gamete. _D_ Male gamete. _B_, _E_
Fertilisation. _F_ Zygote. _G_ Germinating zygote.]
Sexual reproduction has only been discovered in a few cases, and takes
place by means of gametes (oogamous fertilisation perhaps occurs in the
Tilopteridæ). The gametes have the same structure as the zoospores,
and arise in multilocular gametangia; these, like the zoosporangia,
are outgrowths from the external surface, or arise as modifications
from it. The conjugating gametes may be similar (_e.g. Ectocarpus
pusillus_), or there may be a more or less pronounced difference of
sex, an indication of which is found in _Ectocarpus siliculosus_ (Fig.
66). When the gametes in this species have swarmed for a time, some,
which are generally larger, are seen to attach themselves by one of the
cilia, which by degrees is shortened to form a kind of stalk (compare
the upper gamete in Fig. 66 _II_); these are the female gametes, which
now become surrounded by a number of males endeavouring to conjugate
with them, but only one succeeds in effecting fertilisation. The
protoplasm of the two gametes coalesces (Fig. 66 _III_), and a zygote
(_e_) is formed. The male gametes which do not conjugate may germinate,
but the plants derived from them are much weaker than those produced
by the zygotes. Strongly pronounced sexual differences are found in
the Cutleriaceæ, in which order the male and female gametes arise in
separate gametangia (Fig. 67 _A_). The male gametes (Fig. 67 _D_) are
much smaller than the female gamete (Fig. 67 _C_); the latter, after
swarming for a short time, withdraws the cilia, and is then ready to
become fertilised (Fig. 67 _B_, _E_), thus we have here a distinct
transition to the oogamous fertilisation which is found in the Fucaceæ.
Alternation of generations is rarely found.