to their natural affinities is an attempt to construct for
them the genealogical tree by which their relationships can be
traced. Algae are, however, so heterogeneous a class, of
which the constituent groups are so inadequately known, that
it is at present futile to endeavour thus to exhibit their
pedigree. A synoptical representation of the present state
of knowledge would be expressed by a network rather than by a
tree. The following table is an adaptation of a scheme
devised by Klebs, and indicates the inter-relationships
PROTOZOA Peridiniaceae......Diatomaceae
| |
Cryptomonadaceae--Hydruraceae--EUPHAEOPHYCEAE
Flagellata
protomastigina......Bacteriaceae--Endosphaeraceae
|
CYANOPHYCEAE........Bangiacaeae--EUFLORIDEAE
Eugleneae
Chloromonadinae Pleurococcaceae--Endosphaeraceae
Volvocaceae Chlorosphaeraceae CONGUGATAE SIPHONALES
Tetrasporaceae----------Ulvaceae----------CONFERVALES......CHARACEAE
FUNGI BRYOPHYTA
of the various constituent groups. The area included in
the thick boundary line represents algae in the widest
sense in which the term is used, and the four included areas
the four main subdivisions. A continuous line indicates a
close affinity, and a dotted line a doubtful relationship.
Alternation of generations.
In comparing algae with the great archegoniate series which
has doubtless sprung from them, it is natural to inquire to
what extent, if any, they present evidence of the existence
of the marked alternation of generations which dominates
the life-history of the higher plants. Turning first to the
Rhodophyceae, both on account of the high place which they
occupy among algae and also the remarkable uniformity in their
reproductive processes, it is clear that, as is the case among
Archegoniatae, the product of the sexual act never germinates
directly into a plant which gives rise to the sexual organs.
Even among Bangiaceae the carpospores arise from the fertilized
cell by division, while in all other Rhodophyceae the oospore,
as it may be called, gives rise to a filamentous structure,
varying greatly in its dimensions, epiphytic, and to a large
extent parasitic upon the egg-bearing parent plant, and in the
end giving rise to carpospores in the terminal cells of certain
branches. There is here obviously a certain parallelism
with the case of Bryophyta, where the sporogonium arising
from the oospore is epiphytic and partially parasitic upon
the female plant, and always culminates in the production of
spores. Not even Riccia, with its rudimentary sporogonium,
has so simple a corresponding stage as Bangia, for, while
there is some amount of sterile tissue in Riccia, in Bangia
the oospore completely divides to form carpospores. Excluding
Bangiaceae, however, from consideration, the Euflorideae
present in the product of the development of the oospore like
Bryophyta a structure partly sterile and partly fertile. There
is, nevertheless, this important difference between the two
cases. While the spore of Bryophyta on germination gives rise
to the sexual plant, the carpospore of the alga may give rise
on germination to a plant bearing a second sort of asexual
cells, viz. the tetraspores, and the sexual plant may only be
reached after a series of such plants have been successively
generated. It is possible, however, that the tetraspore
formation should be regarded as comparable with the prolific
vegetative reproduction of Bryophyta, and in favour of this
view there is the fact that the tetraspores originate on the
thallus in a different way from carpospores with which the
spores of Bryophyta are in the first place to be compared;
moreover, in certain Nemalionales the production of tetraspores
does not occur, and the difficulty referred to does not arise
in such cases. Altogether it is difficult on morphological
grounds to resist the conclusion that Florideae present the
same fundamental phenomenon of alternation of generations
as prevails in the higher plants. It is by means of the
cytological evidence, however, that this problem will finally be
solved. As is well known, the dividing nuclei of the cells
of the sporophyte generation of the higher plants exhibit a
double number of chromosomes, while the dividing nuclei of
the cells of the gametophyte generation exhibit the single
number. In a fern-plant, for example, which is a sporophyte,
every karyokinesis divulges the double number, while in the
prothallium, which is the gametophyte generation, the single
number appears. The doubling process is provided by the
act of fertilization, where an antherozoid with the single
number of chromosomes fuses with an oosphere also with the
single number to provide a fertilized egg with the double
number. The reduction stage, on the other hand, is the
first division of the mother-cell of the spore. From egg to
spore-mother-cell is sporophyte; from spore-mother-cell to
egg is gametophyte. And since this rule has been found to
hold good for all the archegoniate series and also for the
flowering plants where, however, the gametophyte generation
has become so extremely reduced as to be only with difficulty
discerned, it is natural that when alternation of generation
is stated to occur in any group of Thallophyta it should be
required that the cytological evidence should support the
view. The genus Nemalion has been recently investigated by
Wolfe with the object of examining the cytological evidence.
He finds that eight chromosomes appear in karyokinesis in the
ordinary thallus cells, but sixteen in the gonimoblast filaments
derived from the fertilized carpogonium. Eight chromosomes
appear again in the ultimate divisions which give rise to the
carpospores. Upon the evidence it would seem therefore
that so far as Nemalion is concerned an alternation occurs
comparable with that existing in the lower Bryophyta where
the sporophyte is relatively small, being attached to and to
some extent parasitic upon the gametophyte. Nemalion is,
however, one of those Florideae in which tetraspores do not
occur. What is the case with those Florideae which have been
described as trioecious? If the sporophyte generation is confined
to the cystocarp, is the tetrasporiferous plant, as has been
suggested, merely a potential gametophyte reproducing by a
process analogous to the bud- formation of the Bryophyta? In
answer to this question a recent writer, Yamanouchi, states
in a preliminary communication that he has found that in
Polysiphonia violacea the germinating carpospores exhibit
forty chromosomes, and the germinating tetraspores twenty
chromosomes. From this it would seem that in this plant
reduction takes place in the tetraspore mother- cell, and that
the tetrasporiferous plants are sporophytes which alternate with
sexual plants. Novel as this result may seem, the tetraspores
of Florideae become hereby comparable with the tetraspores of
Dictyota, to which reference will be made hereafter. But it
is clear that it becomes on this view increasingly difficult
to explain the occasional occurrence of tetraspores on male,
female and monoecious plants or the role of the carpospores
in the life-cycle of Florideae. The results of future research
on the cytology of the group will be awaited with interest.
Among Phaeophyceae it is well known that the oospore of
Fucaceae germinates directly into the sexual plant, and there
is thus only one generation. Moreover, it is known that
the reduction in the number of chromosomes which occurs at
the initiation of the gametophyte generation in Pteridophyta
occurs in the culminating stage of Fucus, where the oogonium
is separated from the stalk-cell, so that unless it be
contended that the Fucus is really a sporophyte which does
not produce spores, and that the gametophyte is represented
merely by the oogonium and antheridium, there is no semblance
of alternation of generation in this case. The only case
among Phaeophyceae which has been considered to point to the
existence of such a phenomenon is Cutleria. Here the asexual
cells are borne upon the so-called Aglaozonia reptans and the
sexual cells upon the plants known as Cutleria. The spores
of the Aglaozonia form are known to give rise to sexual
plants, and the oospore of Cutleria has been observed to
grow into rudimentary Aglaozonia. Latterly, however, as the
result of the cytological investigations of Mottler and Lloyd
Williams, great advance has been made in our knowledge of the
conditions existing in Dictyota. Mottler first observed that
a reduction in the number takes place in the mother-cells of the
tetraspore. It will be remembered that, as in most Florideae,
the male, female and asexual plants are distinct in this
genus. Mottler's observation has been confirmed by Lloyd
Williams, who has shown, moreover, that the single number
occurs in germlings from the tetraspore, and also in the
adult stages of all sexual plants, while the double number
occurs in germlings from the oospore, and in adult stages of
all asexual plants. It is probable, therefore, that we have
here a sharp alternation of generations, both generations
being, however, precisely similar to the eye up to point of
reproduction. Among Chlorophyceae it is often the case that the
oospore on germination divides up directly to form a brood of
zoospores. In Coleochaete this seems to be preceded by the
formation of a minute parenchymatous mass, in each cell of which
a zoospore is produced. In Sphaeroplea it is only at this
stage that zoospores are formed at all; but in most cases, such
as Oedogogonium, Ulothrix, Coleochaete, similar zoospores
are produced again and again upon the thallus, and the product
of the oospore may be regarded as merely a first brood of a
series. It has been held by some, however, that the first
brood corresponds to the sporophyte generation of the higher
plants, and that the rest of the cycle is the gametophyte
generation. Were the case of Sphaeroplea to stand alone,
the phenomenon might perhaps be regarded as an alternation of
generations, but still only comparable with the case of Bangia,
and not the case of the Florideae. But it is difficult to
apply such a term at all to those cases in which there intervene
between the oospore and the next sexual stage a series of
generations, the zoospores of which are all precisely similar.
Polymorphism.
The difficulty of tracing the relationships of algae is largely
due to the inadequacy of our knowledge of the conditions
under which they pass through the crucial stages of their
life-cycle. Of the thousands of species which have been
distinguished, relatively few have been traced from spore
to spore, as the flowering plants have been observed from
seed to seed. The aquatic habit of most of the species
and the minute size of many of them are difficulties which
do not exist in the case of most seed-plants. From the
analogy of the higher plants observers have justly argued
that when they have seen and marked the characters of the
reproductive organs they have found the plant at the stage
when it exhibits its most noteworthy features, and they have
named and classified the species in accordance with these
observations. While even in such cases it is obvious that
interesting stages in the life of the plant may escape notice
altogether, in the cases of those plants the reproduction of
which is unknown, and which have been named and placed on the
