analogy of the vegetative parts alone, there is considerable
danger that a plant may be named as a distinct species which
is only a stage in the life of another distinct and perhaps
already known species. To take an example, Lemanea and
Batrachospermum are Florideae which bear densely-whorled
branches, but which, on the germination of the carpospore,
give rise to a laxly-filamentous, somewhat irregularly-branched
plant, from which the ordinary sexual plants arise at a later
stage. This filamentous structure has been attributed to the
genus Chantransia, which it greatly resembles, especially
when, as is said to be the case in Batrachospermum, it
bears similar monospores. The true Chantransia, however,
bears its own sexual organs as well as monospores. To the
specific identity of Haplospora globosa and Scaphospora
speciosa, and of Cutleria muitifida and Aglaozonia
reptans, reference has already been made. Again, many Green
Algae--some unicellular, like Sphaerella and Chlamydomonas;
some colonial forms, like Volvox and Hormotila; some
even filamentous forms, like Ulothrix and Stigeoclonium--
are known to pass into a condition resembling that of a
Palmella, and might escape identification on this account.
It is, on the other hand, a danger in the opposite sense to
conclude that all Chantransia species are stages in the
life-cycle of other plants, and, similarly, that all irregular
colonial forms, like Palmella, represent phases in the life
of other Green Algae. Long ago Kutzing went so far as to
express the belief that the lower algae were all capable of
transformations into higher forms, even into moss-protonemata.
Later writers have also thought that in all four groups
of algae transformations of a most far-reaching character
occur. Thus Borzi finds that Protoderma viride passes
through a series of changes so varied that at different times
it presents the characters of twelve different genera. Chodat
does not find so general a polymorphism, but nevertheless
holds that Raphidium passes through stages represented by
Protococcus, Characium, Dactylococcus and Sciadium. Klebs
has, however, recently canvassed the conclusions of both
these investigators; and as the result of his own observations
declares that algae, so far from being as polymorphic as
they have been described, vary only within relatively narrow
limits, and present on the whole as great fixity as the higher
plants. It certainly supports his view to discover, on
subjecting to a careful investigation Botrydium granulatum,
a siphonaceous alga whose varied forms had been described
by J. Rostafinski and M. Woronin, that these authors had
included in the life-cycle stages of a second alga described
previously by Kutzing, and now described afresh by Klebs as
Protosiphon bolryoides. In Botrydium the chromatophores
are small, without pyrenoids, and oil-drops are present;
in Protosiphon the chromatophores form a net-work with
pyrenoids, and the contents include starch. Klebs insists
that the only solution of such problems is the subjection of
the algae in question to a rigorous method of pure culture.
It is interesting to learn that G. Senn, pursuing the methods
described by Klebs, has confirmed Chodat's observation of the
passage of Raphidium into a Dactylococcus-stage, although
he was unable to observe further metamorphosis. He has
also seen Pleurococcus viridis dividing so as to form a
filament, but has not succeeded in seeing the formation of
zoospores as described by Chodat. While, therefore, there is
much evidence of a negative character against the existence
of an extensive polymorphism among algae, some amount of
metamorphosis is known to occur. But until the conditions
under which a particular transformation takes place have
been ascertained and described, so that the observation
may be repeated by other investigators, scant credence is
likely to be given to the more extreme polymorphistic views.
Physiology.
In comparison with the higher plants, algae exhibit so much
simplicity of structure, while the conditions under which
they grow are so much more readily controlled, that they have
frequently been the subject of physiological investigation with
a view chiefly to the application of the results to the study
of the higher plants. (See PLANTS: Physiology of.) In the
literature of vegetable physiology there has thus accumulated
a great body of facts relating not only to the phenomena of
reproduction, but also to the nutrition of algae. With
reference to their chemical physiology, the gelatinization
of the cell-wall, which is so marked a feature, is doubtless
attributable to the occurrence along with cellulose of pectic
compounds. There is, however, considerable variation in
the nature of the membrane in different species; thus the
cell-wall of Gedogonium, treated with sulphuric acid and
iodine, turns a bright blue, while the colour is very faint
in the case of Spirogyra, the wall of which is said to
consist for the most part of pectose. While starch occurs
commonly as a cell-content in the majority of the Green
Algae no trace of it occurs in Vaucheria and some of its
allies, nor is it known in the whole of the Phaeophyceae and
Rhodophyceae. In certain Euphaeophyceae bodies built up of
concentric layers, and attached to the chromatophores, were
described by Schmitz as phaeophycean-starch; they do not,
however, give the ordinary starch reaction. Other granules,
easily mistaken for the ``starch'' granules, are also found in
the cells of Phaeophyceae; these possess a power of movement
apart from the protoplasm, and are considered to be vesicles
and to contain phloroglucin. The colourless granules of
Florideae, which are supposed to constitute the carbohydrate
reserve material, have been called floridean-starch. A white
efflorescence which appears on certain Brown Algae (Saccorhiza
bulbosa, Laminaria saccharina), when they are dried in
the air, is found to consist of mannite. Mucin is known in
the cell-sap of Acetabularia. Some Siphonales (Codium)
give rise to proteid crystalloids, and they are of constant
occurrence among Florideae. The presence of tannin has been
established in the case of a great number of freshwater algae.
Colouring matters.
By virtue of the possession of chlorophyll all algae are
capable of utilizing carbonic acid gas as a source of carbon
in the presence of sunlight. The presence of phycocyanin,
phycophaein and phycoerythrin considerably modifies the
absorption spectra for the plants in which they occur. Thus
in the case of phycoerythrin the maximum absorption, apart
from the great absorption at the blue end of the spectrum,
is not, as in the case where chlorophyll occurs alone, near
the Fraunhofer line B, but farther to the right beyond the
line D. By an ingenious method devised by Engelmann, it
may be shown that the greatest liberation of oxygen, and
consequently the greatest assimilation of carbon, occurs in
that region of the spectrum represented by the absorption
bands. In this connexion Pfeffer points out that the penetrating
power of light into a clear sea varies for light of different
colours. Thus red light is reduced to such an extent as
to be insufficient for growth at a depth of 34 metres,
yellow light at a depth of 177 metres and green light at 322
metres. It is thus an obvious advantage to Red Algae, which
flourish at considerable depths, to be able to utilize yellow
light rather than the red, which is extinguished so much
sooner. The experiment of Engelmann referred to deserves
to be mentioned here, if only in illustration of the use
to which algae have been put in the study of physiological
problems. Engelmann observed that certain bacteria were motile
only in the presence of oxygen, and that they retained their
motility in a microscopic preparation in the neighbourhood
of an algal filament when they had come to rest elsewhere
on account of the exhaustion of oxygen. After the bacteria
had all been brought to rest by being placed in the dark,
he threw a spectrum upon the filament, and observed in what
region the bacteria first regained their motility, owing to the
liberation of oxygen in the process of carbon-assimilation.
He found that these places corresponded closely with the
region of the absorption band for the algae under experiment.
Although algae generally are able to use carbonic acid gas
as a source of carbon, some algae, like certain of the higher
plants, are capable of utilizing organic compounds for this
purpose. Thus Spirogyra filaments, which have been
denuded of starch by being placed in the dark, form starch
in one day if they are placed in a 10 to 20% solution of
dextrose. According to T. Bokorny, moreover, it appears
that such filaments will yield starch from formaldehyde
when they are supplied with sodium oxymethyl sulphonate, a
salt which readily decomposes into formaldehyde and hydrogen
sodium sulphite, an observation which has been taken to
mean that formaldehyde is always a stage in the synthesis of
starch. With reference to the assimilation of nitrogen,
it would seem that algae, like other green plants, can
best use it when it is presented to them in the form of a
nitrate. Some algae, however, seem to flourish better
in the presence of organic compounds. In the case of
Scenedesmus acutus it is said that the alga is unable
to take up nitrogen in the form of a nitrate or ammoniacal
salt, and requires some such substance as an amide or a
peptone. On the other hand, it has been held by Bernhard
Frank and other observers that atmospheric nitrogen is fixed
by the agency of Green Algae in the soil: (For the remarkable
symbiotism between algae and fungi see FUNGI and LICHENS.)
Habitat.
Most algae, particularly Phaeophyceae and Rhodophyceae, spend
the whole of the life-cycle immersed in water. In the case of
the freshwater algae, however, belonging to the Chlorophyceae
and Cyanophyceae, although they required to be immersed during
the vegetative period, the reproductive cells are often capable
of resisting a considerable degree of desiccation, and in this
condition are dispersed through great distances by various
agencies. Again, as is well known, many species of marine
algae growing in the region between the limits of high and low
water are so constituted that they are exposed to the air twice
a day without injury. The occurrence of characteristic algae
at different levels constituting the zones to which reference
has already been made, is probably in part an expression of
the fact that different species vary in the capacity to resist
desiccation from exposure. Thus Laminaria digitata, which
characterizes the lowest zone, is only occasionally exposed at
all, and then only for short periods of time. On the other
hand, Pelvetia canaliculata, which marks the upper belt, is
exposed for longer periods, and during neap tides may not be
reached by the water for many days. Algae of more delicate
texture than either Fucaceae or Laminariaceae also occur in
the region exposed by the ebb of the tide, but these secure
their exemption from desiccation either by retaining water
in their meshes by capillary attraction, as in the case of
Pilayella, or by growing among the tangles of the larger
Fucaceae, as in the case of Polysiphonia fastigiata, or by
growing in dense masses on rocks, as in the case of Laurencia
pinnatifida. Such a species as Delesseria sanguinea or
Callophyllis laciniata would on the contrary run great risk
by exposure for even a short period. A few algae approach
the ordinary terrestrial plants in their capacity to live in
