a true parenchyma; elsewhere it consists of free filaments,
or filaments so compacted together, as in Cutleriaceae and
Desmarestiaceae, as to form a false parenchyma. In Fucaceae
and Laminariaceae the inner tissue is differentiated into
a conducting system. In Laminariaceae the inflation of
the ends of conducting cells gives rise to the so-called
trumpet-hyphae. In Nereocystis and Macrocystis a zone
of tubes occurs, which present the appearance of sieve-tubes
even to the eventual obliteration of the perforations by a
callus. While there is a general tendency in the group to
mucilaginous degeneration of the cell-wall, in Laminaria
digitata there are also glands secreting a plentiful
mucilage. Secondary growth in thickness is effected by
the tangential division of superficial cells. The most
fundamental external differentiation is into holdfast and
shoot. In Laminariaceae secondary cylindrical props arise
obliquely from the base of the thallus. In epiphytic forms
the rhizoids of the epiphyte often penetrate into the tissue
of the host, and certain epiphytes are not known to occur
excepting in connexion with a certain host; but to what
extent, if any, there is a partial parasitism in these cases
has not been ascertained. In filamentous forms there is
a differentiation into branches of limited and branches of
unlimited growth (Sphacelaria.) In Laminariaceae there is a
distinction of stipe and blade. The blade is centrally-ribbed
in Alaria and laterally-ribbed in Macrocystis. It is
among the Sargassaceae that the greatest amount of external
differentiation, rivalling that of the higher leafy plants, is
reached. A characteristic feature of the more massive
species is the occurrence of air-vesicles in their tissues.
In Fucus vesiculosus they arise in lateral pairs; in
Ascophyllum they are single and median; in Macrocystis
one vesicle arises at the base of each thallus segment; in
Sargassum and Halidrys the vesicles arise on special
branches. They serve to buoy up the plant when attached to the
sea-bottom, and thus light is admitted into the forest-like
growths of the gregarious species. When such plants are
detached they are enabled to float for great distances, and
the great Sargasso Sea of the North Atlantic Ocean is probably
only renewed by the constant addition of plants detached
from the shores of the Caribbean Sea and Gulf of Mexico.
Growth in length is effected in a variety of ways. In Dictyota
Sphacelariaceae and Fucaceae there is a definite apical
cell. In the first it is a biconvex lens, from which segments
are continually cut off parallel to the posterior surface;
and in the second an elongated dome, from which segments are
cut off by a transverse wall. While, however, in Dictyota
the product of the subsequent division in the segment enlarges
with each subdivision, the divisions in the cylindrical
segment of Sphacelariaceae are such that the whole product
after subdivision, however many cells it may consist of, does
not exceed in bulk the segment as cut off from the apical
cell. In Dictyotaceae the apical cell occasionally divides
longitudinally, and thus the dichotomous branching is provided
for. In some Sphacelariaceae branches may appear at their
inception as lateral protuberances of the apical cell itself.
In Fucaceae an apical cell is situate at the surface of the
thallus in a slit-like depression at the apex. From this cell
segments are cut off in three or four lateral oblique planes.
A peculiar manner of growth in length is that to which the term
trichothallic has been applied. It may readily be observed
that in the hair-like branches of Ectocarpaceae, the point at
which most rapid division occurs is situate near the base of the
hair. In Desmarestia and Arthrocladia, for example, it is
found that the thallus ends in a tuft of such hairs, each of
them growing by means of an intercalated growing point. In these
cases, however, the portions of the hairs behind the growing
region become agglutinated together into a solid cylindrical
pseudo-parenchymatous axis. In Cutleria the laminated thallus
is formed in the same way. The intercalated growing region of
Laminaria affords an example of another variety of growth in
Phaeophyceae. While the laminated portion of the thallus is
being gradually worn off in our latitudes during the autumnal
storms, a vigorous new growth appears at the junction of
the stipe and the blade, as the result of which a new piece
is added to the stipe and the lamina entirely renovated.
Both asexual and sexual reproduction occur among Euphaeophyceae.
Fucaceae are marked by an entire absence of the asexual
method. The sexual organs--oogonia and antheridia---are
borne on special portions of the thallus in cavities known as
conceptacles. Both organs may occur in one conceptacle, as
in Pelvetia, or each may be confined to one conceptacle
or even one plant, as in Fucus vesiculosus. The oogonia
arise on a stalk cell from the lining layer of the cavity, the
contents dividing to form eight oospheres as in Fucus, four
as in Ascophyllum, two as in Pelvetia, or one only as in
Hallidrys. It would seem that eight nuclei primarily arise
in all Fucaceae, and that a number corresponding to the number
of oospheres subsequently formed is reserved, the rest being
discharged to the periphery, where they may be detected at a late
stage. On the maturation of the oospheres the outer layer of
the oogonial wall ruptures, and the oospheres, still surrounded
by a middle and inner layer, pass out through the mouth of the
conceptacle. Then usually these layers successively give
way, and the spherical naked oospheres float free in the
water. The antheridia, which arise in the conceptacular cavity
as special cells of branched filaments, are similarly discharged
whole, the antherozoids only escaping when the antheridia
are clear of the conceptacle. The antherozoids are attracted
to the oospheres, round each of which they swarm in great
numbers. Suddenly the attraction ceases, and the oosphere is
fertilized, probably at that moment, by the entry of a single
antherozoid into the substance of the oosphere; a cell-wall
is formed thereupon, in some cases in so short an interval
as five minutes. Remarkable changes of size and outline of
the oosphere have recently been described as accompanying
fertilization in Hallidrys. Probably the act of fertilization
in plants has nowhere been observed in such detail as in
Fucaceae. Dictyotaceae resemble Fucaceae in their pronounced
oogamy. They differ, however, in being also asexually
reproduced. The asexual cells are immotile spores arising in
fours in sporangia from superficial cells of the thallus. In
Dictyota the oospheres arise singly in oogonia, crowded together
in sori on the surface of the female plant. The antheridia
have a similar origin and grouping on the male plant. Until the
recent discovery by Williams of motility, by means of a single
cilium, of the antherozoids of Dictyota and Taonia, they
were believed to be immotile bodies, like the male cells of
red seaweeds. in Dictyota the unfertilized oosphere is found
to be capable of undergoing a limited number of divisions,
but the body thus formed appears to atrophy sooner or later.
Of the small family of the Tilopteridaceae our knowledge is
as yet inadequate, but they probably present the only case of
pronounced oogamy among Phaeosporeae. They are filamentous
forms, exhibiting, however, a tendency to division in more
than one plane, even in the vegetative parts. The discovery
by Brebner of the specific identity of Haplospora globosa and
Scaphospora speciosa marks an important step in the advance of
our knowledge of the group. Three kinds of reproductive organs
are known: first, sporangia, which each give rise to a single
tetra-, or multi-nucleate non-motile, probably asexual spore;
second, plurilocular sporangia, which are probably antheridia,
generating antherozoids; and third, sporangia, which are
probably oogonia, giving rise to single uni- nucleate non-motile
oospheres. No process of fertilization has as yet been observed.
The Cutleriaceae exhibit a heterogamy in which the female sexual
cell is not highly specialized, as is in the groups already
described. From each locule of a plurilocular sporangium there
is set free an oosphere, which, being furnished with a pair
of cilia, swarms for a time. In similar organs on separate
plants the much smaller antherozoids arise. Fertilization has
been observed at Naples; but it apparently depends on climatic
conditions, as at Plymouth the oospheres have been observed to
germinate parthenogenetically. The asexual organs in the case
of Cutleria multifida arise on a crustaceous form, Aglaozonia
reptans, formerly considered to be a distinct species. They
are unilocular, each producing a small number of zoospores.
The possession of two kinds of reproductive organs, unilocular
and plurilocular sporangia, is general among the rest of the
Phaeosporeae. Bornet, however, called attention in 1871 to
the fact that two kinds of plurilocular sporangia occurred in
certain species of the genus Ectocarpus--somewhat transparent
organs of an orange tint producing small zoospores, and also
more opaque organs of a darker colour producing relatively larger
zoospores. On the discovery of another such species by F.
H. Buffham, Batters in 1892 separated the three species,
Ectocarpus secundus, E. fenestratus, E. Lebelii, together
with the new species, into a genus, Giffordia, characterized
by the possession of two kinds of plurilocular sporangia.
The suspicion that a distinction of sex accompanied this
difference of structure has been justified by the discovery
by Sauvageau of undoubted fertilization in Gidordia secunda
and G. fenestrata. The conjugation of similar gametes,
arising from distinct plurilocular sporangia, was observed
by Berthold in Ectocarpus siliculosus and Scytosiphon
lomentarius in 1880; and these observations have been recently
confirmed in the case of the former species by Sauvageau,
and in the case of the latter by Kuckuck. In these cases,
however, the potential gametes may, failing conjugation,
germinate directly, like the zoospores derived from unilocular
sporangia. The assertion of Areschoug that conjugation
occurs among zoospores derived from unilocular sporangia,
in the case of Dictyosiphon hippuroides, is no doubt to be
ascribed to error of observation. It would thus seem that
the explanation of the existence of two kinds of sporangia,
unilocular and plurilocular, among Phaeosporeae, lies in the
fact that unilocular sporangia are for asexual reproduction,
and that plurilocular sporangia are gametangia--potential or
actual. It must, however, be remembered that so important
a generalization is as yet supported upon a somewhat narrow
base of observation. Moreover, for the important family
of the Laminariaceae only unilocular sporangia are known to
occur; and for many species of other families, only one or
other kind, and in some cases neither kind, has hitherto been
observed. The four species--Ectocarpus siliculosus,
Giffordia secunda, Cutleria multifida and Haplospora
globosa--may be taken to represent, within the phaeosporeae,
successive steps in the advance from isogamy to oogamy.
The Peridiniaceae have been included among Flagellata under
the title of Dinoflagellata. The majority of the species
belong to the sea, but many are found in fresh water. The
thallus is somewhat spherical and unicellular, exhibiting
a distinction between anterior and posterior extremities,
and dorsal and ventral surfaces. The wall consists of a
