The individuals are multicellular, with growth by an apical cell. The
thallus--often bilateral--is differentiated into a root-like structure
(attachment-disc), and stem, sometimes also into leaves (_Sargassum_).
Sometimes a differentiation occurs into various tissue-systems, viz. an
external assimilating tissue, a storing tissue, a mechanical tissue
of thickened, longitudinal, parenchymatous, strengthening cells, and
a conducting tissue of sieve-cells, or of short sieve-tubes with
perforated walls. Colouring material, as in Phæosporeæ. Vegetative
reproduction can only take place by means of detached portions of the
thallus (_Sargassum_), which are kept floating by means of bladders
(Fig. 70 _A_, _a_, Fig. 72). Zoospores are wanting.
The sexual reproduction takes place by oogamous fertilisation. The
oogonia and antheridia are formed inside special organs (conceptacles),
and are surrounded by paraphyses. The conceptacles (Fig. 70 _B_,
Fig. 71 _b_) are small, pear-shaped or spherical depressions,
produced by a special ingrowth of the surface cells of the thallus,
and their mouths (_ostioles_) project like small warts; they are
either situated near the end of the ordinary branches of the thallus
(_Fucus serratus_, Fig. 71 _a_) which may be swollen on this account
(_Fucus vesiculosus_, Fig. 70 _A_, _b_), or on special short branches
(_Ascophyllum_, _Sargassum_). The vertical section of a conceptacle is
seen in Fig. 70 _B_ (see also Fig. 71 _b_) where, in addition to the
paraphyses, oogonia only are seen (_F. vesiculosus_ is diœcious--male
plant, yellow-brown; female plant, olive-brown); but in some
species antheridia, together with oogonia, are produced in the same
conceptacle. The oogonia are large, almost spherical cells, situated on
a short stalk, in each of which are formed from 1–8 (in _Fucus_, 8; in
_Ascophyllum_, 4; in _Halidrys_, 1; in _Pelvetia_, 2) rounded, immotile
oospheres. The wall of the oogonium ruptures, and the oospheres,
still enclosed in the inner membrane, are ejected through the mouth
of the conceptacle, and float about in the water, being finally set
free by the bursting of the inner membrane. The antheridia are oblong
cells (Fig. 70 _C_, _a_), many of which are produced on the same
branched antheridiophore (Fig. 70 _C_); the numerous spermatozoids are
provided with 2 cilia and are very small (Fig. 70 _D_, two antheridia
surrounded by spermatozoids, one being open). The spermatozoids, still
enclosed by the inner membrane of the antheridium, are similarly
set free, and fertilisation takes place in the water, numerous
spermatozoids collecting round the oosphere (Fig. 70 _E_), which is
many times larger, and by their own motion causing it to rotate. After
fertilisation, the oospore surrounds itself with a cell-wall and
germinates immediately, attaching itself (Fig. 70 _F_) to some object,
and by cell-division grows into a new plant.
[Illustration: FIG. 70.--_Fucus vesiculosus. A_ Portion of
thallus with swimming bladders (_a_) and conceptacles (_b_). _B_
Section of a female conceptacle; _h_ the mouth; _p_ the inner
cavity; _s_ oogonia. _C_ Antheridiophore; _a_ antheridium; _p_
sterile cells. _D_ Antheridia out of which the spermatozoids are
escaping. _E_ Fertilisation. _F_ Germinating oospore.]
[Illustration: FIG. 71.--_Fucus serratus_. _a_ Portion of a male
plant which has been exposed to the action of the open air for
some time; small orange-yellow masses, formed by the antheridia,
are seen outside the mouths of the male conceptacles (nat. size).
_b_ Cross section through the end of a branch of a female plant,
showing the female conceptacles (× 4).]
[Illustration: FIG. 72.--_Sargassum bacciferum_. A portion of the
thallus, natural size.]
Order 1. =Fucaceæ.= The following species are common on our coasts:
_Fucus vesiculosus_ (Fig. 70) has a thallus with an entire margin,
and with bladders arranged in pairs; _F. serratus_ (Fig. 71) without
bladders, but with serrated margin; _Ascophyllum nodosum_ has
strap-like shoots, which here and there are swollen to form bladders;
_Halidrys siliquosa_ has its swimming bladders divided by transverse
walls; _Himanthalia lorea_, which is found on the west coast of Norway,
and the south coast of England, has a small perennial, button-shaped
part, from the centre of which proceeds the long and sparsely branched,
strap-like, annual shoot, which bears the conceptacles. The Gulf-weed
(_Sargassum bacciferum_, Fig. 72) is well known historically from the
voyage of Columbus; it is met with in large, floating, detached masses
in all oceans, and is found most abundantly in the Atlantic, off the
Canary Islands and the Azores, and towards the Bermudas. The stalked,
spherical air-bladders are the characteristic feature of this genus.
The thallus is more highly developed than in _Fucus_, and there is a
contrast between the stem and leaf-like parts. The portions which are
found floating are always barren, only those attached are fertile.
USES. The Fucaceæ, like the Laminariaceæ, are used as manure
(the best kinds being _Fucus vesiculosus_ and _Ascophyllum
nodosum_), for burning to produce kelp, and as food for domestic
animals (_Ascophyllum nodosum_ is especially used for this
purpose).