CLASS III.--THE RED ALGÆ (_Rhodophyceæ_).
These are among the most beautiful and interesting members of the
plant kingdom, both on account of their beautiful colors and the
exquisitely graceful forms exhibited by many of them. Unfortunately
for inland students they are, with few exceptions, confined to salt
water, and consequently fresh material is not available. Nevertheless,
enough can be done with dried material to get a good idea of their
general appearance, and the fruiting plants can be readily preserved
in strong alcohol. Specimens, simply dried, may be kept for an
indefinite period, and on being placed in water will assume perfectly
the appearance of the living plants. Prolonged exposure, however, to
the action of fresh water extracts the red pigment that gives them
their characteristic color. This pigment is found in the chlorophyll
bodies, and usually quite conceals the chlorophyll, which, however,
becomes evident so soon as the red pigment is removed.
The red seaweeds differ much in the complexity of the plant body, but
all agree in the presence of the red pigment, and, at least in the
main, in their reproduction. The simpler ones consist of rows of
cells, usually branching like _Cladophora_; others form cell plates
comparable to _Ulva_ (Fig. 30, _C_, _D_); while others, among which is
the well-known Irish moss (_Chondrus_), form plants of considerable
size, with pretty well differentiated tissues. In such forms the outer
cells are smaller and firmer, constituting a sort of rind; while the
inner portions are made up of larger and looser cells, and may be
called the pith. Between these extremes are all intermediate forms.
They usually grow attached to rocks, shells, wood, or other plants,
such as the kelps and even the larger red seaweeds. They are most
abundant in the warmer seas, but still a considerable number may be
found in all parts of the ocean, even extending into the Arctic
regions.
[Illustration: FIG. 29.--_A_, a red seaweed (_Callithamnion_), of the
natural size. _B_, a piece of the same, × 50. _t_, tetraspores. _C_
i-v, successive stages in the development of the tetraspores, × 150.
_D_ I, II young procarps. _tr._ trichogyne. iii, young; iv, ripe spore
fruit. I, III, × 150. iv, × 50. _E_, an antheridium, × 150. _F_, spore
fruit of _Polysiphonia_. The spores are here surrounded by a case,
× 50.]
The methods of reproduction may be best illustrated by a specific
example, and preferably one of the simpler ones, as these are most
readily studied microscopically.
The form here illustrated (_Callithamnion_) grows attached to wharves,
etc., below low-water mark, and is extremely delicate, collapsing
completely when removed from the water. The color is a bright rosy
red, and with its graceful form and extreme delicacy it makes one of
the most beautiful of the group.
If alcoholic material is used, it may be mounted for examination
either in water or very dilute glycerine.
The plant is composed of much-branched, slender filaments, closely
resembling _Cladophora_ in structure, but with smaller cells
(Fig. 29, _B_). The non-sexual reproduction is by means of special
spores, which from being formed in groups of four, are known as
tetraspores. In the species under consideration the mother cell of
the tetraspores arises as a small bud near the upper end of one of
the ordinary cells (Fig. 29, _C_ i). This bud rapidly increases in
size, assuming an oval form, and becoming cut off from the cell of
the stem (Fig. 29, _C_ ii). The contents now divide into four equal
parts, arranged like the quadrants of a sphere. When ripe, the wall
of the mother cell gives way, and the four spores escape into the
water and give rise to new plants. These spores, it will be noticed,
differ in one important particular from corresponding spores in most
algæ, in being unprovided with cilia, and incapable of spontaneous
movement.
Occasionally in the same plant that bears tetraspores, but more
commonly in special ones, there are produced the sexual organs, and
subsequently the sporocarps, or fruits, developed from them. The
plants that bear them are usually stouter that the non-sexual ones,
and the masses of ripe carpospores are large enough to be readily
seen with the naked eye.
If a plant bearing ripe spores is selected, the young stages of the
female organ (procarp) may generally be found by examining the
younger parts of the plant. The procarp arises from a single cell of
the filament. This cell undergoes division by a series of
longitudinal walls into a central cell and about four peripheral
ones (Fig. 29, _D_ i). One of the latter divides next into an upper
and a lower cell, the former growing out into a long, colorless
appendage known as a trichogyne (Fig. 29, _D_, _tr._).
The antheridia (Fig. 29, _E_) are hemispherical masses of closely
set colorless cells, each of which develops a single spermatozoid
which, like the tetraspores, is destitute of cilia, and is dependent
upon the movement of the water to convey it to the neighborhood of
the procarp. Occasionally one of these spermatozoids may be found
attached to the trichogyne, and in this way fertilization is
effected. Curiously enough, neither the cell which is immediately
fertilized, nor the one beneath it, undergo any further change; but
two of the other peripheral cells on opposite sides of the filament
grow rapidly and develop into large, irregular masses of spores
(Fig. 29, _D_ III, IV).
While the plant here described may be taken as a type of the group,
it must be borne in mind that many of them differ widely, not only in
the structure of the plant body, but in the complexity of the sexual
organs and spores as well. The tetraspores are often imbedded in the
tissues of the plant, or may be in special receptacles, nor are they
always arranged in the same way as here described, and the same is
true of the carpospores. These latter are in some of the higher forms,
_e.g._ _Polysiphonia_ (Fig. 29, _F_), contained in urn-shaped
receptacles, or they may be buried within the tissues of the plant.
[Illustration: FIG. 30.--Marine red seaweeds. _A_, _Dasya_. _B_,
_Rhodymenia_ (with smaller algæ attached). _C_, _Grinnellia_. _D_,
_Delesseria_. _A_, _B_, natural size; the others reduced one-half.]
The fresh-water forms are not common, but may occasionally be met with
in mill streams and other running water, attached to stones and
woodwork, but are much inferior in size and beauty to the marine
species. The red color is not so pronounced, and they are, as a rule,
somewhat dull colored.
[Illustration: FIG. 31.--Fresh-water red algæ. _A_, _Batrachospermum_,
× about 12. _B_, a branch of the same, × 150. _C_, _Lemanea_, natural
size.]
The commonest genera are _Batrachospermum_ and _Lemanea_ (Fig. 31).