Part 3

Order 4. =Hydruraceæ.= The individuals are attached, without cilia, multicellular, branched, and with apical growth. The cells are spherical, but in the final stage almost spindle-shaped, and embedded in large masses of mucilage. Asexual reproduction by zoospores which are tetrahedric, with 1 cilia, and by resting akinetes. _Hydrurus_ is most common in mountain brooks.

Class 2. =Dinoflagellata.=

The individuals are of a very variable form, but always unicellular, and floating about in free condition. The cell is _dorsiventral_, _bilateral_, _asymmetric_ and generally surrounded by a colourless membrane, which has _no silica_ embedded in it, but is formed of a substance allied to _cellulose_. The membrane, which externally is provided with pores and raised borders, easily breaks up into irregularly-shaped pieces. In the forms which have longitudinal and cross furrows, _two cilia_ are fixed where these cross each other, and project through a cleft in the membrane; one of these cilia _projects freely_ and is directed longitudinally to the front or to the rear, the other one _stretches crosswise_ and lies close to the cell, often in a furrow (cross furrow). The chromatophores are coloured brown or green and may either be two parallel (_Exuviella_), or several radially placed, discs, which sometimes may coalesce and become a star-shaped chromatophore. The coloring material (pyrrophyl) consists, in addition to a modification of chlorophyl, also of _phycopyrrin_ and _peridinin_; this colour is sometimes more or less masked by the products of assimilation which consist of yellow, red or colourless oil (?) and starch. Cell-nucleus one: in _Polydinida_ several nuclei are found; contractile vacuoles many, which partly open in the cilia-cleft (Fig. 13 _gs_). In some an eye-spot, coloured red by hæmatochrome, is found. Pyrenoids occur perhaps in _Exuviella_ and _Amphidinium_.

THE REPRODUCTION takes place as far as is known at present, only by division. This, in many salt water forms, may take place in the swarming condition, and, in that case, is always parallel to the longitudinal axis. The daughter-individuals, each of which retains half of the original shell, sometimes do not separate at once from each other, and thus chains (_e.g._ in _Ceratium_) of several connected individuals may be formed. In others, the division occurs after the cilia have been thrown off and the cell-contents rounded. The daughter-cells then adopt entirely new cell-walls. A palmella-stage (motionless division-stage) sometimes appears to take place, and also aplanospores (?) with one or two horn-like elongations (_e.g._ in _Peridinium cinctum_ and _P. tabulatum_); at germination one, or after division, two or more, new individuals may be formed.

Sexual reproduction has not been observed with certainty.

The Dinoflagellata move forward or backward, turning round their longitudinal axes; in their motion they are influenced by the action of light. The motion possibly may be produced only by the transverse cilium, which vibrates rapidly; whilst the longitudinal cilium moves slowly, and is supposed to serve mainly as a steering apparatus. They live principally in salt water, but also in fresh.

Besides the coloured forms, which are able to make their own organic compounds by the splitting up of the carbonic acid contained in the water, there are a few colourless forms (_e.g. Gymnodinium spirale_), or such as do not possess chromatophores (_Polykrikos_); these appear to live saprophytically, and may be able to absorb solid bodies with which they come in contact.

Dinoflagellata occur in the “Plankton” of the open sea, where they form together with Diatomaceæ the basis for the animal life. It is known with certainty that some salt water forms (like the _Noctiluca_, which belongs to the animal kingdom and to which they are perhaps related) produce light, known as phosphorescence.

_Dinoflagellata_ (_Peridinea_, _Cilioflagellata_) are allied through their lowest form (_Exuviella_) to the Syngeneticæ and especially to the order Chrysomonadinaceæ. They may be divided into three orders.

Order 1. =Adinida.= Without transverse or longitudinal furrows, but enclosed in two shells, and with two parallel chromatophores in each cell. _Exuviella_, _Prorocentrum_.

Order 2. =Dinifera.= With tranverse and generally longitudinal furrow. Many radially-placed, disc-formed chromatophores. The most common genera are--_Ceratium_ (Fig. 13), _Peridinium_, _Glenodinium_ (Fig. 13), _Gymnodinium_, _Dinophysis_.

Order 3. =Polydinida.= With several transverse furrows, no chromatophores, and several cell-nuclei. Only one genus--_Polykrikos_.

The order _Polydinida_ deviates in a high degree from the other Dinoflagellata, not only by its many tranverse furrows, each with its own transverse cilium, and by the absence of chromatophores, but also in having several cell-nuclei and a kind of stinging capsule, which otherwise does not occur within the whole class. It may therefore be questionable whether this order should really be placed in the vegetable kingdom.

Class 3. =Diatomeæ.=

The individuals--each known as a _frustule_--assume very various forms and may be unicellular or multicellular, but present no differentiation; many similar cells may be connected in chains, embedded in mucilaginous masses, or attached to mucilaginous stalks. The cells are bilateral or centric, often asymmetrical, slightly dorsiventral and have no cilia; those living in the free condition have the power of sliding upon a firm substratum. The cell contains 1 cell-nucleus and 1–2 plate-shaped or several disc-shaped chromatophores. The colouring material “_Melinophyl_” contains, in addition to a modification of chlorophyl, a brown colouring matter, _diatomin_. 1 or 2 pyrenoids sometimes occur. Starch is wanting and the first product of assimilation appears to be a kind of oil (?).

The cell-walls are _impregnated with silica_ to such a degree that they are imperishable and are therefore able to contribute in a great measure to the formation of the earth’s crust. The structure of their cell-wall is most peculiar and _differs from all other plants_ (except certain Desmidiaceæ); it does not consist of a single piece but is made up of two--the “shells”--(compare _Exuviella_ and _Prorocentrum_ among the Dinoflagellata) which are fitted into each other, one being a little larger than the other and embracing its edge, like a box with its lid (Fig. 14 _B_). The two parts which correspond to the bottom and lid of the box are known as _valves_. Along the central line of the valves a longitudinal _rib_ may often be found, interrupted at its centre by a small cleft (perhaps homologous with the cilia-cleft of the Dinoflagellata), through which the protoplasm is enabled to communicate with the exterior (Fig. 14 _A_). It is principally by reason of the valves, which bear numerous fine, transverse ribs, striæ or warts, etc. (Figs. 14, 15, 17), that the Diatomeæ have become so well known and employed as test objects in microscopical science. When the division takes place, the two shells are separated a little from each other, and after the cell-contents have divided into two masses, two new shells are formed, one fitting into the larger valve, the other one into the smaller valve of the original frustule. The latter cell (frustule) is thus, upon the whole, smaller than the mother-cell, and as the cells do not increase in size, some frustules are smaller than the ones from which they are derived, and thus, by repeated divisions, it follows that smaller and smaller frustules are produced. This continued diminution in size is, however, compensated for by the formation, when the cells have been reduced to a certain minimum, of _auxospores_, 2–3 times larger. These may either be formed _asexually_ by the protoplasm of a cell increasing, rounding off and surrounding itself with a new wall (_e.g. Melosira_) or after _conjugation_, which may take place with various modifications: 1. Two individuals unite after the secretion of a quantity of mucilage, and the valves then commence to separate from each other, on the side which the two individuals turn towards each other. The protoplasmic bodies now release themselves from their cell-wall, and each rounds off to form an ellipsoidal mass; these two protoplasmic masses (gametes) coalesce to form a zygote, the cell-nuclei and chromatophores also fusing together. The zygote increases in size, and surrounds itself with a firm, smooth, siliceous wall--the _perizonium_. The auxospores, whichever way they arise, are not resting stages. The germination of the zygote commences by the protoplasm withdrawing itself slightly from the cell-wall and constructing first the larger valve, and later on the smaller one; finally the membrane of the zygote bursts (_e.g. Himantidium_). 2. The conjugation occurs in a similar manner, but the protoplasm of the cells divides transversely before conjugation into two daughter-cells. Those lying opposite one another conjugate (Fig. 16) and form two zygotes. The formation of the perizonium, and germination take place as in the preceding instance (_e.g. Epithemia_). 3. Two cells place themselves parallel to each other, and each of the two cell-contents, without coalescing, becomes an auxospore. The formation of the wall takes place as in the preceding case. This is found in the Naviculeæ, Cymbelleæ, the Gomphonemeæ (_e.g. Frustulia_, _Cocconema_).

The Diatomaceæ may be found in salt as well as in fresh water (often in such masses that the colour of the water or mud becomes yellow or brown; in the same manner the genera _Chætoceros_, _Rhizosolenia_, _Coscinodiscus_, and several others, form large slime-masses, “Plankton” on the surface of the sea), on damp soil and in dust blown by the wind. They occur as fossils in the recent formations, often in large deposits (siliceous earth, mountain meal), as in the cement lime in Jutland, the alluvial deposits beneath Berlin, in clay strata beneath peat bogs, in guano, etc. These accumulations of fossilized diatoms are used in the manufacture of dynamite and in various manufactures.

The Diatomaceæ appear nearest to, and must be placed as a group co-ordinate with the Dinoflagellata, as they doubtless may be supposed to derive their origin from forms resembling _Exuviella_, and to have lost the cilia. The resemblances to the Desmidiaceæ which are striking in many respects, can only be conceived as analogies, and cannot be founded upon homologies, and it is therefore impossible to regard them as proof of genetic relationship. The family contains only one order.

Order 1. =Diatomaceæ.= This order may be divided into two sub-orders, viz.--

Sub-Order 1. =Placochromaticæ.= The chromatophores are discoid, large, 1 or 2 in each cell; the structure of the valves is bilateral and always without reticulate markings. The following groups belong to this sub-order: _Gomphonemeæ_, _Cymbelleæ_, _Amphoreæ_, _Achnantheæ_, _Cocconeideæ_, _Naviculeæ_, _Amphipleureæ_, _Plagiotropideæ_, _Amphitropideæ_, _Nitzchieæ_, _Surirayeæ_, and _Eunotieæ_.

Sub-Order 2. =Coccochromaticæ.= The chromatophores are granular, small and many in each cell. The structure of the cells is zygomorphic or centric, often with reticulate markings. The following groups belong to this sub-order: _Fragilarieæ_, _Meridieæ_, _Tabellarieæ_, _Licmophoreæ_, _Biddulphieæ_, _Anguliferæ_, _Eupodisceæ_, _Coscinodisceæ_, and _Melosireæ_.

Class 4. =Schizophyta, Fission-Algæ.=

The individuals are 1--many celled; the thallus consists in many of a single cell, in others of chains of cells, the cells dividing in only one definite direction (Figs. 18, 21). In certain Fission-Algæ the cell-chain branches (Fig. 30) and a difference between the anterior and the posterior ends of the chain is marked; in some, the cells may be united into the form of flat plates by the cell-division taking place in two directions; and in others into somewhat cubical masses, or rounded lumps of a less decided form, by the divisions taking place in three directions; or less defined masses may be formed by the divisions taking place in all possible directions.

The cell-walls rarely contain cellulose, they often swell considerably (Figs. 20, 22), and show distinct stratifications, or they are almost completely changed into a mucilaginous mass in which the protoplasts are embedded, _e.g._ in _Nostoc_ (Fig. 22), and in the “Zooglœa” stage of the Bacteria (Fig. 27). Sexual reproduction is wanting. Vegetative reproduction by division and the separation of the divisional products by the splitting of the cell-wall or its becoming mucilaginous; among the Nostocaceæ, Lyngbyaceæ, Scytonemaceæ, etc., “Hormogonia” are found; in _Chamæsiphon_ and others single reproductive akinetes are formed. Many Fission-Algæ conclude the growing period by the formation of resting akinetes or aplanospores.

The Schizophyta may be divided into 2 families:

1. SCHIZOPHYCEÆ.

2. BACTERIA.

Family 1. =Schizophyceæ,[5] Blue-Green Algæ.=

All the Blue-green Algæ are able to assimilate carbon by means of a colouring material containing chlorophyll (cyanophyll); but the chlorophyll in this substance is masked by a blue (phycocyan), or red (phycoerythrin, _e.g._ in _Trichodesmium erythræum_ in the Red Sea) colouring matter which may be extracted from them in cold water after death. The colouring matter, in most of them, permeates the whole of the protoplasm (excepting the cell-nucleus), but in a few (_e.g._ _Glaucocystis_, _Phragmonema_), slightly developed chromatophores are to be found. Where the cells are united into filaments (cell-chains) a differentiation into apex and base (_Rivulariaceæ_) may take place, and also between ordinary vegetative cells and heterocysts; these latter cannot divide, and are distinguished from the ordinary vegetative cells (Fig. 22 _h_) by their larger size, yellow colour, and poverty of contents. Branching sometimes occurs and is either true or spurious.

The cell-chain in the spurious branching divides into two parts, of which either one or both grow beyond the place of division (Fig. 18) and often out to both sides (_e.g. Scytonema_), the divisions however, always take place transversely to the longitudinal direction of the cell-chain. In the true branching a cell elongates in the direction transverse to the cell-chain, and the division then takes place nearly at right angles to the former direction (_Sirosiphoniaceæ_).

Cilia are wanting, but the filaments are sometimes self-motile (_e.g._ hormogonia in _Nostoc_) and many partly turn round their axes, partly slide forward or backward (_Oscillaria_).

Reproduction takes place by spores and hormogonia in addition to simple cell-division. Hormogonia are peculiar fragments of a cell-chain capable of motion, and often exhibit a vigorous motion in the sheath, until at last they escape and grow into a new individual (Fig. 18). The spores are reproductive akinetes (_Chamæsiphon_, etc.) or resting akinetes; these latter arise by the vegetative cells enlarging and constructing a thick cell-wall (Fig. 19 _e f_). On germination, this cell-wall bursts and the new cell-chain elongates in the same longitudinal direction as before (Fig. 19 _b c_). Many (_e.g._ _Oscillaria_) may however winter in their ordinary vegetative stage. Aplanospores are wanting.

The Fission-Algæ are very prevalent in fresh water and on damp soil, less so in salt water; they also often occur in water which abounds in decaying matter. Some are found in warm springs with a temperature as high as 50° C.

The Family may be divided into 2 sub-families:

1. HOMOCYSTEÆ (heterocysts are wanting): _Chroococcaceæ_, _Lyngbyaceæ_ and _Chamœsiphonaceæ_.

2. HETEROCYSTEÆ (heterocysts present): _Nostocaceæ_, _Rivulariaceæ_, _Scytonemaceæ_ and _Sirosiphoniaceæ_.

Order 1. =Chroococcaceæ.= The individuals are 1--many-celled, but all the cells are uniform, united to form plates or irregular masses, often surrounded by a mucilaginous cell-wall, but never forming cell-chains. Multiplication by division and sometimes by resting akinetes, but reproductive akinetes are wanting. _Chroococcus_, _Aphanocapsa_, _Glœocapsa_ (Fig. 20), _Cœlosphærium_, _Merismopedium_, _Glaucocystis_, _Oncobyrsa_, _Polycystis_, _Gomphosphæria_.

Order 2. =Lyngbyaceæ (Oscillariaceæ).= The cells are discoid (Fig. 21), united to straight or spirally twisted, free filaments, which are unbranched, or with spurious branching. The ends of the cell-chains are similar. Heterocysts absent. Reproduction by synakinetes, resting akinetes are wanting. _Oscillaria_ (Fig. 21), _Spirulina_, _Lyngbya_, _Microcoleus_, _Symploca_, _Plectonema_.

Order 3. =Chamæsiphonaceæ.= The individuals are 1--many-celled, attached, unbranched filaments with differentiation into apex and base, without heterocysts. Multiplication by reproductive akinetes; resting akinetes are wanting. _Dermocarpa_, _Clastidium_, _Chamæsiphon_, _Godlewskia_, _Phragmonema_.

Order 4. =Nostocaceæ.= The individuals are formed of multicellular, unbranched filaments, without differentiation into apex and base; heterocysts present. Reproduction by synakinetes and resting akinetes.

Some genera are not mucilaginous, _e.g. Cylindrospermum_ (Fig. 19). The cell-chains in others, _e.g. Nostoc_, wind in between one another and are embedded in large structureless jelly-like masses, which may attain the size of a plum or even larger (Fig. 22); sometimes they are found floating in the water, sometimes attached to other bodies. Other genera as follows: _Aphanizomenon_ and _Anabæna_ (in lakes and smaller pieces of water); _Nodularia_ is partly pelagic. Some occur in the intercellular spaces of higher plants, thus _Nostoc_-forms are found in _Anthoceros_, _Blasia_, _Sphagnum_, _Lemna_, and in the roots of _Cycas_ and _Gunnera_; _Anabæna_ in _Azolla_.

Order 5. =Rivulariaceæ.= The individuals are multicellular filaments, with differentiation into apex and base; spurious branching, and a heterocyst at the base of each filament, reproduction by synakinetes and resting akinetes, rarely by simple reproductive akinetes. _Rivularia_, _Glœotrichia_, _Isactis_, _Calothrix_.

Order 6. =Scytonemaceæ.= The individuals are formed of multicellular filaments with no longitudinal division; differentiation into apex and base very slight or altogether absent; branching spurious; heterocysts present. Reproduction by synakinetes, rarely by resting akinetes and ordinary reproductive akinetes. _Tolypothrix_, _Scytonema_, _Hassalia_, _Microchæte_.

Order 7. =Sirosiphoniaceæ.= The individuals are formed of multicellular threads with longitudinal divisions; true branching and heterocysts, and often distinct differentiation into apex and base. Reproduction by synakinetes, rarely by resting akinetes and ordinary reproductive akinetes. _Hapalosiphon_, _Stigonema_, _Capsosira_, _Nostocopsis_, _Mastigocoleus_.

Family 2. =Bacteria.=[6]

The Bacteria (also known as Schizomycetes, and Fission-Fungi) are the smallest known organisms, and form a parallel group to the Blue-green Algæ, but separated from these Algæ by the absence of their colouring material; chlorophyll is only found in a few Bacteria.

The various forms under which the vegetative condition of the Bacteria appear, are termed as follows:

1. GLOBULAR FORMS, COCCI (Figs. 27, 30 _c_): spherical or ellipsoidal, single cells, which, however, are usually loosely massed together and generally termed “_Micrococci_.”

2. ROD-LIKE FORMS: more or less elongated bodies; the shorter forms have been styled “_Bacterium_” (in the narrower sense of the word), and the term “_Bacillus_” has been applied to longer forms which are straight and cylindrical (Figs. 28, 29, 30 _E_).

3. THREAD-LIKE FORMS: unbranched, long, round filaments, resembling those of _Oscillaria_, are possessed by _Leptothrix_ (very thin, non-granular filaments; Fig. 30 _A_, the small filaments) and _Beggiatoa_ (thicker filaments, with strong, refractile grains or drops of sulphur (Fig. 31); often self-motile). Branched filaments, with false branching like many _Scytonemaceæ_, are found in _Cladothrix_ (Fig. 30 _B_, _G_).

4. SPIRAL FORMS: Rod-like or filamentous bodies, which more or less strongly resemble a corkscrew with a spiral rising to the left. In general these are termed _Spirilla_ (Fig. 23); very attenuated spirals, _Vibriones_ (standing next to Fig. 30 _M_); if the filaments are slender and flexible with a closely wound spiral, _Spirochætæ_ (Fig. 24).

5. The MERISMOPEDIUM-FORM, consisting of rounded cells arranged in one plane, generally in groups of four, and produced by divisions perpendicular to each other.

6. The SARCINA-FORM, consisting of roundish cells which are produced by cellular division in all the three directions of space, united into globular or ovoid masses (“parcels”) _e.g. Sarcina ventriculi_ (Figs. 25, 26).

All Bacteria are unicellular. In the case of the micrococci this is self-evident, but in the “rod,” “thread,” and “spiral” Bacteria, very often numerous cells remain united together and their individual elements can only be recognised by the use of special reagents.