Order 14. =Ralfsiaceæ.= _Ralfsia verrucosa_ is common as a red-brown
incrustation on stones and rocks at the water’s edge.
Order 15. =Lithodermataceæ.= Some species of the genus _Lithoderma_
occur in fresh water.
[Illustration: FIG. 69.--_Laminaria digitata_ (much reduced in
size).]
Order 16. =Laminariaceæ.= The thallus is more or less leathery, and
has generally a root-like lower part (Fig. 69) which serves to attach
it, and a stalk or stem-like part, terminated by a large leaf-like
expansion. Meristematic cells are situated at the base of the leaf,
and from these the new leaves are derived. The older leaf thus pushed
away by the intercalary formation of the younger ones, soon withers
(Fig. 69). Gametes are wanting. Zoosporangia are developed from
the lower part of a simple, few-celled sporangiophore, which is an
outgrowth from a surface-cell and has a large club-formed apical cell.
The sporangia are aggregated into closely packed sori, which cover
the lower part of the terminal leaf, or occur on special, smaller,
lateral, fertile fronds (_Alaria_). Most of the species belonging to
this order live in seas of moderate or cold temperature and occur
in the most northern regions that have yet been explored, forming
their organs of reproduction during the cold and darkness of the
arctic night. _Laminaria_ is destitute of a midrib and has only one
terminal leaf. _L. digitata_ has a broad leaf, which, by the violence
of the waves, is torn into a number of palmate strips (Fig. 69). _L.
saccharina_ has a small, undivided leaf. _Alaria_ has a midrib and
special fertile fronds. _A. esculenta_ occurs plentifully on the west
coast of Norway and on the shores of Great Britain. _Chorda filum_, a
common seaweed, is thick, unbranched, and attains a length of several
metres, without any strong demarcation between stalk and leaf. Some
attain quite a gigantic size, _e.g. Macrocystis pyrifera_, whose
thallus is said sometimes to be more than 300 metres in length. The
_Lessonia_-species, like the above, form submarine forests of seaweed
on the south and south-west coasts of South America, the Cape, and
other localities in the Southern Hemisphere.
USES. The large Laminarias, where they occur in great numbers,
are, like the Fuci, used for various purposes, for example, in
the production of iodine and soda, and as an article of food
(_Laminaria saccharina_, _Alaria esculenta_, etc.). _Laminaria
saccharina_ contains a large quantity of sugar (mannit) and is
in some districts used in the preparation of a kind of syrup;
in surgical operations it is employed for the distension of
apertures and passages, as for instance the ear-passage. It
is by reason of the anatomical peculiarities and structure of
the cell-walls, that they are employed for this purpose. The
cell-walls are divided into two layers, an inner one which
has very little power of swelling, and an outer one, well
developed and almost gelatinous--the so-called “intercellular
substance”--which shrivels up when dried, but can absorb water
and swell to about five times its size. The stalks of _Laminaria
clustoni_ are officinal.
Order 17. =Cutleriaceæ.= The thallus is formed by the union of the
originally free, band-shaped shoots. The growth is intercalary. Sexual
reproduction by the conjugation of male and female gametes. An asexual
generation of different appearance, which produces zoospores, arises
from the germination of the zygote. _Cutleria_, _Zanardinia_.