CLASSIFICATION AND ADDITIONAL STUDIES OF THE ALGÆ.
In order to show the general relationship of the algæ studied, the
principal classes are here enumerated as well as some of the families.
In some of the groups not represented by the examples studied above, a
few species are described which may serve as the basis of additional
studies if desired. The principal classes[17] of algæ are as follows:
Class Chlorophyceæ.
=331.= These are the green algæ, so called because the chlorophyll
green is usually not masked by other pigments, though in some forms it
is. There are three subclasses.
=332. Subclass PROTOCOCCOIDEÆ.=—In the Protococcoideæ are found
the simplest green plants. Many of them consist of single cells which
live an independent life. Others form “colonies,” loose aggregations
of individuals not yet having attained the permanency of even a simple
plant body, for the cells often separate readily and are able to form
new colonies. The colonies are often held together by a gelatinous
membrane, or matrix. Some are motile, while others are non-motile. A
few of the families are here enumerated.
=333. Family Volvocaceæ.=—These are all motile, during the
vegetative stage. The individuals are single or form more or less
globose colonies.
=334. The “red snow” plant (Sphærella nivalis).=—This is often
found in arctic and alpine regions forming a red covering over more or
less large areas of snow or ice. For this reason it is called the “red
snow plant.”
=335. Sphærella lacustris=, a closely related species, is very
widely distributed in temperate regions along streams or on the borders
of lakes and ponds. Here in dry weather it is often found closely
adhering to the dry rock surface, and giving it a reddish color as
if the rock were painted. This is especially the case in the shallow
basins formed over the uneven surface of the rock near the water’s
edge. These places during heavy rains or in high water are provided
with sufficient water to fill the basins. During such times the red
snow plant grows and multiplies, loses its red color and becomes green,
and, being motile, is free swimming. It is a single-celled plant,
oval in form, surrounded by a gelatinous sheath and with two cilia or
flagella at the smaller end, by the vibration of which it moves (fig.
162). The single cell multiplies by dividing into two cells. When the
water dries out of the basin, the motile plant comes to rest, and many
of the cells assume the red color. To obtain the plant for study,
scrape some of the red covering from these rock basins and place it in
fresh spring water, and in a day or so the swarmers are likely to be
found. Under certain conditions small microzoids are formed.
[Illustration: Fig. 162.
Sphærella lacustris (Girod.) Wittrock. _A_, mature free swimming
individual with central red spot. _B_, division of mother individual to
form two. _C_, division of a red one to form four. _D_, division into
eight. _E_, a typical resting cell, red. _F_, same beginning to divide.
_G_, one of four daughter zoospores after swimming around for a time
losing its red color and becoming green. (After Hazen.)]
=336. Chlamydomonas= is a very interesting genus of motile
one-celled green algæ, because the species are closely related to
the Flagellates among the lower animals. The plant is oval, with a
single chloroplast and surrounded by a gelatinous envelope through
which the two cilia or flagella extend. One-celled organisms of this
kind are sometimes called _monads_, i.e., a one-celled being. This
one has a gelatinous cloak and is, therefore, a _cloaked monad_
(_Chlamydomonas_). The species often are found as a very thin green
film on fresh water. C. pulvisculus is shown in fig. 163. When it
multiplies the single cell divides into two, as shown in _B_. Sometimes
a non-motile palmella stage is formed, as shown in _C_ and _D_.
Reproduction takes place by gametes which are of unequal size, the
smaller one representing the sperm and the larger one the egg, as in
_E_ and _F_. These conjugate as in _G_ and _H_, the protoplasm of the
smaller one passing over into the larger one, and a zygospore is thus
formed.
[Illustration: Fig. 163.
_Chlamydomonas pulvisculus_ (Müll.) Ehrb. _A_, an old motile
individual; _n_, nucleus; _p_, pyrenoid; _s_, red eye spot; _v_,
contractile vacuole; _B_, motile individual has drawn in its cilia
and divided into two; _C_, mother plant has drawn in its cilia and
divided into four non-motile cells; _D_, pamella stage; _E_, female
gamete—egg; _F_, male gamete—sperm; _G_, early stage of conjugation;
_H_, zygospore with conjugating tube and empty male cell attached.
(After Wille.)]
=337. Of those which form colonies=, Pandorina morum is widely
distributed and not rare. It consists of a sphere formed of sixteen
individuals enclosed in a thin gelatinous membrane. Each cell possesses
two cilia (or flagella), which extend from the broader end out through
the enveloping membrane. By the movement of these flagella the colony
goes rolling around in the water. When the plant multiplies each
individual cell divides into sixteen small cells, which then grow and
form new colonies. Reproduction takes place when the individual cells
of the young colonies separate, and usually a small individual unites
with a larger one and a zygospore is formed (see fig. 164). Eudorina
elegans is somewhat similar, but when the gametes are formed certain
mother cells divide into sixteen small motile males or sperms, and
certain other mother cells divide into sixteen large motile females or
eggs. These separate from the colonies, and the sperms pair with the
eggs and fuse to form zygospores. This plant as well as Chlamydomonas
pulvisculus foreshadows the early differentiation of sex in plants.
[Illustration: Fig. 164.
Pandorina morum (Müll.) Bory. I, motile colony; II, colony divided into
16 daughter colonies; III, sexual colony, gametes escaping; IV, V,
conjugating gametes; VI, VII, young and old zygospore; VIII, zygospore
forming a large swarm spore, which is free in IX; X, same large swarm
spore divided to form young colony. (After Pringsheim.)]
[Illustration: Fig. 165. Pleurococcus (protococcus) vulgaris.]
=338. Family Tetrasporaceæ.=—This family is well represented by
Tetraspora lubrica forming slimy green net-like sheets attached to
objects in slow-running water. It is really a single-celled plant. The
rounded cells divide by cross walls into four cells, and these again,
and so on, large numbers being held in loose sheets by the slime in
which they are imbedded.
=339. Family Pleurococcaceæ.=—The members of this family are
all non-motile in the vegetative stage. They consist of single
individuals, or of colonies. Pleurococcus vulgaris (Protococcus
vulgaris) is a single-celled alga, usually obtained with little
difficulty. It is often found on the shaded, and cool, or moist side of
trees, rocks, walls, etc., in damp places. This plant is not motile. It
multiplies by fission (Fig. 165) into two, then four, etc. These cells
remain united for a time, then separate. Sometimes the cells are found
growing out into filaments, and it is thought by some that P. vulgaris
may be only a simple stage of a higher alga. Eremosphæra viridis is
another single-celled alga found in fresh water among filamentous
forms. The cells are large and globose.
[Illustration: Fig. 166.
Pediastrum boryanum. _A_, mature colony, most of the young colonies
have escaped from their mother cells; at _g_, a young colony is
escaping; _sp_, empty mother cells; _B_, young colony; _C_, same colony
with spores arranged in order. (After Braun.)]
=340. Family Hydrodictyaceæ.=—These plants form colonies of
cells. Hydrodictyon reticulatum, the water net, is made up of large
numbers of cylindrical cells so joined at their ends as to form a large
open mesh or net. Pediastrum forms circular flat colonies, as shown in
fig. 166. Both of these plants are rather common in fresh-water pools,
the latter one intermingled with filamentous algæ, while the former
forms large sheets or nets. Multiplication in Hydrodictyon takes place
by the protoplasm in one of the cells dividing into thousands of minute
cells, which gradually arrange themselves in the form of a net, escape
together from the mother cell, and grow into a large net. In Pediastrum
multiplication takes place in a similar way, but the protoplasm in each
cell usually divides into sixteen small cells, and escaping together
from the mother cell arrange themselves and grow to full size (fig.
166).
=341. The Conjugateæ= include several families of green algæ,
which probably should be included among the Chlorophyceæ. They have
probably had their origin from some of the more simple members of the
Protococcoideæ. They are represented by Spirogyra, Zygnema, and the
desmids, studied in Chapter 14.
=342. Subclass CONFERVOIDEÆ.=—These are mostly filamentous algæ,
the filaments being composed of cells firmly united, and, with the
exception of the simplest forms, there is a definite growing point. A
few of the families are as follows:
=343. Family Ulvaceæ.=—These contain the sea wracks, or sea
lettuce, like Ulva, forming expanded green, ribbon-like growths in the
sea.
[Illustration: Fig. 167.
Ulothrix zonata. _A_, base of thread. _B_, cells with zoospores, _C_,
one cell with zoospores escaping another cell with small biciliate
gametes escaping and some fusing to form zygospores, _E_, zoospores
germinating and forming threads: _F_, _G_, zygospore growing and
forming zoospores. (After Caldwell and Dodel-Port.)]
=344. Family Ulotrichaceæ=, represented by Ulothrix zonata, not
uncommon in slow-running water or in ponds of fresh water attached
to rocks or wood. It consists of simple threads of short cells.
Multiplication takes place by zoospores. Reproduction takes place by
motile sexual cells (gametes) which fuse to form a zygospore (fig. 167).
=345. Family Chætophoraceæ=, represented by Chætophora (in Chapter
15) and Drapernaudia in fresh water.
=346. Family Œdogoniaceæ=, represented by Œdogonium (Chapter 16).
=347. Family Coleochætaceæ=, represented by Coleochæte (Chapter
17).
=348. Subclass SIPHONEÆ.=—There are several families.
=349. Family Botrydiaceæ.=—This is represented by Botrydium
granulatum (Chapter 15, p. 146).
=350. Family Vaucheriaceæ=, represented by Vaucheria (Chapter 15),
with quite a large number of species, is widely distributed.
Class Schizophyceæ (= Cyanophyceæ).
[Illustration: Fig. 168. Glœocapsa.]
=351. The Blue-Green Algæ=, or =Cyanophyceæ= form slimy
looking thin mats on damp wood or the ground, or floating mats or
scum on the water. The color is usually bluish green, but in some
species it is purple, red or brown. All have chlorophyll, but it is
not in distinct chloroplasts and is more or less completely guised
by the presence of other pigments. Two orders and eight families are
recognized. The following include some of our common forms:
=352. ORDER COCCOGONALES (COCCOGONEÆ).=—Single-celled plants,
occurring singly or in colonies, in some forms forming short threads.
One of the two families is mentioned.
=353. Family Chroococcaceæ.=—The plants multiply only through cell
division. Chroococcus, forms rounded, blue-green cells enclosed in a
thick gelatinous coat, in fresh water and in damp places; certain
species form “lichen-gonidia” in some genera of lichens. Glœocapsa is
similar to Chroococcus, but the colonies are surrounded by an additional
common gelatinous envelope (fig. 168); on damp rocks, etc.
[Illustration: Fig. 169.
_A_, Oscillatoria princeps, _a_, terminal cell; _b_, _c_, portions from
the middle of a filament. In _c_, a dead cell is shown between the
living cells; _B_, Oscillatoria froelichii, _b_, with granules along
the partition walls.]
=354. ORDER HORMOGONALES (HORMOGONEÆ).=—Plants filamentous,
simple celled or with false or true branching, usually several celled
(Spirulina is single celled). Multiplication takes place through
_hormogones_, short sections of the threads becoming free; also through
resting cells. Two of the six families are mentioned.
=355. Family Oscillatoriaceæ.=—This family is represented by
the genus Oscillatoria, and by several other genera common and widely
distributed. Oscillatoria contains many species. They are found on the
damp ground or wood, or floating in mats in the water. They often form
on the soil at the bottom of the pool, and as gas becomes entangled
in the mat of threads, it is lifted from the bottom and floated to
the surface of the water. The plant is thread-like, and divided up
into many short cells. The threads often show an oscillating movement,
whence the name _Oscillatoria_.
=356. Family Nostocaceæ.=—This family is represented by Nostoc,
which forms rounded, slimy, blue-green masses on wet rocks. The
individual plants in the slimy ball resemble strings of beads, each
cell being rounded, and several of these arranged in chains as shown
in fig. 170. Here and there are often found larger cells (heterocysts)
in the chain. Nostoc punctiforme lives in the intercellular spaces
of the roots of cycads (often found in greenhouses), and in the
stems of Gunnera. N. sphæricum lives in the spaces between the cells
in many species of liverworts (in the genera Anthoceros, Blasia,
Pellia, Aneura, Riccia, etc.), and in the perforated cells of Sphagnum
acutifolium. Anabæna is another common and widely distributed genus.
The species occur in fresh or salt water, singly or in slimy masses.
Anabæna azollæ lives endophytically in the leaves of the water fern,
Azolla.
[Illustration: Fig. 170.
Nostoc linckii. _A_, filament with two heterocysts (_h_), and a large
number of spores (_sp_); _B_, isolated spore beginning to germinate;
_C_, young filament developed from spore. (After Bornet.)]
[Illustration: Fig. 171.
Bacteria. _A_, Bacillus subtilis. Spores in threads, unstained rods,
and stained rods showing cilia; _B_, Bacillus tetani, the tetanus
or lockjaw bacillus, found in garden soil and on old rusty nails.
Spores in club-shaped ends. _C_, Micrococcus; _D_, Sarcina; _E_,
Streptococcus; _F_, Spirillum. (After Migula.)]
Class Schizomycetes.
=357. Bacteriales.=—The bacteria are sometimes classified
with the Cyanophyceæ, under the name Schizophyta, and represent the
subdivision Schizomycetes, or fission fungi, because many of them
multiply by a division of the cells just as the blue-green algæ do.
For example, Bacillus forms rods which increase in length and divide
into two rods, or it may grow into a long thread of many short rods.
Micrococcus consists of single rounded cells. Streptococcus forms
chains of rounded cells, Sarcina forms irregular cubes of rounded
cells, while others like Spirillum are spiral in form. Bacillus
subtilis may be obtained by making an infusion from hay and allowing
it to stand for several days. Bacillus tetani occurs in the soil, on
old rusty nails, etc. It is called the tetanus bacillus because it
causes a permanent spasm of certain muscles, as in “lockjaw.” This
bacillus grows and produces this result on the muscles when it occurs
in deep and closed wounds such as are caused by stepping on an old nail
or other object which pierces the flesh deeply. In such a deep wound
oxygen is deficient, and in this condition the bacillus is virulent.
Opening the wounds to admit oxygen and washing them out with a solution
of bichloride of mercury prevents the tetanus. Many bacteria are of
great importance in bringing about the decay of dead animal and plant
matter, returning it to a condition for plant food. (See also nitrate
and nitrite bacteria, Chapter IX.) While most bacteria are harmless
there are many which cause very serious diseases of man and animals,
as typhoid fever, diphtheria, tuberculosis, etc., while some others
produce disease in plants. Others aid in certain fermentations of
liquids and are employed for making certain kinds of wines or other
beverages. Some work in symbiosis with yeasts, as in the kephir yeast,
used in fermenting certain crude beverages by natives of some countries.
=357=_a_. =Myxobacteriales (Myxobacteriaceæ
Thaxter[18]).=—These plants consist of colonies of bacteria-like
organisms, motile rods, which multiply by cross-division and secrete
a gelatinous substance or matrix which surrounds the colonies. They
form plasmodium-like masses which superficially resemble the slime
moulds. In the fruiting stage some species become elevated from the
substratum into cylindrical, clavate, or branched forms, which bear
cysts of various shapes containing the rods in a resting stage, or the
rods are converted into spore-like masses. Ex., Chondromyces crocatus
on decaying plant parts, Myxobacter aureus on wet wood and bark,
Myxococcus rubescens on dung, decaying lichens, paper, etc.
Class Flagellata.
=358. The flagellates= are organisms of very low organization
resembling animals as much as they do plants. They are single celled
and possess two cilia or flagella, by the vibration of which they
move. Some are without a cell wall, while others have a well-defined
membrane, but it rarely consists of cellulose. Some have chromatophores
and are able to manufacture carbohydrates like ordinary green plants.
These are green in Euglena, and brown in Hydrurus. Some possess a
mouth-like opening and are able to ingest solid particles of food
(more like animals), while others have no such opening and absorb food
substances dissolved in water (more like plants). The Euglena viridis
is not uncommon in stagnant water, often forming a greenish film on the
water.
Class Peridineæ.
=358=_a_. These are peculiar one-celled organisms provided with
two flagella and show some relationship to the Flagellates. They
usually are provided with a cellulose membrane, which in some forms
consists of curiously sculptured plates. In the higher forms this
cellulose membrane consists of two valves fitting together in such a
way as to resemble some of the diatoms. Like the Flagellates, some
have green chromatophores, which in some are obscured by a yellow or
brown pigment (resembling the diatoms), while still others have no
chlorophyll. The Peridineæ are abundant in the sea, while some are
found in fresh water.
Class Diatomaphyceæ (Bacillariales, Diatomaceæ).
[Illustration: Fig. 171_a_.
A group of Diatoms: _c_ and _d_, top and side views of the same form;
_e_, colony of stalked forms attached to an alga; _f_ and _g_, top and
side views of the form shown at _e_; _h_, a colony; _i_, a colony, the
top and side view shown at _k_ and _n_, forming auxospores. (After
Kerner.)]
=358=_b_. =The diatoms= are minute and peculiar organisms
believed to be algæ. They live in fresh, brackish, and salt water. They
are often found covering the surface of rocks, sticks, or the soil
in thin sheets. They occur singly and free, or several individuals
may be joined into long threads, or other species may be attached to
objects by slender gelatinous stalks. Each protoplast is enclosed in a
silicified skeleton in the form of a box with two halves, often shaped
like an old-fashioned pill box, one half fitting over the other like
the lid of a box. It is evident that in this condition the plant cannot
increase much in size.
They multiply by fission. This takes place longitudinally, i.e., in the
direction of the two halves or _valves_ of the box. Each new plant then
has a valve only on one side. A new valve is now formed over the naked
half, and fits inside the old valve. At each division the individuals
thus become smaller and smaller until they reach a certain point, when
the valves are cast off and the cell forms an _auxospore_, i.e., it
grows alone, or after conjugation with another, to the full size again,
and eventually provides itself with new valves. The valves are often
marked, with numerous and fine lines, often making beautiful figures,
and some are used for test objects for microscopes.
The free forms are capable of movement. The movement takes place in the
longitudinal direction of the valves. They glide for some time in one
direction, and then stop and move back again. It is not a difficult
thing to mount them in fresh water and observe this movement.
The diatoms have small chlorophyll plates, but the green color is
disguised by a brownish pigment called diatomin. The relationships of
the diatoms are uncertain, but some, because of the color, think they
are related to the Phæophyceæ.
Class Phæophyceæ.
=359. The brown algæ. (Phæophyceæ).=—The members of this class
possess chlorophyll, but it is obscured by a brown pigment. The plants
are accessible at the seashore, and for inland laboratories may be
preserved in formalin (2½ per cent). (See also Chapter LVI.)
[Illustration: Fig. 172.
_A_, Ectocarpus siliculosus; _B_, branch with a young and a ripe
plurilocular sporangium; _E_, gametes fusing to form zygospore, (_B_,
after Thuret; _E_, after Berthold.)]
=360. Ectocarpus.=—The genus Ectocarpus represents well some
of the simpler forms of the brown algæ (fig. 172). They are slender,
filamentous branched algæ growing in tufts, either epiphytic on other
marine algæ (often on Fucaceæ), or on stones. The slender threads are
only divided crosswise, and thus consist of long series of short cells.
The sporangia are usually plurilocular (sometimes unilocular), and
usually occur in the place of lateral branches. The zoospores escape
from the apex of the sporangium and are biciliate, and they fuse to
form zygospores.
=361. Sphacelaria.=—The species of this genus represent an
advance in the development of the thallus. While they are filamentous
and branched, division takes place longitudinally as well as crosswise
(fig. 173).
=362. Leathesia difformis= represents an interesting type because
the plant body is small, globose, later irregular and hollow, and
consists of short radiately arranged branches, the surface ones in the
form of short, crowded, but free, trichome-like green branches. This
trichothallic body recalls the similar form of Chætophora pisiformis
(Chapter 16) among the Chlorophyceæ.
[Illustration: Fig. 173. Sphacelaria, portion of plant showing
longitudinal division of cells, and brood bud (plurilocular
sporangium).]
[Illustration: Fig. 174. Laminaria digitata, forma cloustoni, North
Sea. (Reduced.)]
=363. The Giant Kelps.=—Among the brown algæ are found the
largest specimens, some of the laminarias or giant kelps, rivaling in
size the largest land plants, and some of them have highly developed
tissues. _Postelsia palmæformis_ has a long, stout stem, from the
free end of which extend numerous large and long blades, while the
stem is attached to the rocks by numerous “root” like outgrowths, the
holdfasts. It occurs along the northern Pacific coast, and appears to
flourish where it receives the shock of the surf beating on the shore.
Several species of Laminaria occur on our north Atlantic coast. In L.
digitata, the stem expands at the end into a broad blade, which becomes
split into several smaller blades (fig. 174). _Macrocystis pyrifera_
inhabits the ocean in the southern hemisphere, and sometimes is found
along the north American coast. It is said to reach a length of 200-300
meters.
=364. Fucus, or Rockweed.=—This plant is a more or less branched
and flattened thallus or “frond.” One of them, illustrated in fig. 119,
measures 15-30 _cm_ (6-12 inches) in length. It is attached to rocks
and stones which are more or less exposed at low tide. From the base
of the plant are developed several short and more or less branched
expansions called “holdfasts,” which, as their name implies, are organs
of attachment. Some species (F. vesiculosus) have vesicular swellings
in the thallus.
The fruiting portions are somewhat thickened as shown in the figure.
Within these portions are numerous oval cavities opening by a circular
pore, which gives a punctate appearance to these fruiting cushions.
Tufts of hairs frequently project through them.
[Illustration: Fig. 175. Portion of plant of Fucus showing conceptacles
in enlarged ends; and below the vesicles (Fucus vesiculosus).]
[Illustration: Fig. 176. Section of conceptacle of Fucus, showing
oogonia, and tufts of antheridia.]
=365. Structure of the conceptacles.=—On making sections of the
fruiting portions one finds the walls of the cavities covered with
outgrowths. Some of these are short branches which bear a large rounded
terminal sac, the oogonium, at maturity containing eight egg-cells.
More slender and much-branched threads bear narrowly oval antheridia.
In these are developed several two-ciliated spermatozoids.
[Illustration: Fig. 177. Oogonium of Fucus with ripe eggs.]
[Illustration: Fig. 178. Antheridia of Fucus, on branched threads.]
=366. Fertilization.=—At maturity the spermatozoids and egg-cells
float outside of the oval cavities, where fertilization takes place.
The spermatozoid sinks into the protoplasm of the egg-cell, makes its
way to the nucleus of the egg, and fuses with it as shown in fig. 181.
The fertilized egg then grows into a new plant. Nearly all the brown
algæ are marine.
[Illustration: Fig. 179. Antheridia of Fucus with escaping
spermatozoids.]
[Illustration: Fig. 180. Eggs of Fucus surrounded by spermatozoids.]
[Illustration: Fig. 181.
Fertilization in Fucus; _fn_, female nucleus; _mn_, male nucleus;
_n_, nucleolus. In the left figure the male nucleus is shown moving
down through the cytoplasm of the egg; in the remaining figures the
cytoplasm of the egg is omitted. (After Strasburger.)]
=367. The Gulf weed= (=Sargassum bacciferum=) in the warmer
Atlantic ocean unites in great masses which float on the water, whence
comes the name “Sargassum Sea.” The Sargassum grows on the coast where
it is attached to the rocks, but the beating of the waves breaks many
specimens loose and these float out into the more quiet waters, where
they continue to grow and multiply vegetatively.
=368. Uses.=—Laminaria japonica and L. angustata are used as food
by the Chinese and Japanese. Some species of the Laminariaceæ are used
as food for cattle and are also used for fertilizers, while L. digitata
is sometimes employed in surgery.
_Classification._—Kjellman divides the Phæophyceæ into two orders.
=369. Order Phæosporales (Phæosporeæ)= including 18 families.
One of the most conspicuous families is the Laminariaceæ, including
among others the Giant Kelps mentioned above (Laminaria, Postelsia,
Macrocystis, etc.).
=370. Order Cyclosporales (Cyclosporeæ).=—This includes one
family, the _Fucaceæ_ with Ectocarpus, Sphacelaria, Læathesia, Fucus,
Sargassum, etc.
Class Rhodophyceæ.
=371. The red algæ (Rhodophyceæ).=—The larger number of the
so-called red algæ occur in salt water, though a few genera occur in
fresh water. The plants possess chlorophyll, but it is usually obscured
by a reddish or purple pigment.
=372. Nemalion.=—This is one of the lower marine forms, though
its thallus is not one of the simplest in structure. The plant body
consists of a slender cylindrical branched shoot, sometimes very
profusely branched. The central strand is rather firm, while the cortex
is composed of rather loose filaments.
[Illustration: Fig. 182.
A red alga (Nemalion). _A_, sexual branches, showing antheridia (_a_);
carpogonium or procarp (_o_) with its trichogyne (_t_), to which are
attached two spermatia (_s_); _B_, beginning of a cystocarp (_o_),
the trichogyne (_t_) still showing; _C_, an almost mature cystocarp
(_o_), with the disorganizing trichogyne (_t_). (After Vines.)]
=373. Batrachospermum.=—This genus occurs in fresh water, and the
species are found in slow-running water of shallow streams or ditches.
There is a central slender strand which is more or less branched,
and on these branches are whorls of densely crowded slender branches
occurring at regular intervals. The plants are usually very slippery.
Gonidia are formed on the ends of some of these branches in globose
sporangia, called monosporangia, since but a single spore or gonidium
is developed in each. Other branches often terminate in long slender
hyaline setæ.
=374. Lemanea.=—This genus also occurs in fresh water. The
species develop only during the cold winter months in rapids of streams
or where the water from falls strikes the rocks and is thoroughly
aerated. They form tufts of greenish threads, cylindrical or whiplike,
which in the summer are usually much broken down. The threads are
hollow and have a firm cortex. These are the sexual shoots, and they
arise as branches from a sterile filamentous-branched, Chantransia-like
form.
=375. Fertilization in the lower red algæ.=—The sexual organs in
the red algæ consist of antheridia and carpogonia. The antheridia are
usually borne in crowded clusters, or surfaces, and bear terminally
the small non-motile sperm cells. The carpogonium is a branch of one
or several cells, the terminal cell (procarp) extending into a long
slender process, the trichogyne. The sperm cell comes in contact with
the trichogyne, and in the case of Nemalion and some others the nucleus
has been found to pass down the inside and fuse with the nucleus of the
procarp.
[Illustration: Fig. 183.
_A_, part of a shoot showing whorls of branches with clusters of
carpospores. _B_, carpogonic branch or procarp. _c_, procarp cell;
_tr_, trichogyne. _C_, same with sperm (_sp_) uniting with trichogyne.
_D_, same with carpospores developing from procarp cell. _E_, male
branch with one-celled antheridia. _F_, same with some of antheridia
empty. (After Schmitz.)]
From this point in the lower red algæ like Nemalion, Batrachospermum
and Lemanea the formation of the spores is very simple. The procarp
is stimulated to growth, and buds in different directions, producing
branched chains of spores (carpospores). The carpospores form a rather
compact cluster called the sporocarp, which means spore-fruit or
spore-fruit body. In Batrachospermum it is seen as a compact tuft in
the loose branching, in Nemalion it lies in the surface of the cortex,
while in Lemanea the sporocarps lie at different positions in the
hollow tube of the sexual shoot.
[Illustration: Fig. 184.
A red alga (Callithamnion), showing sporangium _A_, and the tetraspores
discharged _B_. (After Thuret.)]
[Illustration: Fig. 185. Gracilaria, portion of frond, showing position
of cystocarps.]
[Illustration: Fig. 186. Gracilaria, section of cystocarp showing
spores.]
=376. Gonidia in the red algæ.=—The common type of gonidium
in the red algæ is found in the _tetraspores_. A single mother cell
divides into four cells arranged usually in the form of tetrads within
the _tetrasporangium_. In Callithamnion the tetrasporangium is exposed.
In Polysiphonia, Rhabdonia, Gracilaria, etc., it is imbedded in the
cortex. In Batrachospermum there are monosporangia, each monosporangium
containing a single gonidium, while in Lemanea, and according to some
also in Nemalion, gonidia are wanting.
=377. Gracilaria.=—Gracilaria is one of the marine forms, and
one species is illustrated in fig. 185. It measures 15-20_cm_ or more
long, and is profusely branched in a palmate manner. The parts of the
thallus are more or less flattened. The fruit is a cystocarp, which
is characteristic of the Rhodophyceæ (Florideæ). In Gracilaria these
fruit bodies occur scattered over the thallus. They are somewhat
flask-shaped, are partly sunk in the thallus, and the conical end
projects strongly above the surface. The carpospores are grouped in
radiating threads within the oval cavity of the cystocarp. These
cystocarps are developed as a result of fertilization. Other plants
bear gonidia in groups of four, the so-called _tetraspores_.
=378. Rhabdonia.=—This plant is about the same size as the
gracilaria, though it possesses more filiform branches. The cystocarps
form prominent elevations, while the carpospores lie in separated
groups around the periphery of a sterile tissue within the cavity. (See
figs. 187, 188.) Gonidia in the form of tetraspores are also developed
in Rhabdonia.
[Illustration: Fig. 187. Rhabdonia, branched portion of frond showing
cystocarps.]
[Illustration: Fig. 188. Section of cystocarp of rhabdonia, showing
spores.]
=379. Fertilization of the higher red algæ.=—The process of
fertilization in most of the red algæ is very complicated, chiefly
because the fertilized egg-cell (procarp) does not develop the spores
directly, as in Nemalion, Lemanea, etc., but fuses directly, or by a
short cell or long filament with one or more auxiliary cells before
the sporocarp is finally formed. Examples are Rhabdonia, Polysiphonia,
Callithamnion, Dudresnaya, etc. (fig. 189). The auxiliary cell then
develops the sporocarp. See fig. 189 for conjugation of a filament from
the fertilized procarp with an auxiliary cell.
[Illustration: Fig. 189.
Dudresnaya purpurifera. _tr_, trichogyne, with spermatozoids attached;
_ct_, connecting-tube which grows out from below the base of the
trichogyne, and comes in contact with the fertile branches _f_, _f_;
_ct′_, young connecting-tube. (After Thuret and Bornet.)]
=380. Uses of the red algæ.=—Many species produce a great amount
of gelatinous substance in their tissues, and several of these are used
for food, for the manufacture of gelatines and agar-agar. Some of these
are Gracilaria lichenoides and wrightii, the former species occurring
along the coast of India and China. The plant is easily converted into
gelatinous substance (agar-agar). Chondrus crispus, widely distributed
in the northern Atlantic is known as “Irish” moss and is used for food
and for certain medicinal purposes. Gigartina mamillosa in the Atlantic
and Arctic oceans is similarly employed. The following orders are
recognized in the red algæ:
=381. Order Bangiales.=—Example, Bangia atropurpurea (= Conferva
atropurpurea) in springs and brooks in North America and Europe.
Porphyra contains a number of species forming broad, thin, leaf-like
purple sheets in the sea.
=382. Order Nemalionales.=—Including Lemanea, Batrachospermum,
Nemalion, described above, and many others.
=383. Order Gigartinales.=—In this order occurs the common
Iceland moss (Chondrus crispus) in the sea, and Rhabdonia and Gigartina
mentioned above.
=384. Order Rhodomeniales.=—In this order occurs Gracilaria and
Polysiphonia mentioned above, also the beautiful marine forms like
Ceramium.
=385. Order Cryptonemiales.=—Examples are Dudresnaya, Melobesia,
Corallina, etc., the last two genera include many species with a wide
distribution.
Class Charophyceæ, Order Charales.
=386.= The Charales are by some thought to represent a distinct
class of algæ standing near the mosses, perhaps, because of the
biciliate character of the spermatozoids. There is one family, the
Characeæ. The plants occur in fresh and brackish water. Aside from the
peculiarity of the reproductive organs they are remarkable for the
large size of the cells of the internodes and of the “leaves,” and the
protoplasm exhibits to a remarkable degree the phenomenon of “cyclosis”
(see paragraphs 17-20). Three of the genera are found in North America
(Chara, Nitella (Fig. 8) and Tolypella).
[Illustration: Fig. 172_a_.
Reproductive organs of _Chara fragilis_. _A_, a central portion of a
leaf, _b_, with an antheridium, _a_, and a carpogonium, _s_, surrounded
by the spirally twisted enveloping cells; _c_, crown of five cells at
apex; β, sterile lateral leaflets; β′, large lateral leaflet near the
fruit; β″, bracteoles springing from the basal node of the reproductive
organs. _B_, a young antheridium, _a_, and a young carpogonium, _sk_;
_w_, nodal cell of leaf; _u_, intermediate cell between _w_ and the
basal-node cell of the antheridium; _l_, cavity of the internode of
the leaf; _br_, cortical cells of the leaf. _A_ × about 33; _B_ × 240.
(After Sachs.)]
=386a.= The complicated structure of the sexual organs shows a
higher state of organization than any of the other living algæ known.
While the internodes in Nitella are composed of a single, stout cell,
some times a foot or more in length, the nodes in all are composed of
a group of smaller cells. From the lateral cells of this group lateral
axes (sometimes called leaves) arise in whorls.
In Nitella the internodes are naked, but in most species of Chara
they are _corticated_, i.e., they are covered by a layer of numerous
elongated cells which grow downward from the nodes at the base of the
whorl of lateral shoots.
=386b.= The sexual organs are situated at the nodes of the whorled
lateral shoots, and consist of antheridia and carpogonia. Most of the
plants are monœcious, and both antheridia and carpogonia are often
attached to the same node, the antheridium projecting downward while
the carpogonium is more or less ascending. The sexual organs are
visible to the unaided eye. The antheridium is a globose red body of
an exceedingly complicated structure. The sperms are borne in several
very long coiled slender threads which are divided transversely into
numerous cells. The carpogonium is oval or elliptical in outline, the
wall of which is composed of several closely coiled spiral threads
enclosing the large egg.
FOOTNOTES:
[17] In Engler & Prantl’s Pflanzenfamilien, Wille uses the term class
for these principal subdivisions of the algæ. Systematists are not yet
agreed upon a uniform use of the terms.
[18] See Bot. Gaz., 17, 389, 1892.