Chapter III: Appendix: 574 (2)

All chlorophyll-free organisms act in a transforming and disturbing manner on the organic compounds from which they obtain their nourishment, and while they themselves grow and multiply, they produce, each after its kind, compounds of a less degree of complexity, _i.e._ they produce _fermentation_, _putrefaction_, sometimes the formation of _poisons_, and in living beings often _disease_.

Those organisms which produce fermentation are called _ferments_; this word, however, is also employed for similar transformations in purely chemical materials (inorganic ferments or enzymes). Many organic (“living”) ferments, among which are Yeast-cells and Bacteria, give off during their development certain inorganic and soluble ferments (enzymes) which may produce other transformations without themselves being changed. Different organisms may produce in the same substratum different kinds of transformation; alcoholic fermentation may for instance be produced by different species of Fungi, but in different proportions, and the same species produces in different substrata, different transformations (_e.g._ the Vinegar-bacteria oxydize diluted alcohol to vinegar, and eventually to carbonic acid and water).

In the study of Bacteria it is absolutely necessary to sterilize
the vessels employed in cultivation, the apparatus, and nutrient
solutions, _i.e._ to free them from Bacteria germs and
also to preserve the cultures from the intrusion of any foreign
germs (“pure-cultures”). A firm, transparent, nutritive medium
is frequently employed. This may be prepared by adding to
the nutrient solutions (broth) either gelatine, or--when the
Bacteria are to be cultivated at blood-heat--serum of sheep’s or
calf’s blood, agar-agar or carragen; serum alone may in itself
serve as a nutrient medium. The so-called “plate-cultures” are
frequently employed, _i.e._ the germs are isolated by
shaking them with the melted liquid nutrient gelatine, which
is then spread on a glass plate and allowed to coagulate; when
later on the individual germs grow into colonies, these remain
separate in the solid substratum and it is easy to pursue
their further development. Similar plate-cultures may also be
cultivated in test-tubes and on microscopic slides. The slides
and glass plates must be placed in “moist chambers” free from
Bacteria. By sowing a few cells (if possible one) using a fine
platinum wire, pure cultures for further investigation may be
obtained.

In order to prove the relationship between pathogenic Bacteria
and certain diseases, the experimental production of pathogenic
Bacteria by the inoculation of Bacteria from pure cultures into
healthy animals, is very important.

It has not so far been possible to establish a _classification_ of the Bacteria, as the life-history of many species, has not yet been sufficiently investigated.[7] The opinions of botanists are at variance, in many cases, about the forms of growth of a particular kind. Some species are pleomorphic (many-formed) while others possess only one form.

The following Bacteria are =Saprophytes=:--

_Cladothrix dichotoma_ is common in stagnant and running water which is impregnated with organic matter; the cell-chains have false branching. According to Zopf, _Leptothrix ochracea_ is one of the forms of this species which, in water containing ferrous iron (_e.g._ as FeCO_{3}), regularly embeds ferric-oxide in its sheath by means of the activity of the protoplasm. _Leptothrix ochracea_ and other Iron-bacteria, according to Winogradsky (1888), do not continue their growth in water free from protoxide of iron; while they multiply enormously in water which contains this salt of iron. The large masses of ochre-coloured slime, found in meadows, bogs, and lakes, are probably due to the activity of the Iron-bacteria.

Those forms which, according to Zopf’s views, represent the forms of development of _Cladothrix dichotoma_ are placed together in Fig. 30. A represents a group of plants, seventy times magnified, attached to a Vaucheria. The largest one is branched like a tree, with branches of ordinary form; a specimen with spirally twisted branches is seen to the right of the figure, at the lower part some small _Leptothrix_-like forms. _B_ shows the manner of branching and an incipient _Coccus_-formation. _C_ a _Coccus_-mass whose exit from the sheath has been observed. _D_ the same mass as _C_ after the course of a day, the Cocci having turned into _rods_. _E_ a group of Cocci in which some have developed into shorter or longer rods. _F_ one of these rods before and after treatment with picric acid, which causes the chain-like structure to become apparent. _G_ a portion of a plant with conspicuous sheath, two lateral branches are being formed. _H_ part of a plant, whose cells have divided and form Cocci. The original form of the cells in which the Cocci are embedded may still be recognised. I. _Leptothrix_-filaments with conspicuous mucilaginous sheath, from which a series of rods is about to emerge; the rod near the bottom is dead, and has remained lying in the sheath. _K_ part of a plant which is forming Cocci, those at the top are in the zooglœa-stage, at the base they are elongating to form rods and _Leptothrix_-filaments. _L_ a portion of a branched _Cladothrix_, which divides into motile _Bacillus_-forms; the rays at the free ends indicate the currents which the cilia produce in the water. _M_ a spirally-twisted, swarming filament, before and after division into halves. _N_ part of a tree-like zooglœa with Cocci and short rods.--All of these spirilla, zooglœa, etc., which Zopf has connected with _Clad. dichotoma_, are according to Winogradsky, independent organisms.

_Micrococcus ureæ_ produces _urinal fermentation_ (transformation of urinal matter into ammonium carbonate); aerobic; round cells generally united to form bent chains or a zooglœa.--Several other kinds of Bacteria have the same action as this one: in damp soil containing ammonia-compounds, _saltpetre-formations_ are produced by _M. nitrificans_ and several different kinds of Bacteria.

_Micrococcus prodigiosus_ is found on articles of food containing starch; “bleeding bread” is caused by this Bacterium, which has the power of forming a red pigment; it also occurs in milk, and produces lactic acid.

_Leuconostoc mesenterioides_ is the frog-spawn Bacterium (Fig. 27) which is found in sugar manufactories, and has the power of producing a viscous fermentation in saccharine solutions which have been derived from plants, _e.g._ in beetroot-sugar manufactories, where large accumulations of mucilage are formed at the expense of the sugar, with an evolution of carbonic acid. The cell-rows, resembling somewhat a pearl necklace, have thick mucilaginous cell-walls, and form white “Nostoc”-lumps. The mucilage eventually deliquesces and the cells separate from each other; arthrospores?--Similar viscous deteriorations occur in beer and wine, which may then be drawn out into long, string like filaments--“ropiness.”

_Bacterium aceti_, the Vinegar-bacterium, oxidizes alcohol into acetic acid (acetous-fermentation) and forms a greyish covering of Bacteria (“Vinegar-mother”) on the surface of the liquid; the acetic acid formed, becomes by continued oxidization by _B. aceti_, again transformed into carbonic acid and water. Aerobic; short cylindrical cells, often united into chains, or to form a zooglœa; sometimes also rod-and spindle-shaped. The Vinegar-bacteria and other kinds with ball- or rod-forms sometimes become swollen, spindle-shaped, or oval links; they are supposed to be diseased forms[8] (“Involution-forms”).

_Bacillus lacticus_ (_Bacterium acidi lactici_, Zopf) is always found in milk which has stood for some time, and in sour foods (cabbage, cucumbers, etc.); it turns the milk sour by producing lactic acid fermentation in the sugar contained in the milk; the lactic acid formed, eventually causes the coagulation of the casein. It resembles the Vinegar-bacteria, occurring as small cylindrical cells, rarely in short rows; not self-motile.--Several other Bacteria appear to act in the same way, some occurring in the mouth of human beings; some of these Bacteria give to butter its taste and flavour.

The _kefir-grains_ which are added to milk for the preparation of kefir, contain in large numbers a Bacterium (_Dispora caucasica_) in the zooglœa-form, a Yeast-fungus, and _Bacillus lacticus_. Kefir is a somewhat alcoholic sour milk, rich in carbonic acid; it is a beverage manufactured by the inhabitants of the Caucasus, from the milk of cows, goats, or sheep, and is sometimes used as a medicine. In the production of kefir, lactic acid fermentation takes place in one part of the sugar contained in the milk, and alcoholic fermentation in another part, and the casein which had become curdled is partially liquefied (peptonised) by an enzyme of a Zooglœa-bacterium.

_Bacillus amylobacter_ (_Bacillus butyricus_), the Butyric-acid-bacterium (Fig. 29), is a very common anaerobic which produces fermentation in sugar and lactic-acid salts, and whose principal product is _butyric acid_. It destroys articles of food and (together with other species) plays a part in the butyric acid fermentation which is necessary in the making of cheese; it is very active wherever portions of plants are decaying, in destroying the cellulose in the cell-walls of herbaceous plants, and is thus useful in the preparation of flax and hemp. The cells are self-motile, generally cylindrical, sometimes united into short rows; endosporous; the spore-forming cells swell, assume very different forms, and show granulose reaction. The germ-tube grows out in the direction of the long axis of the spore.

_Bacillus subtilis_, the Hay-bacillus, is developed in all decoctions of hay; a slender, aerobic, self-motile Bacillus; endosporous (aplanospores); the spore-wall ruptures transversely on germination.

_Crenothrix kuehniana_ occurs in the springs of many baths, in wells, in water or drain-pipes.

_Beggiatoa_ (parallel with the Blue-green Alga _Oscillaria_). Long filaments formed of cylindrical cells which are attached by one of the ends, but which are nearly always free when observed. The filaments, like those of _Oscillaria_, describe conical figures in their revolutions, the free filaments slide upwards and parallel with one another; sheaths are wanting; strongly refractive sulphur drops are found in the interior. The Beggiatoas are the most prevalent _Sulphur-bacteria_. They occur, very commonly in large numbers, wherever plant or animal remains are decaying in water in which sulphuretted hydrogen is being formed; thus, for example, _B. alba_ (Fig. 31) occurs frequently as a white covering or slimy film on mud containing organic remains. ~_B. mirabilis_ is remarkable for its size and its strong peristaltic movements.~ The Sulphur-bacteria oxidize the sulphuretted hydrogen, and accumulate sulphur in the shape of small granules of soft amorphic sulphur, which in the living cell never passes over into the crystalline state. They next oxidize this sulphur into sulphuric acid, which is immediately rendered neutral by absorbed salts of calcium, and is given off in the form of a sulphate, thus CaCO_{3} is principally changed into CaSO_{4}. In the absence of sulphur the nutritive processes are suspended, and consequently death occurs either sooner or later. The Sulphur-bacteria may exist and multiply in a fluid which only contains traces of organic matter, in which organisms devoid of chlorophyll are not able to exist. The Beggiatoas very frequently form white, bulky masses in sulphur wells and in salt water, the traces of organic material which the sulphur water contains proving sufficient for them. ~The cellulose-fermentation, to which the sulphur wells in all probability owe their origin, mainly procures them suitable conditions for existence. The CaCO_{3} and H_{2}S, formed during the cellulose fermentation by the reduction of CaSO_{4} is again changed into CaSO_{4} and CO_{2} by the Sulphur-bacteria (Winogradsky, 1887).--Other Sulphur-bacteria, the so-called purple Sulphur-bacteria, _e.g._ _B. roseo-persicina_, _Spirillum sanguineum_ (Fig. 23), _Bacterium sulfuratum_, etc., have their protoplasm mixed with a red colouring matter (bacterio-purpurin) which, like chlorophyll, has the power, in the presence of light, of giving off oxygen (as proved by T. W. Englemann, 1888, in oxygen-sensitive Bacteria). The three purple Sulphur-bacteria mentioned, are, according to Winogradsky, not pleomorphic kinds but embrace numerous species.~

Many _Spirilli_ (_Spirillum tenue_, _S. undula_, _S. plicatile_, and others) are found prevalent in decaying liquids.

Bacteria (especially Bacilli) are the cause of many substances emitting a foul odour, and of various changes in milk.

=Parasitic Bacteria= live in other living organisms; but the relation between “host” and parasite may vary in considerable degree. Some parasites do no injury to their host, others produce dangerous contagious diseases; some choose only a special kind as host, others again live equally well in many different ones. There are further specific and individual differences with regard to the _predisposition_ of the host, and every individual has not the same receptivity at all times.

THE HARMLESS PARASITES OF HUMAN BEINGS. Several of the above mentioned saprophytes may also occur in the alimentary canal of human beings; _e.g._, the Hay-bacillus, the Butyric-acid-bacillus, etc.; but the gastric juice prevents the development of others, at all events in their vegetative condition. _Sarcina ventriculi_, “packet-bacterium,” is only known to occur in the stomach and intestines of human beings, and makes its appearance in certain diseases of the stomach (dilation of the stomach, etc.) in great numbers, without, however, being the cause of the disease. It occurs in somewhat cubical masses of roundish cells (Fig. 25).

LESS DANGEROUS PARASITES. In the mouth, especially between and on the teeth, a great many Bacteria are to be found (more than fifty species are known), _e.g. Leptothrix buccalis_ (long, brittle, very thin filaments which are united into bundles), Micrococci in large lumps, _Spirochæte cohnii_, etc. Some of them are known to be injurious, as they contribute in various ways to the decay of the teeth (_caries dentium_); a _Micrococcus_, for instance, forms lactic acid in materials containing sugar and starch, and the acid dissolves the lime salts in the external layers of the teeth: those parts of the teeth thus deprived of lime are attacked by other Bacteria, and become dissolved. Inflammation in the tissues at the root of a tooth, is probably produced by septic materials which have been formed by Bacteria in the root-canal.

DANGEROUS PARASITES. In a large number of the infectious diseases of human beings and animals, it has been possible to prove that parasitic Bacteria have been the cause of the disease. Various pathogenic Bacteria of this nature, belonging to the coccus, rod, and spiral Bacteria groups, are mentioned in the following:--

=Pathogenic Micrococci.= _Staphylococcus pyogenes aureus_ produces abscesses of various natures (boils, suppurative processes in internal organs). The same effects are produced by--

_Streptococcus pyogenes_, which is the most frequent cause of malignant puerperal fever; it is perhaps identical with--

_Streptococcus erysipelatis_, which is the cause of erysipelas in human beings.

_Diplococcus pneumoniæ_ (A. Fränkel) is the cause of pneumonia, and of the epidemic cerebro-spinal meningitis.

_Gonococcus_ (Neisser) is the cause of gonorrhea and inflammation of the eyes.

=Pathogenic Rod-Bacteria.= _Bacterium choleræ gallinarum_, an aerobic, facultative parasite which produces fowl-cholera among poultry; it is easily cultivated on various substrata as a saprophyte. The disease may be conveyed both through wounds and by food, and may also be communicated to mammals.

_Bacillus anthracis_, the _Anthrax bacillus_ (Fig. 32), chiefly attacks mammals, especially herbivorous animals (house mice, guinea-pigs, rabbits, sheep, cattle), in a less degree omnivorous animals (including human beings), and in a still less degree the Carnivores. Aerobic. Cylindrical cells, 3–4 times as long as broad, united into long rod-like bodies, which may elongate into long, bent, and twisted filaments. Not self-motile. Endosporous. Germination takes place without the throwing off of any spore-membrane (compare Hay-bacillus p. 37 which resembles it). Contagion may take place both by introduction into wounds, and from the mucous membrane of the intestines or lungs, both by vegetative cells and by spores; in intestinal anthrax, however, only by spores. The Bacillus multiplies as soon as it has entered the blood, and the anthrax disease commences. The Bacilli not only give off poison, but also deprive the blood of its oxygen. Vegetative cells only occur in living animals. This species is a _facultative parasite_ which in the first stage is a saprophyte, and only in this condition forms spores.

_Bacillus tuberculosis_ produces tuberculosis in human beings, also in domestic animals (_perlsucht_). It is a distinct parasite, but may also live saprophytically. It is rod-formed, often slightly bent, and is recognised principally by its action with stains (when stained with an alkaline solution of methyl-blue or carbolic fuchsin, it retains the colour for a long time even in solutions of mineral acids, in contrast with the majority of well-known Bacteria): it probably forms spores which are able to resist heat, dryness, etc.

_Bacillus lepræ_ produces leprosy; _Bacillus mallei_ produces
glanders; _Bacillus tetani_, tetanus (the tetanus bacillus
is very common in soil; anaerobic); _Bacillus diphtheriæ_,
diphtheria; _Bacillus typhosus_, typhoid fever, etc.

=Pathogenic Spiral Bacteria.= _Spirochæte obermeieri_ (Fig. 24) produces intermittent fever (febris recurrens); it makes its appearance in the blood during the attacks of fever, but it is not to be found during intervals when there is no fever. Obligate parasite.

_Spirillum choleræ asiaticæ_ (_Microspira comma_) without doubt produces Asiatic cholera; an exceedingly motile spirillum, which is also found in short, bent rods (known as the “Comma-bacillus”), it lives in the intestines of those attacked by the disease, and gives off a strong poison which enters the body. It is easily cultivated as a saprophyte.

A great many circumstances seem to show that a number of other infectious diseases (syphilis, small-pox, scarlet-fever, measles, yellow-fever, etc.) owe their origin to parasitic Bacteria, but this has not been proved with certainty in all cases.

It has been possible by means of special cultivations (ample supply of oxygen, high temperature, antiseptic materials) to produce from the parasitic Bacteria described above (_e.g._ the fowl-cholera and the anthrax Bacteria) _physiological varieties_ which are distinct from those appearing in nature and possess a less degree of “virulence,” _i.e._ produce fever and less dangerous symptoms in those animals which are inoculated with them. The production of such physiological varieties has come to be of great practical importance from the fact that they are used as vaccines, _i.e._ these harmless species produce in the animals inoculated with them _immunity_ from the malignant infectious Bacteria from which they were derived. This immunity is effected by the change of the products of one or more of the Bacteria, but we do not yet know anything about the way in which they act on the animal organism. The white blood corpuscles, according to the Metschnikoff, play the part of “Phagocytes” by absorbing and destroying the less virulent Bacteria which have entered the blood, and by so doing they are gradually enabled to overcome those of a more virulent nature.

Class 5. =Conjugatæ.=

The Algæ belonging to this class have chlorophyll, and pyrenoids round which starch is formed. The cells divide only in one direction, they live solitarily, or united to form filaments which generally float freely (seldom attached). Swarm-cells are wanting. _The fertilisation is isogamous (conjugation) and takes place by means of aplanogametes._ The zygote, after a period of rest, produces, immediately on germination, one or more new vegetative individuals; sometimes akinetes or aplanospores are formed in addition. They only occur in fresh or slightly brackish water.

Order 1. =Desmidiaceæ.= The cells generally present markings on the outer wall, and are mostly divided into two symmetrical halves by a constriction in the middle, or there is at least a symmetrical division of the protoplasmic cell-contents. The cell-wall consists nearly always of two layers, the one overlapping the other (Fig. 35 _C_). The cells either live solitarily or are united into unbranched filaments. The mass of protoplasm formed by the fusion of the two conjugating cells becomes the zygote, which on germination produces one (or after division 2, 4 or 8) new vegetative individual. The chromatophores are either star-, plate-, or band-shaped, and regularly arranged round the long axis of the cell.

The Desmidiaceæ are not able to swim independently, many species, however, show movements of different kinds by rising and sliding forward on the substratum. These movements, which are partly dependent upon, and partly independent of light and the force of gravitation, are connected with the protrusion of a mucilaginous stalk. The mucilage, which sometimes surrounds the whole individual, may acquire a prismatic structure, it is secreted by the protoplasmic threads which project through certain pores definitely situated in the walls (Fig. 35 _A_, _B_).

VEGETATIVE MULTIPLICATION takes places by division. A good example of this is found in _Cosmarium botrytis_ (Fig. 36 _A-D_). The nucleus and chromatophores divide, and simultaneously the central indentation becomes deeper, the outer wall is then ruptured making a circular aperture through which the inner wall protrudes forming a short, cylindrical canal between the two halves to which it is attached (Fig. 36 _C_). After elongation the canal is divided by a central transverse wall, which commences as a ring round its inner surface and gradually forms a complete septum. The dividing wall gradually splits, and the two individuals separate from each other, each one having an old and a new half. The two daughter-cells bulge out, receive a supply of contents from the parent-cells, and gradually attain their mature size and development (Fig. 36 _B-D_). Exceptions to this occur in some forms.

CONJUGATION takes place in the simplest way in _Mesotænium_, where the two conjugating cells unite by a short tube (conjugation-canal), which is not developed at any particular point. The aplanogametes merge together after the dissolution of the dividing wall, like two drops of water, almost without any trace of preceding contraction, so that the cell-wall of the zygote generally lies in close contact with the conjugating cells. The conjugating cells in the others lie either transversely (_e.g. Cosmarium_, Fig. 37 _d_; _Staurastrum_, etc.), or parallel to one another (_e.g. Penium_, _Closterium_, etc.), and emit a short conjugation-canal (Fig. 37 _d_) from the centre of that side of each cell which is turned towards the other one. These canals touch, become spherical, and on the absorption of the dividing wall the aplanogametes coalesce in the swollen conjugation-canal (Fig. 37 _e_), which is often surrounded by a mucilaginous envelope. The zygote, which is often spherical, is surrounded by a thick cell-wall, consisting of three layers; the outermost of these sometimes bears thorn-like projections, which in some species are simple (Fig. 37 _f_), in others branched or variously marked; in some, however, it remains always smooth (_e.g._ _Tetmemorus_, _Desmidium_). Deviation from this mode of conjugation may occur within certain genera (_e.g._ _Closterium_, _Penium_). Upon germination the contents of the zygote emerge, surrounded by the innermost layers of the wall (Fig. 37 _g_, _h_) and generally divide into two parts which develop into two new individuals, placed transversely to each other (Fig. 37 _i_); these may have a somewhat more simple marking than is generally possessed by the species.

The most frequent genera are:--

_A._ Solitary cells: MESOTÆNIUM, PENIUM (Fig. 38 _B_),
CYLINDROCYSTIS, EUASTRUM (Fig. 38 _D_), MICRASTERIAS (Fig. 38
_C_), COSMARIUM (Fig. 36, 37), XANTHIDIUM, STAURASTRUM (Fig. 38
_E_), PLEUROTÆNIUM, DOCIDIUM, TETMEMORUS, CLOSTERIUM (Fig. 38
_A_), SPIROTÆNIA.

_B._ Cells united into filaments: SPHÆROZOSMA, DESMIDIUM,
HYALOTHECA, GYMNOZYGA, ANCYLONEMA, GONATOZYGON.

Order 2. =Zygnemaceæ.= Cell-wall without markings. The cells are cylindrical, not constricted in the centre, and (generally) united into simple, unbranched filaments. The whole contents of the conjugating cells take part in the formation of the zygote, which on germination grows out directly into a new filament.

_Spirogyra_ is easily recognised by its spiral chlorophyll band; _Zygnema_ has two star-like chromatophores in each cell (Fig. 40); both these genera are very common Algæ in ponds and ditches.

The conjugation among the Zygnemaceæ takes place in the following manner: the cells of two filaments, lying side by side, or two cells, the one being situated above the other in the same filament (Fig. 41), push out small protuberances opposite each other (Fig. 39 _A_, _a_, _b_); these finally meet, and the dividing wall is absorbed so that a tube is formed connecting one cell with the other; the protoplasmic contents round off, and the whole of these contents of one of the cells glides through the conjugation-tube and coalesces with that of the other (Fig. 39 _B_), the aggregate mass then rounds off, surrounds itself with a cell-wall, and becomes a zygote. A distinct difference may be found between the cells in the two filaments, those in the one whose protoplasmic contents pass over being cylindrical, while those of the recipient one are more barrel-shaped, and of a larger diameter. The former may be regarded as a male, the latter as a female plant. The zygote germinates after a period of rest, and grows out into a new filament (Fig. 42).

Order 3. =Mesocarpaceæ.= The cell-walls are glabrous, unconstricted in the centre, and united into simple unbranched filaments. The chromatophore consists of an axial chlorophyll-plate, with several pyrenoids. The zygote is formed by the coalescence of two cells (Fig. 43) (sometimes three or four), but the whole protoplasmic contents of the cells do not take part in this process, a portion always remaining behind; the aplanogametes coalesce in the conjugation-canal. The zygote thus formed appears incapable of germination until after 3–5 divisions. Of the cells so formed, only one is fertile, the sterile cells, according to Pringsheim, constituting a rudimentary sporocarp. The germinating cells grow out into a new filament. In this order, conjugation has been observed between two cells of the same filament. The Mesocarpaceæ thrive best in water which contains lime.

Class 6. =Chlorophyceæ (Green Algæ).=

These Algæ are coloured green by chlorophyll, seldom in combination with other colouring matter, and then especially with red. The product of assimilation is frequently starch, which generally accumulates round certain specially formed portions of protoplasm termed pyrenoids. The thallus is uni- or multicellular; in the higher forms (certain Siphoneæ) the organs of vegetation attain differentiation into stem and leaf. The asexual reproduction takes place in various ways; the sexual reproduction is effected by conjugation of motile gametes, or by oogamous fertilisation. The swarm-cells (zoospores, gametes, and spermatozoids) are constructed symetrically, and have true protoplasmic cilia, these generally being attached to the front end of the swarm-cells. Most of these Algæ live in water (fresh or salt); some are found upon damp soil, stones, or tree-stems, and some live enclosed in other plants.

The Class is divided into three families:--

1. PROTOCOCCOIDEÆ: Volvocaceæ, Tetrasporaceæ, Chlorosphæraceæ, Pleurococcaceæ, Protococcaceæ, Hydrodictyaceæ.

2. CONFERVOIDEÆ: Ulvaceæ, Ulothricaceæ, Chætophoraceæ, Mycoideaceæ, Cylindrocapsaceæ, Œdogoniaceæ, Coleochætaceæ, Cladophoraceæ, Gomontiaceæ, Sphæropleaceæ.

3. SIPHONEÆ: Botrydiaceæ, Bryopsidaceæ, Derbesiaceæ, Vaucheriaceæ, Phyllosiphonaceæ, Caulerpaceæ, Codiaceæ, Valoniaceæ, Dasycladaceæ.

Family 1. =Protococcoideæ.=

The Algæ which belong to this group are uni- or multicellular with the cells more or less firmly connected, sometimes in a definite, sometimes in an indefinite form (Fig. 47). Colonies are formed either by division or by small unicellular individuals becoming united in a definite manner; the colonies formed in this latter way are termed _Cœnobia_. Apical cells and branching are absent. Multiplication by division; asexual reproduction by zoospores, rarely by akinetes. Sexual reproduction may be wanting, or it takes place by isogamous, rarely by oogamous fertilisation.

Some are attached by means of a stalk to other objects (_Characium_, Fig. 49), others occur as “Endophytes” in the tissues of certain Mosses or Phanerogams, _e.g. Chlorochytrium lemnæ_, in _Lemna trisulca_; _Endosphæra_, in the leaves of _Potamogeton_, _Mentha aquatica_, and _Peplis portula_; _Phyllobium_, in the leaves of _Lysimachia nummularia_, _Ajuga_, _Chlora_, and species of Grasses; _Scotinosphæra_ in the leaves of _Hypnum_ and _Lemna trisulca_; the majority, however, live free in water and in damp places. Many species which were formerly considered to belong to this family have been proved to be higher Algæ in stages of development.

Order 1. =Volvocaceæ.= The individuals in this order are either uni- or multicellular, and during the essential part of their life are free-swimming organisms. They are generally encased in a mucilaginous envelope, through which 2–6 cilia project from every cell. The vegetative reproduction takes place by the division of all, or a few, of the cells of the individual; in some a palmella-stage is found in addition. The sexual reproduction takes place by isogamous or oogamous fertilisation.

The Volvocaceæ may be considered to include the original forms
of the Chlorophyceæ, because, among other reasons, the motile
stage is here the most prominent; they also form the connecting
link between the animal Flagellata, and forms intermediate to
the _Syngeneticæ_ may perhaps be found amongst them. Three
series of green Algæ may be supposed to have taken their origin
from the Volvocaceæ: CONJUGATEÆ (_Desmidiaceæ_) which have lost
the swarming stage, but whose conjugation is the nearest to the
fertilisation in _Chlamydomonas pulvisculus_: the PROTOCOCCACEÆ
in which the vegetative divisions have disappeared, while the
swarming stage continues to be present, though of shorter
duration; and TETRASPORACEÆ, in which the vegetative divisions
are more prominent, whilst the swarming stage is less so.

A. UNICELLULAR INDIVIDUALS. The principle genera are: _Chlamydomonas_, _Sphærella_, _Phacotus_.--_Sphærella nivalis_ is the Alga which produces the phenomenon of “Red Snow,” well known on high mountains and on ice and snow fields in the polar regions. The red colouring matter which appears in this and other green Algæ, especially in the resting cells, is produced by the alteration of the chlorophyll.

_Phacotus lenticularis_ has an outer covering incrusted with lime, which, at death, or after division, opens out into two halves. Species may be found among _Chlamydomonas_, in which conjugation takes place between gametes of similar size without cell-wall, but in _C. pulvisculus_ conjugation takes place between male and female aplanogametes which are surrounded by a mucilaginous envelope.

B. MULTICELLULAR INDIVIDUALS. The most important genera are _Gonium_, _Stephanosphæra_, _Pandorina_, _Eudorina_, _Volvox_.--_Gonium_ has 4 or 16 cells arranged in a definite pattern in a flat plate (Fig. 44). _Pandorina_ (Fig. 45), has 16 cells arranged in a sphere (Fig. 45 _A_). The vegetative reproduction takes place in this way: each cell, after having rounded off, and after the withdrawal of the cilia, divides itself into 16 new ones (Fig. 45 _B_), each forming a new individual, which soon grows to the size of the mother-individual. It was in this Alga that the conjugation of self-motile gametes was first discovered by Pringsheim, 1869. When conjugation is about to take place, each cell divides into sixteen, as in vegetative reproduction, but the 16 × 16 cells all separate from one another (Fig. 45 _C_, female gametes, and _D_, male gametes), and swarm solitarily in the water. The male are, most frequently, smaller than the female, but otherwise they are exactly alike; they are more or less pear-shaped, with a colourless anterior end, 2 cilia, a red “eye-spot,” etc. After swarming for some time they approach each other, two and two, generally a large and a smaller one, and come into contact at their colourless end; in a few moments they coalesce and become one cell (Fig. 45 _E_, _F_), this has at first a large colourless anterior end, 4 cilia, and 2 “eye-spots” (Fig. 45 _G_), but these soon disappear and the cell becomes uniformly dark-green and spherical, and surrounds itself with a thick cell-wall, losing at the same time its power of motion: the zygote (Fig. 45 _H_) is formed, and becomes later on a deep red colour. On the germination of the zygote, the protoplasmic cell-contents burst open the wall (Fig. 45 _J_), and emerge as a large swarmspore (Fig. 45 _K_) which divides into 16 cells, and the first small individual is formed (Fig. 45 _L_, _M_).

_Eudorina_ is like _Pandorina_ in structure, but stands somewhat higher, since the contrast between the conjugating sexual cells is greater, the female one being a motionless oosphere.

The highest stage of development is found in _Volvox_ (Fig. 46). The cells are here arranged on the circumference of a sphere, and enclose a cavity filled with mucilage. The number of these cells may vary from 200–22,000, of which the majority are vegetative and not reproductive, but some become large, motionless oospheres (Fig. 46 _b_); others, which may appear as solitary individuals, divide and form disc-shaped masses of from 8–256 small spermatozoids (Fig. 46 _a_). After the oosphere has been fertilised by these, the oospore surrounds itself by a thick, sometimes thorny cell-wall, and on germination becomes a new individual of few cells. A few cells conspicuous by their larger size may be found (1–9, but generally 8) in certain individuals, and these provide the vegetative reproduction, each forming by division a new individual.

Order 2. =Tetrasporaceæ= reproduce both by vegetative divisions and swarmspores, some have also gamete-conjugation. The principal genera are: _Tetraspora_, _Apiocystis_, _Dactylococcus_, _Dictyosphærium_, _Chlorangium_.

Order 3. =Chlorosphæraceæ.= _Chlorosphæra._

Order 4. =Pleurococcaceæ.= In this order the swarm-stages and sexual reproduction are entirely absent. Vegetative reproduction by division. The principal genera are: _Pleurococcus_ (Fig. 47), _Scenedesmus_ (Fig. 48), _Raphidium_, _Oocystis_, _Schizochlamys_, _Crucigenia_, _Selenastrum_.--_Pleurococcus vulgaris_ (Fig. 47) is one of the most common Algæ throughout the world, occurring as green coverings on tree-stems, and damp walls, and it is one of the most common lichen-gonidia.

Order 5. =Protococcaceæ.= The cells are motionless, free or affixed on a stalk (_e.g. Characium_, Fig. 49), either separate or loosely bound to one another; they never form multicellular individuals. Multiplication by division is nearly always wanting. Reproduction takes place by swarmspores, which have 1 or 2 cilia, and sexual reproduction in some by gamete-conjugation. The principal genera are: _Chlorococcum_, _Chlorochytrium_, _Chlorocystis_, _Scotinosphæra_, _Endosphæra_, _Phyllobium_, _Characium_, _Ophiocytium_, _Sciadium_.

Order 6. =Hydrodictyaceæ.= The individuals are unicellular but several unite after the zoospore-stage into definitely formed families (cœnobia). Ordinary vegetative division is wanting, but asexual reproduction takes place by zoospores (or by motionless cells without cilia), which unite and form a family similar to the mother-family, inside the mother-cell, or in a mucilaginous envelope. Where sexual reproduction is found it takes place by gamete-conjugation. The principal genera are: _Pediastrum_ (Fig. 50), _Cœlastrum_, _Hydrodictyon_ (Fig. 51).

The cœnobium of _Hydrodictyon reticulatum_ (Water-net) is formed of a large number of cells which are cylindrical, and attached to one another by the ends (Fig. 51). The asexual reproduction takes place by zoospores, which are formed in large numbers (7,000–20,000) in each mother-cell, within which they move about for a time, and then come to rest and arrange themselves into a new net (Fig. 51 _A_) which is set free by the dissolution of the wall of the mother-cell, grows, and becomes a new cœnobium. The sexual reproduction takes place by gamete-conjugation. The gametes are formed in the same manner as the zoospores, but in larger numbers (30,000–100,000), and swarm out of the mother-cell (Fig. 51 _B_). The zygote forms, on germination, 2–5 large zoospores, each with one or two cilia, these generally swarm about for a time, and after a period of rest become irregular thorny bodies (polyhedra); their contents again divide into zoospores, the thorny external coating of the polyhedra is cast off, and the zoospores, surrounded by the dilated internal coating, unite to form a small family, which produces several others in the manner described.

Family 2. =Confervoideæ.=

The individuals are always multicellular, the cells firmly bound together and united into unbranched or branched filaments, expansions, or masses of cells which grow by intercallary divisions or have apical growth. In the first seven orders the cells are uninuclear, but the cells of the remaining three orders contain several nuclei. Asexual reproduction by zoospores, akinetes or aplanospores. Sexual reproduction by isogamous or oogamous fertilisation.

The Confervoideæ, through the Ulvaceæ, are connected with the
Tetrasporaceæ, and from the _Coleochætaceæ_, which is the most
highly developed order, there are the best reasons for supposing
that the Mosses have taken their origin. The _Cladophoraceæ_
show the nearest approach to the _Siphoneæ_.

Order 1. =Ulvaceæ.= The thallus consists of one or two layers of parenchymatous cells, connected together to form either a flat membrane (_Monostroma_, _Ulva_) or a hollow tube (_Enteromorpha_), and may be either simple, lobed, or branched. Reproduction takes place by detached portions of the thallus; or asexually by zoospores or akinetes. Gamete-conjugation is known to take place in some members of this order, the zygote germinating without any resting-stage. The majority are found in salt or brackish water.

Order 2. =Ulothricaceæ.= The thallus consists normally of a simple unbranched filament (sometimes a small expansion consisting of one layer of cells is formed, as in _Schizomeris_ and _Prasiola_ which were formerly described as separate genera). Asexual reproduction takes place by means of zoospores (with 1, 2, or 4 cilia), akinetes or aplanospores; the last named may germinate immediately, or only after a period of rest. Sexual reproduction takes place by the conjugation of gametes of about the same size, each having two cilia (Fig. 52 _d_). The zygote of _Ulothrix_, on germination, produces a brood of zoospores which swarm for a time and then elongate to become _Ulothrix_-filaments (alternation of generations). The gametes may also germinate without conjugation in the same manner as the zoospores. The principal genera are: _Ulothrix_, _Hormidium_, _Conferva_, _Microspora_.--_Ulothrix zonata_ is very common in running fresh water. Nearly all the species of _Hormidium_ occur on damp soil, tree-stems and stones.

Order 3. =Chætophoraceæ.= The thallus consists of a single, branched, erect or creeping filament of cells, often surrounded by mucilage. The cells have only one nucleus. Asexual reproduction by zoospores with 2 or 4 cilia, by akinetes, or aplanospores. In many, conjugation between gametes with 2 cilia may be found. They approach on one side, Ulothricaceæ, and on the other, Mycoideaceæ. The principal genera are: _Stigeoclonium_, _Draparnaldia_, _Chætophora_, _Entoderma_, _Aphanochæte_, _Herposteiron_, _Phæothamnion_, _Chlorotylium_, _Trichophilus_, _Gongrosira_, _Trentepohlia_. Most of the species of _Trentepohlia_ are coloured red by the presence of a red colouring material, which occurs in addition to the chlorophyll. They are aerial Algæ which live on stones (_T. jolithus_, “violet stone,” so named on account of its violet-like odour in rainy weather), on bark and old wood (_T. umbrina_), or on damp rocks (_T. aurea_). _Trichophilus welckeri_ lives in the hair of Bradypus.

Order 4. =Mycoideaceæ.= The thallus is discoid, consisting of one or more cell-layers, and is always attached. Asexual reproduction by zoospores with 2 or 4 cilia. Sexual reproduction in some species by the conjugation of gametes with 2 cilia. This order forms the connecting link between _Chætophoraceæ_ and _Coleochætaceæ_. The species occur in fresh water (_Chætopeltis_) as well as in salt (_Pringsheimia_), on the carapace of tortoises (_Dermatophyton_ = _Epiclemmydia_), or endophytic between the cuticle and the epidermal cells of the leaves of tropical plants, destroying the leaf-tissue (_Mycoidea_).

Order 5. =Cylindrocapsaceæ.= The thallus consists of a simple (rarely, in parts, formed of many rows) unbranched filament, attached in the young condition, which has short cells with a single nucleus, and is enveloped in a thick envelope with a laminated structure. Asexual reproduction by zoospores with 2 cilia, which are formed 1, 2, or 4 in each vegetative cell. The antheridia are produced by a single cell, or a group of cells, in a filament, dividing several times without increasing in size. Two egg-shaped spermatozoids, each with 2 cilia (Fig. 53 _D_), are formed in each antheridium, and escape through an aperture in the side; in the first stages they are enclosed in a bladder-like membrane (Fig. 53 _B_, _C_). Other cells of the filament swell out and form oogonia (Fig. 53 _A_), which resemble those of _Œdogonium_. After fertilisation, the oospore surrounds itself with a thick wall, and assumes a reddish colour. The germination is unknown. The unfertilised oospheres remain green, divide often into 2–4 daughter-cells, and grow into new filaments.

This order, which only includes one genus, _Cylindrocapsa_, forms the connecting link between _Ulothricaceæ_ and _Œdogoniaceæ_. The few species (4) occur only in fresh water.

Order 6. =Œdogoniaceæ.= The thallus consists of branched (_Bulbochæte_) or unbranched (_Œdogonium_) filaments, attached in the early stages. The cells may be longer or shorter, and have one nucleus. Asexual reproduction by zoospores, which have a chaplet of cilia round the base of the colourless end (Fig. 6 _a_). Sexual reproduction takes place by oogamous fertilisation. On the germination of the oospore, 4 zoospores are formed (Fig. 54 _F_). They occur only in fresh or slightly brackish water. The division of the cells takes place in quite a peculiar and unusual manner. At the upper end of the cell which is about to divide, a ring-shaped thickening of soft cellulose is formed transversely round the wall; the cell-nucleus of the mother-cell and the protoplasm then divide by a transverse wall into two portions of similar size, and the cell-wall bursts transversely along the central line of the thickened ring. The cell-wall thus divides into two parts--the upper one short, the “cap,” and the lower one much longer, the “sheath.” The portions of the original cell-wall now separate from each other, the cellulose ring extending, and supplying an additional length of cell-wall between them. The cap and sheath will project a little in front of the piece thus inserted. The dividing wall between the two new cells is formed near to the uppermost edge of the sheath, and gradually becomes thicker and firmer. The inserted piece of wall forms the larger part of the wall of the upper cell: the remainder is formed by the cap. This mode of division is repeated exactly in the same way, and new caps are formed close below the first one, one for every division.

Fertilisation takes place in the following way. The oogonium is a large ellipsoidal, swollen cell (_og_, in Fig. 54 _A_), whose contents are rounded off into an oosphere with a colourless receptive-spot (see _B_); an aperture is formed in the wall of the oogonium, through which the spermatozoids are enabled to enter (_B_). The spermatozoids are produced either directly, as in _D_ (in pairs), in basal cells of the filament, or indirectly. In the latter case a swarmspore (_androspore_) is formed which comes to rest, attaches itself to an oogonium, germinates, and gives rise to a filament of a very few cells--_dwarf-male_ (_A_, _B_, _m_). The spermatozoids are formed in the upper cell of the dwarf-male (_m_), and are set free by the summit of the antheridium lifting off like a lid. On the germination of the oospore (_C_), which takes place in the following spring, 4 zoospores are produced (_F_) (_i.e._ the sexual generation); these swarm about for a time, and ultimately grow into new filaments.

Order 7. =Coleochætaceæ.= The thallus is always attached, and of a disc- or cushion-shape, formed by the dichotomous branching of filaments of cells united in a pseudo-parenchymatous manner. Each cell has only one nucleus. Asexual reproduction by zoospores with 2 cilia (Fig. 55 _D_), which may arise in all the cells. Sexual reproduction by oogamous fertilisation. The spermatozoids resemble the swarmspores, but are smaller (_E_), and originate singly (in the species figured) in small conical cells (_c_, _d_ in _A_). The oogonia are developed at the extremities of certain branches: they are bottle-shaped cells with very long and thin necks (_trichogyne_), open at the end (_a_ in _A_); at the base of each oogonium is a spherical oosphere. The spermatozoids reach the oosphere through the trichogyne, or through an aperture in the wall when the trichogyne is absent, and fertilisation having taken place, the oogonium becomes surrounded by a cell-layer (envelope), which grows out from the cells near its base (_b_ in _A_), and in this way a kind of fruit is formed (_B_) (_spermocarp_, _cystocarp_).

The oospore, next spring, divides and forms a parenchymatous tissue (homologous with the Moss-sporophyte); this bursts open the envelope (_C_), and a zoospore (homologous with the spores of the Moss-capsule) arises in each of the cells, and produces a new _Coleochæte_. We have then, in this case, a still more distinct alternation of generations than in _Œdogonium_. Only one genus, _Coleochæte_, is known, but it contains several species, all living in fresh water.

Order 8. =Cladophoraceæ.= This order is probably derived from the Ulothricaceæ. The thallus consists of a single, unbranched or branched filament, generally with an apical cell. The cells have each 2 or more nuclei. Asexual reproduction by zoospores with 2 or 4 cilia, and by akinetes. Conjugation of gametes with 2 cilia is found in some genera. They occur in salt as well as in fresh water. The principal genera are: _Urospora_, _Chætomorpha_, _Rhizoclonium_, _Cladophora_; of the last named genus the species _C. lanosa_ and _C. rupestris_ are common in salt water; _C. fracta_ and _C. glomerata_ in fresh water.

Order 9. =Gomontiaceæ.= _Gomontia polyrrhiza_, the only species hitherto known, is found on old calcareous shells of certain salt water Molluscs.

Order 10. =Sphæropleaceæ.= The thallus consists of free, unbranched filaments, with very elongated multinuclear cells. The vegetative cells form no zoospores. Sexual reproduction by oogamous fertilisation (see page 13, Fig. 10 _B_). The oospore has a thick wall (Fig. 10 _D_) studded with warts, and assumes a colour resembling red lead. It germinates only in the following spring, and produces 1–8 zoospores, each with 2 cilia (Fig. 10 _E_), which grow into new filaments. Only one species, _Sphæroplea annulina_, is known.

Family 3. Siphoneæ.

The thallus has apical growth, and in the vegetative condition consists generally of one single (in the Valoniaceæ most frequently of more) multinuclear cell, which may be much branched, and whose separate parts in the higher forms (_e.g. Bryopsis_, Fig. 57; _Caulerpa_, Fig. 59, etc.) may be differentiated to perform the various physiological functions (as root, stem and leaf). Vegetative multiplication by detached portions of the thallus (gemmæ); asexual reproduction by zoospores, akinetes, or aplanospores. Sexual reproduction by gamete-conjugation, rarely by oogamous fertilisation. The zygote or oospore germinates as a rule without any resting-stage.

Most of the Siphoneæ occur in salt water or on damp soil. Many (_e.g._ _Dasycladaceæ_) are very much incrusted with lime, and occur, in the fossilized condition, in the deposits from the Cretaceous period to the present time. The Siphoneæ are connected by their lowest forms (_Botrydiaceæ_ or _Valonia_) with the Protococcaceæ, but show also, through the Valoniaceæ, points of relationship to the _Cladophoraceæ_.

Order 1. =Botrydiaceæ.= The thallus in the vegetative condition is unicellular, club-shaped, with a small single (_Codiolum_) or repeatedly dichotomously branched system of colourless rhizoids (_Botrydium_, Fig. 56 _a_), by which it is attached to objects immersed in salt water (_Codiolum_) or to damp clay soil (_Botrydium_). Asexual reproduction by zoospores with one (_Botrydium_) or two cilia, and by aplanospores. The sexual reproduction is only known in _Botrydium_, and takes place in the following manner: in the part of the thallus which is above ground and in an active vegetative condition, several round cells (Fig. 56 _c_) are formed, which may be green or red according as they grow under water, or exposed to the strong light of the sun. These cells must be considered as “gametangia” as they produce many gametes (_d_) provided with two cilia. The zygote (_h_, _i_) formed by the conjugation (_e_, _f_, _g_) may either germinate immediately, or become a thick-walled resting-cell of an irregular, angular form.

Order 2. =Bryopsidaceæ.= The thallus in the vegetative condition
is unicellular, and consists at the lower extremity of branched
rhizoids, while the upper portion is prolonged into a stem-like
structure of unlimited growth, producing, acropetally, branches
and leaf-like structures. The latter have limited growth,
and are separated by a cross wall from the stem, and become
gametangia, or drop off. The gametes have two cilia, and are of
two kinds: the female, which are green and large and the male,
which are of brownish colour and smaller. Zoospores or any other
method of asexual reproduction are unknown. Only one genus,
_Bryopsis_, living in salt water.

Order 3. =Derbesiaceæ.= Only one genus, _Derbesia_, living in
saltwater. The zoospores, which are formed in a few lateral,
swollen zoosporangia, possess one nucleus which has arisen
through the coalescence of several, and they resemble the
zoospores of _Œdogonium_ by having a circle of cilia attached at
the base of the colourless spot.

Order 4. =Vaucheriaceæ.= The thallus consists, in the vegetative condition, of a single irregularly or dichotomously branched cell, without differentiation into stem or leaf; root-like organs of attachment may however occur. Asexual reproduction by zoospores, which are formed singly in the extremity of a branch cut off by a transverse wall. They contain many nuclei, and bear small cilia situated in pairs, which give the appearance of a fine “pile” covering the whole or a great part of the surface. Akinetes, aplanospores, and phytoamœbæ (naked masses of protoplasm, without cilia, which creep like an amœba on a substratum) may occur under certain conditions.

The sexual reproductive organs are formed on short lateral branches, and are separated from the vegetative cell (Fig. 58 _A_) by cell-walls. Numerous spermatozoids, each with two cilia, are developed in the coiled antheridium (_A_, _b_). The oogonium is a thick, egg-shaped, often oblique cell, with its protoplasm rounded into an oosphere, which has a hyaline “receptive-spot” (_A_, _a_) immediately beneath the aperture formed in the wall of the oogonium. A slimy mass, which serves to receive the spermatozoids, is formed in some species in this aperture. The spermatozoids when liberated swim towards and enter the oosphere, which then immediately surrounds itself with a thick cell-wall. The mature oospore (_B_) contains a large quantity of oil. At germination the outer cell-wall bursts and a new plant is formed. There is only one genus, _Vaucheria_, with species living in salt as well as in fresh water and on damp soil.

Order 5. =Phyllosiphonaceæ= are parasites in the leaves and stalks of Flowering-plants.

Order 6. =Caulerpaceæ.= The thallus has distinct differentiation into root, stem and leaf-like members (Fig. 59); it is unicellular. Within the cell, strong, branched threads of cellulose extend from one side to the other serving as stays to support the thallus. Reproduction takes place by detached portions of the thallus; no other modes of reproduction are known. This order may most approximately be classed with the _Bryopsidaceæ_. The genus _Caulerpa_ consists of more than seventy species which inhabit the tropical seas.

Order 7. =Codiaceæ.= The thallus has various forms, but without distinct differentiation in stem- or leaf-structures, sometimes (_e.g._ _Halimeda_) it is very much incrusted with lime. In the early stages it is unicellular (later, often multicellular), very much branched, with the branches, at any rate partly, so united or grown in amongst one another (Fig. 60) that an apparently parenchymatous cellular body is formed. Akinetes or aplanospores are wanting; zoospores (or gametes?) may be developed in some species, however, in special swollen sporangia. Fertilisation similar to that in _Bryopsis_ occurs perhaps in _Codium_. They are all salt water forms.

Order 8. =Valoniaceæ.= The thallus is generally multicellular, without differentation into stem- or leaf-structures, but the cells are sometimes united together and form a leaf-like reticulate expansion (_e.g. Anadyomene_). Zoospores are known in some, and they are then formed directly in the vegetative cells. In others (_e.g. Valonia_), a mass of protoplasm, which maybe separated through the damaging of a cell, can surround itself with a cell-wall, and grow into a new plant. No other modes of reproduction are known. The most important genera are: _Valonia_, _Siphonocladus_, _Chamædoris_, _Struvea_, _Microdictyon_, _Anadyomene_. They are all salt water forms.

As already pointed out, the _Valoniaceæ_ occupy a somewhat
central position among the Siphoneæ, and present points
of similarity and contrast with the _Botrydiaceæ_ and the
_Bryopsidaceæ_ through _Valonia_, with the _Dasycladaceæ_
through _Chamædoris_, and also with the _Cladophoraceæ_ through
_Siphonocladus_, and _Struvea_.

Order 9. =Dasycladaceæ.= The thallus consists of an axile longitudinal cell, destitute of transverse walls, attached at the base by root-like organs of attachment, and producing acropetally whorls of united, single or branched, leaf-like structures with limited growth. Asexual reproduction is wanting. Sexual reproduction by conjugation of gametes which arise in separate, fertile leaves, either directly or from aplanospores, which develope into gametangia. The principal genera are: _Acetabularia_, _Dasycladus_, _Neomeris_, _Cymopolia_. All marine.

The curiously shaped _Acetabularia mediterranea_ grows gregariously on limestone rocks, and shells of mussels in the Mediterranean; it resembles a minute umbrella with a small stem, sometimes as much as nine centimetres in height, and a shade which may be more than one centimetre in diameter. The cell-membrane is thick, and incrusted with carbonate and oxalate of lime. Only the lower, root-like part of the thallus, which penetrates the calcareous substratum survives the winter, and may grow up into a new plant. The sterile leaves, which drop off early, are dichotomously branched and formed of cylindrical cells separated from each other by cross-walls, but they are not grown together. The shade is formed by a circle of 70–100 club-shaped rays (fertile leaves) grown together, in each ray 40–80 aplanospores are formed, which become liberated at the breaking of the shade, and later on are changed to gametangia (compare _Botrydium_) which open by a lid and allow a large number of egg-shaped gametes with two cilia to escape. Gametes from various gametangia conjugate with one another; the product of the conjugation swarms about for some time, rounds off, and then surrounds itself with a cell-wall. The zygote germinates after a period of rest and then produces a sexual plant. The aplanospores (gametangia) thus represent the sexual generation.

Class 7. =Characeæ.=

The thallus has a stem with nodes and internodes; and whorls of leaves, on which may be developed the antheridia and oogonia, are borne at the nodes. Vegetative reproduction by bulbils and accessory shoots. Zoospores are wanting. The antheridia are spherical, and contain a number of filaments in which the spirally coiled spermatozoids, each with two cilia, are formed. The oogonium is situated terminally, and is at first naked, but becomes later on surrounded by an investment, and forms after fertilisation the so-called “fruit.” The oospore, after a period of rest, germinates by producing a “proembryo,” from which the young sexual plant arises as a lateral branch. The Characeæ are distinguished by the structure of their vegetative system as well as by the spirally-coiled spermatozoids, and stand as an isolated group among the Thallophytes, of which, however, the Siphoneæ appear to be their nearest relations. They were formerly, but wrongly, placed near the Mosses. The class contains only one order, the Characeæ.