M2. Chara sp., apical region.
N. Botrydium, a simple siphonaceous alga with root-like attachment.
O. Acetabularia Mediterranea, mushroom-like calcareous siphonaceous alga.
(A, C, E, F, G, H, K, M1, M2, from from Engler and Prantl,
Pflanzenfamilien, by permission of Wilhelm Engelmann; B1,
N, from Vines, Student's Text Book of Botany, by permission of
Swan Sonnenschein and Co.; B2, D, O from Oltmanns, Morphologie
u. Biologie der Algen, by permission of Gustav Fischer.)
Dermatophyton grows on the carapace of the tortoise and
Trichophilus in the hairs of the sloth. Certain Protococcales
and Confervales exist as the gonidia of the lichenthallus.
The thallus is of more varied structure in this group than in any
other. In the simplest case it may consist of a single cell,
which may remain free during the whole of the greater part
of its existence, or be loosely aggregated together within a
common mucilage, or be held together by the adhesion of the
cell-walls at the surface of contact. These aggregations or
colonies, as they are termed, may assume the form of a plate, a
ring, a solid sphere, a hollow sphere, a perforate sphere, a
closed net, or a simple or branched filament. It is not easy
in all cases to draw a distinction between a colony of planes
and a multicellular individual. in a Volvox sphere, for
example, there is a marked protoplasmic continuity between
all the cells of the colony. The Ulvaceae, the thallus of
which consists of laminae, one or more cells thick, or hollow
tubes, probably represent a still more advanced stage in the
passaae of a colony into a multicellelar plant. Here there
is some amount of localization of growth and distinction of
parts. It is only in such cases as Volvox and Ulvaceae
that there is any pretension to the formation of a true
parenchyma within the limits of the Chlorophyceae. In the
whole series of the Confervales, the thallus consists of
filaments branched or unbranched, attached at one extremity,
and growing almost wholly at the free end. The branches
end in fine hairs in Chaetophoraceae. In Coleochaetaceae
the branches are often welded into a plate, simulating a
parenchyma. In all Conjugatae and most Protococcales, and
in the bulk of the Confervales, the thallus consists of a
cell or cells, the Protoplast of which contains a single
nucleus. In Hydrodictyaceae, Cladophoraceae, Sphaeropleaceae
and Gomontiaceae this is no longer the case. Instead of a
single relatively large nucleus, each cell is found to contain
many small nuclei, and is spoken of as a coenocyte. This
character becomes still more pronounced in the large group of
the Siphonales. Valoniaceae and Dasycladaceae are partially
septate, but elsewhere no cellulose partitions occur, and the
thallus is more or less the continuous tube from which the
group is named. Yet the siphonaceous algae may assume great
variety of form and reach a high degree of differentiation.
Protosiphon and Botrydium, on the one hand, are minute
vesicles attached to muddy surfaces by rhizoids; Caulerpa, on
the other, presents a remarkable instance of the way in which
much the same external morphology as that of cormophytes has
been reached by a totally different internal structure. Many
Siphonales are encrusted with lime like Corallina among Red
Algae. Penicillus is brush-like, Hallimeda and Cymopolia
are jointed, Acetabularia has much the same external form
as an expanded Coprinus, Neomeris simulates the fertile
shoot of Equisetum with its densely packed whorled branches,
and in Microdictyon, Anadyomene, Struvea and Boodlea the
branches, spreading in one plane, become bound together in
a more or less close network. Characeae are separated from
other Chloroohlceae by a long interval, and present the
highest degree of differentiation of parts known among Green
Algae. Attached to the bottom of pools by means of rhizoids,
the thallus of Characeae grows upwards by means of an apical
cell, giving off whorled appendages at regular intervals.
The appendages have a limited growth; but in connexion with
each whorl there arise, singly or in pairs, branches which
have the same unlimited growth as the main axis. There is
thus a close approach to the external morphology of the higher
plants. The streaming of the protoplasm, known elsewhere among
Chlorophyceae, is a conspicuous feature of the cells of Characeae.
The Chlorophyceae excel all other groups of algae in the
magnitude and variety of form of the chlorophyll-bodies. In
Ulva and Mesocarpus the chromatophore is a single plate,
which in the latter genus places its edge towards the incident
light; in Spirogyra they are spiral bands embedded in the
primordial utricle; in Zygnema they are a pair of stellate
masses, the rays of which branch peripherally; in Oedogonium
they are longitudinally-disposed anastomosing bands; in Desmids
plates with irregular margins; in Cladophora polyhedral
plates: in Vaucheria minute elliptical bodies occurring in
immense numbers. Embedded in the chromatophore, much in the
same way as the nucleus is embedded in the cytoplasm, are the
pyrenoids. Unknown in Cyanophyceae and Phoeophyeeae, known
only in Bangiaceae and Nemalion among Rhodophyceae, they
are of frequent occurrence among Chlorophyceae, excepting
Characeae. Sometimes several pyrenoids occur in each
chloroplast, as in Mesocarpus and Spirogyra; sometimes
only an occasional chloroplast contains pyrenoid at all,
as in Cadophora. The pyrenoid seems to be of proteid
nature and gelatinous consistency, and to arise as a new
formation or by division of pre-existing pyrenoids. When
carbon-assimilation is active, starch-granules crowd upon the
surface of the pyrenoid and completely obscure it from view.
Special provision for vegetative multiplication is not common
among Chlorophyceae. Valonia and Caulerpa among Siphonales
detach portions of their thallus, which are capable of independent
growth. In Caulerpa no other means of multiplication is as yet
known. In Characeae no fewer than four methods of vegetative
reproduction have been described, and the facility with which
buds and branches are in these cases detached has been adduced
as an evidence of affinity with Bryophyta, which, as a class, are
distinguished by their ready resort to vegetative reproduction.
With regard to true reproduction, which is characterized by
the formation of special cells, the group Euchlorophyceae
is characterized by the production of zoospores (Gr. zoon,
animal, spora, seed); that is to say, cells capable of
motility through the agency of cilia. Such ciliary motion
is known in the adult condition of the cells of Volvocaceae,
but where this is not the case the reproductive cells are
endowed with motility for a brief period. The zoospore
is usually a pyriform mass of naked protoplasm, the beaked
end of which where the cilia arise is devoid of colouring
matter. A reddish-brown body, known as the eyespot, is
usually situated near the limits of the hyaline portion, and
in the protoolasm contractile vacuoles similar to those of
lower animals have been occasionally detected. The movement
of the zoospore is effected by the lashing of the cilia and
is in the direction of the beak, while the zoospore slowly
rotates on Botrydium and Hydrodictyon only one is present;
in certain species of Cladophora four; in Dasycladus a
chaplet, and in Oedogonium a ring of many cilia. The
so-called zoospore of Vaucheria is a coenocyte covered over
with paired cilia corresponding in position to nuclei lying
below. In all other cases, zoospores are uninucleate bodies.
Zoospores arise in cells of ordinary size and form termed
zoosporangia. In unicellular forms (Sphaerella) the thallus
becomes transformed into a zoosporangium at the reproductive
stage. In the zoosporangia of Oedogonium, Tetraspora and
Coleochaete the contents become transformed into a single
zoospore. In most cases repeated division seems to take
place, and the final number is represented by some power of
two. In coenocytic forms the zoospores would seem to arise
simultaneously, probably because many nuclei are already
present. The escape of zoospores is effected by the
degeneration of the sporangial wall (Chaetophora), or by
a pore (Cladophora), a slit (Pediastrum ), or a circular
fracture (Oedogonium). Zoospores are of two kinds: (1) Those
which come to rest and germinate to form a new plant; these are
asexual and are zoospores proper. (2) Those which are unable
to germinate of themselves, but fuse with another cell, the
product giving rise to a new individual; these are sexual and
are zoogametes (Gr. zoon, animal, and gametes, gamete,
husband, wife). When two similar zoogametes fuse, the process
is conjugation, and the product a zygospore (Gr. zugon,
yoke). Usually, however, only one of the fusing cells is a
zoogamete, the other gamete being a much larger resting
cell. In such a case the zoogamete is male, is called an
antherozoid or spermatozoid, and arises in an antheridium; the
larger gamete is an oosphere and arises in an oogonium. The
fusion is now known as fertilization, and the product is an
oospore. Reproduction by conjugation is also known as isogamy,
by fertilization as oogamy. When zoospores come to rest,
a new cell is formed and germination ensues at once. When
zygospores and oospores are produced a new cell-wall is also
formed, but a long period of rest ensues. All investigation
goes to show that an essential part of sexual union is the
fusion of the two nuclei concerned. It is interesting to
know, on the authority of Oltmanns, that when the oosphere is
forming in the oogonium of Vaucheria, there is a retrocession
of all the included nuclei but one. that the antherozoid of
Vaucheria contains a single nucleus had been inferred before.
From a comparison of those Euchlorophyceae which have been
most closely investigated, it appears probable that sexual
reproductive cells have in the course of evolution arisen
as the result of specialization among asexual reproductive
cells, and that in turn oogamous reproduction has arisen
as the result of differentiation of the two conjugating
cells into the smaller male gamete and the larger male
gamete. It would further appear that oogamous reproduction
has arisen independently in each of the three main groups
of Euchlorophyceae, viz. Ptotococcales, Siphonales and
Confervales. Thus among Volvocaceae, a family of Protococcales,
while in some of the genera (Chloraster, Sphondylomorum)
no sexual union has as yet been observed, in others
(Pandorina, Chlorogonium, Stephanosphaera, Sphaerella)
conjugation of similar gametes takes place, in others still
(Phacotus, Eudorina, Volvox) the union is of the nature of
fertilization. No other family of Protococcales has advanced
beyond the stage of isogamous reproduction. Again, among
Siphonales only one family (Vaucheriaceae) has reached the
stage of oogamy, although an incipient heterogamy is said
to occur in two other families (Codiaceae, Bryopsidaceae).
Elsewhere among Siphonales, in those cases where reproductive
cells are known, the reproduction is either isogamous or
asexual. Among Confervales there is no family in which
sexual reproduction--isogamy or oogamy--is not known to occur
among some of the component species, and as many as four
families (Cylindrocapsaceae, Sphaeropleaceae, Oedogoniaceae,
Coleochaetaceae) are oogamous. On these, as well as other
grounds. Confervales are regarded as having attained to the
highest rank among Euchlorophyceae. Although the phenomena
attending isogamous and oogamous reproduction respectively
