are essentially the same in all cases, slight variations in
both instances appear in different families, attributable
doubtless to the independent origin of the process in different
groups. Thus, although isogamy consists in typical cases of
a union of naked motile gametes by a fusion which begins at
the beaked ends, and results in the formation of an immotile
spherical zygote surrounded by a cell-wall, in Leptosira it
is noticeable that the fusion begins at the blunt end; in a
species of Chlamydomonas the two gametes are each included
in a cell-wall before fusion; and in many cases the zygote
retains for some time its motility with the double number of
cilia. Again, in oogamous reproduction, while in general only
one oosphere is differentiated in the oogonium, in Sphaeroplea
several oospheres arise in each oogonium; and while the oospheres
usually contract away from the oogonial wall, acquiring for
themselves a new cell-wall after fertilization, in Coleochaete
the oosphere remains throughout in contact with the oogonial
wall. The oosphere is in all cases fertilized while still within
the oogonium, the antherozoids being admitted by means of a
pore. There is usually distinguishable upon the surface
of the oosphere an area free from chlorophyll, known as the
receptive spot, at which the fusion with the antherozoid
takes place; and in many cases, before fertilization, a small
mucilaginous mass has been observed to separate itself off
from the oosphere at this point and to escape through the
pore. In Coleochaete the oogonial wall is drawn out into a
considerable tube, which is provided with an apical pore, and
this tube has a somewhat similar appearance to the imperforate
trichogyne of Florideae to be hereafter described. In certain
species of Oedogonium minute male plantlets, known as dwarf
males, become attached to the female plant in the neighbourhood
of the oogonia, thus facilitating fertilization. Indeed the
genus Oedogonium exhibits a high degree of specialization
in its reproductive system, considering that its thallus
has not advanced beyond the stage of an unbranched filament.
Many Euchlorophyceae are endowed with both asexual and sexual
reproduction. Such are Coleochaete, Oedogonium, Cylindrocapsa,
Ulothrix, Vaucheria, Volvox, &c. In others only the asexual
method is yet known. When a species resorts to both methods,
it is generally found that the asexual method prevails in the
early part of the vegetative period and the sexual towards
the close of that period. This is in consonance with the
facts already mentioned that zoospores germinate forthwith,
and that the sexually-produced cell or zygote enters upon a
period of rest. It is known that zoogametes, which usually
conjugate, may, when conjugation fails, germinate directly
(Sphaerella.) In rare cases the oosphere has been known to
germinate without fertilization (Oedogonium, Cylindrocapsa.)
The germination of a zygospore or oospore is effected by
the rupture of an outer cuticularized exosporium; then the
cell may protrude an inner wall, the endosporium, and grow
out into the new plant ( Vaucheria), or the contents may
break up into a first brood of zoospores. It is held that
in Coleochaetea parenchyma results from the division of
the oospore, from each cell of which a zoospore arises.
Reproduction is also effected among Euchlorophyceae by means of
aplanospores and akinetes. Aplanospores would seem to represent
zoospores arrested in their development; without reaching
the stage of motility, they germinate within the sporangium.
Akinetes are ordinary thallus cells, which on account of their
acquisition of a thick wall are capable of surviving unfavourable
conditions. Both aplanospores and akinetes may germinate with
or without the formation of zoospores at the initial stage.
Among Conjugatae reproduction is effected solely by means
of conjugation of what are literally aplanospores. Among
those Desmidiaceae which live a free life, two plants
become surrounded by a common mucilage, in which they lie
either parallel (Closterium) or crosswise (Cosmarium.)
Gaps then appear in the apposed surfaces, usually at the
isthmus; the entire protoplasts either pass out to melt into
one another clear of the old walls, or partly pass out and
fuse without complete detachment from the old walls. Among
colonial Desmidiaceae, the break-up of the filament is a
preliminary to this conjugation; otherwise the process is the
same. The zygospore becomes surrounded with its own wall,
consisting finally of three layers, the outer of which is
furnished with spicular prominences of various forms. In
Zygnemaceae there is no dissolution of the filaments, but
the whole contents of one cell pass over by means of a
conjugation-tube into the cavity of a cell of a neighbouring
filament, where the zygospore is formed by the fusion of the two
Fig. 3.--Chlorophyceae, variously magnified.
A. Spirogyra sp., in conjugation.
B. Zoospore of Pandorina. B234, stages of conjugation.
C. Ulothrix sp., zoospores escaping. C23, stages of conjugation.
D1. Oedogonium sp., oogonium at moment
of fertilization with dwarf male attached.
D2. Oedogonium sp., zoospore with crown of cilia.
E1. Coleochaete sp., with antheridia and an oogonium.
E2. Coleochaete sp., fertilized egg with investment of filaments.
E3. Coleochaete sp., zoospore.
F123. Protosiphon, conjugation of zoogametes.
G. Derbesia sp., zoospore with chaplet of cilia.
H1. Chara sp., oogonium and antheridium at a node on a lateral appendage.
H2. Chara sp., antherozoid.
K1. Vaucheria sp., oogonium and antheridium before fertilization.
K2. Vaucheria sp., after fertilization.
(A from Cooke, British Freshwater Algae, by permission
of Kegan Paul, Trench Trubner and Co.; C, E, F. G. H. K
from Engler and Prantl, by permission of Wilheim Engelmann;
B1 from Vines, by permission of Swan Sonnenschein and Co.;
B2, D from Oltmanns, by permission of Gustav Fischer.)
protoplasts. In these cases the activity of one of the
gametes, and the passivity of the other, is regarded
as evidence of incipient sex. In Strogonium there is
cell-division in the parent-cell prior to conjugation; and
as two segments are cut off in the case of the active gamete,
and only one in the case of the passive gamete, there is a
corresponding difference of size, marking another step in
the sexual differentiation. In Zygogonium, although no
cell-division takes place, the gametes consist of a portion
only of the contents of a cell, and this is regularly the
case in Mesocarpaceae, which occupy the highest grade among
Conjugatae. Some Zygnemaceae and Mesocarpaceae form either
a short conjugating tube, or none at all, but the filaments
approach each other by a knee-like bend, and the zygospore is
formed at the point of contact, often being partially contained
within the walls of the parent-cell. It would seem that in
some cases the nuclei of the gametes remain distinct in the
zygospore for a considerable time after conjugation. It is
probable that in all cases nuclear fusion takes place sooner or
later. In Zygnemaceae and Mesocarpaceae the zygospore,
after a period of rest, germinates, to form a new filamentous
colony; in Desmidiaceae its contents divide on germination,
and thus give rise to two or more Desmids. Gametes which fail
to conjugate sometimes assume the appearance of zygospores
and germinate in due course. They are known as azygospores.
The reproduction of Characeae is characterized by a pronounced
oogamy, the reproductive organs being the most highly
differentiated among Chlorophyceae. The antheridia and oogonia
are formed at the nodes of the appendages. The oogonium,
seated on a stalk cell, is surrounded by an investment
consisting of five spirally-wound cells, from the projecting
ends of which segments are cut off, constituting the so-called
stigma. The oosphere is not differentiated within the wall of
the oogonium, but certain cells known as wendungszellen, the
significance of which has given rise to much speculation, are
cut off from the basal portion of the parent-cell during its
development. The antheridia are spherical orange-coloured
bodies of very complex structure. The antherozoid is a
spirally-coiled thread of protoplasm, furnished at one
end with a pair of cilia. It much more resembles the
antherozoids of Bryophyta and certain Pteridophyta than any
known among other algae. The fertilized egg charged with
food reserves rests for a considerable period, surrounded
by its cortex, the whole having assumed a reddish-brown
colour. On germination it gives rise to a row of cells in
which short (nodal) and long (internodal) cells alternate.
From the first node arise rhizoids; from the second a lateral
bud, which becomes the new plant. This peculiar product of
germination, which intervenes between the oospore and the
adult form, is the proembryo. It will be remembered that in
Musci, the asexual spore somewhat similarly gives rise to a
protonema, from which the adult plant is produced as a lateral
bud. The proembryonic branches of Characeae, one of the
means of vegetative reproduction already referred to, are so
called because they repeat the characters of the proembryo.
Before leaving the Chlorophyceae, it should be mentioned
that the genus Volvox has been included by some zoologists
(Butschli, for example) among Flagellata; on the other hand,
certain green Flagellata, such as Euglena, are included by
some botanists (for example, van Tieghem) among unicellular
plants. A similar uncertainty exists with reference to certain
groups of Phaeophyceae, and the matter will thus arise again.
A census of the Chlorophyceae is furnished below:--
1. Confervoideae---12 families, 77 genera, 1021 species.
2. Siphoneae--9 families, 26 genera, 271 species.
3. Protococcoideae--2 families, 90 genera, 342 species.
4. Conjugateae--2 families, 33 genera, 1296
species. (De Toni's Sylloge Algarum, 1889.)
5. Characeae--2 families, 6 genera, 181 species.
(Engler and Prantl's Pflanzenfamilien, 1897)
III. PHAEOPHYCEAE.--The Phaeophyceae are distinguished
by the possession of a brown colouring matter, phycophaein,
in addition to chlorophyll. They consist of the following
groups:--Fucaceae, Phaeosporeae; Dictyotaceae, Cryptomonadaceae,
Peridiniaceae and Diatomaceae. Of these the first three
include multicellular plants, some of them of great size; the
last three are unicellular organisms, with little in common
with the rest excepting the possession of a brown colouring
matter. Fucaceae and Phaeosporeae are doubtless closely
allied, and to these Dictyotaceae may be joined, though
the relationship is less close. They constitute the
Euphaeophyceae, and will be dealt with in the first place.
Euphaeophyceae are almost exclusively marine, growing on
rocks and stones on the coast, or epiphytic upon other
algae. In tidal seas they range from the limits of high water
to some distance beyond the low-water line. On the British
coasts zones are observable in passing from high to low water
mark, characterized by the prevalence of different species,
thus:---Pelvetia canaliculata, Fucus platycarpus, Fucus
vesiculosus, Ascophyllum nodosum, Fucus serratus, Laminaria
digitata. Some species are minute filamentous plants, requiring
the microscope for their detection; others, like Lessonia,
are of considerable bulk, or, like Macrocystis, of enormous
length. In Fucaceae, Dictyotacea, and in Laminariaceae and
Sphacelariaceae, among Phaeosporeae, the thallus consists of
