Chapter II: Appendix: 574 (1)

INDEX 593

CORRIGENDA.

Page 9, line 12 from top, for _Hydrodicton_ read _Hydrodictyon_.
„ 14, lines 1 and 2 from top, for _as in the preceding case_
read _in this case_.
„ 14, „ 2 and 15 from top, for _zygote_ read _oospore_.
„ 88, line 15 from bottom, for _Periphyses_ read _periphyses_.
„ 124, „ 7 „ „ for _Chæromyces_ read _Choiromyces_.
„ 142, „ 2 „ „ and in Fig. 137, for _Bœomyces_ read
_Bæomyces_.
„ 152, „ 2 „ top, for _Pirus_ read _Pyrus_.
„ 152, „ 5 „ „ for _Crategus_ read _Cratægus_.
„ 216, Fig. 215, for _Salvina_ read _Salvinia_.
„ 306, line 6 from top, for _Pista_ read _Pistia_.
„ 316, „ 26 „ „ after Dracæna insert a comma.
„ 337, „ 13 „ „ for _end_ read _beginning_.
„ 483, „ 11 „ bottom, for _Lagerstrœmia_ read
_Lagerstrœmeria_.

For ä, ö and ü read æ, œ and ue throughout.

The following are not officinal in the British Pharmacopœia:--page 316, _Dracæna_ (Dragon’s-blood), _Smilax glabra_; p. 321, “Orris-root”; p. 326, species of _Curcuma_, _Alpinia officinarum_; p. 333, _Orchis_-species (“Salep”). On page 296, par. 4, only Pearl Barley is offic. in the Brit. Phar.

CLASSIFICATION OF THE VEGETABLE KINGDOM.

The Vegetable Kingdom is arranged in 5 Divisions.

Division I.--=Thallophyta=, =Stemless Plants=, or those which are composed of a “thallus,” _i.e._ organs of nourishment which are not differentiated into root (in the sense in which this term is used among the higher plants), stem, or leaf. Vascular bundles are wanting. Conjugation and fertilisation in various ways; among most of the Fungi only vegetative multiplication.

In contradistinction to the Thallophytes all other plants are
called “Stem-plants” (“Cormophyta”), because their shoots are
leaf-bearing stems. The name Thallophyta (Stemless-plants) is
to some extent unsuitable, since many of the higher Algæ are
differentiated into stem and leaf.

The Thallophytes are again separated into 3 sub-divisions, namely:

Sub-Division =A.=--=Myxomycetes, Slime-Fungi=, with only 1 class.

Sub-Division =B.=--=Algæ=, with 10 classes:
Class 1. Syngeneticæ.
„ 2. Dinoflagellata, Peridinea.
„ 3. Diatomeæ, Diatoms.
„ 4. Schizophyta, Fission Algæ.
„ 5. Conjugatæ.
„ 6. Chlorophyceæ, Green Algæ.
„ 7. Characeæ, Stone-worts.
„ 8. Phæophyceæ, Brown Algæ.
„ 9. Dictyotales.
„ 10. Rhodophyceæ, Red Algæ.

Sub-Division =C.=--=Fungi=, with 3 classes:
Class 1. Phycomycetes.
„ 2. Mesomycetes.
„ 3. Mycomycetes, Higher Fungi.

Division II.--=Bryophyta or Muscineæ, Mosses.= These have leaf-bearing shoots, but neither true roots nor vascular bundles. The lowest Mosses have, however, a thallus. Fertilisation is accomplished by means of self-motile, spirally coiled spermatozoids, through the agency of water. From the fertilised oosphere a “fruit-body” (capsule) with unicellular organs of reproduction (spores) is produced. The spore on germination gives rise to the vegetative system, which bears the organs of sexual reproduction; and this system is divided into two stages--the protonema, and the leaf-bearing plant produced on it.

Alternation of generations:

I. The protonema and the entire nutritive system which
bears the organs of sexual reproduction.
II. The capsule-like sporangium, with spores.
2 Classes: 1. Hepaticæ, Liverworts.
2. Musci, Leafy Mosses.

Division III.--=Pteridophyta or Vascular Cryptogams=, =Fern-like Plants= having leaf-bearing shoots, true roots, and vascular bundles with tracheides and sieve-tubes. Fertilisation as in the Mosses. From the fertilised oosphere the leaf-bearing shoot arises, which bears on its leaves the reproductive organs, the spores, in capsule-like sporangia. From the germination of the spore a small prothallium is formed, which bears the sexual reproductive organs.

Alternation of generations:

I. Prothallium with organs of sexual reproduction.
II. Leaf-bearing shoot with capsule-like sporangia.
3 Classes: 1. Filicinæ, True Ferns.
2. Equisetinæ, Horsetails.
3. Lycopodinæ, Club-mosses.

Division IV.--=Gymnospermæ.= The vegetative organs are in the main similar to those in the 3rd Division; special shoots are modified into flowers for the service of reproduction. From the oosphere, which is fertilised by means of the pollen-tube, the leaf-bearing plant is derived; this passes the first period of its life as an embryo in the seed, and continues its development when the germination of the seed takes place. The organs corresponding to the spores of the two preceding Divisions, are called respectively the pollen-grain and embryo-sac. The pollen-grains are multicellular; i.e. they contain an indistinct prothallium. In the embryo-sac a prothallium, rich in reserve material (endosperm), with female organs of reproduction, is developed BEFORE FERTILISATION. The pollen-grains are carried by means of the wind to the ovules; these enclose the embryo-sac, and are situated on the open fruit-leaf (carpel), which has no stigma.

Alternation of generations:

I. Prothallium = Endosperm in ovule.
II. Leaf-bearing plant, with flowers which produce the pollen-sac
and ovule.
3 Classes: 1. Cycadeæ.
2. Coniferæ.
3. Gnetaceæ.

Division V.--=Angiospermæ=. The members of this group are very similar to those of Division IV. The ovules are, however, encased in closed fruit-leaves (ovary), which have a special portion (stigma) adapted for the reception and germination of the pollen-grains. The pollen-grains are bicellular, but with only a membrane separating the two nuclei; they are carried to the stigma by animals (chiefly insects), by the wind, or by some other means. Endosperm is not formed till AFTER FERTILISATION. Alternation of generations in the main as in the Gymnosperms, but less distinct; while the sexual generation, the prothallium, with the organs of fertilisation, is also strongly reduced.

2 Classes:[1] 1. Monocotyledones. Embryo with one seed-leaf.
2. Dicotyledones. Embryo with two seed-leaves.

For a long time the vegetable kingdom has been divided
into. CRYPTOGAMS (so called because their organs
of reproduction remained for some time undiscovered), and
PHANEROGAMS or Flowering-plants which have evident
sexual organs.

The first three divisions belong to the Cryptogams, and the
third and fourth divisions to the Phanerogams. This arrangement
has no systematic value, but is very convenient in many ways.

The Cryptogams are also known as Spore-plants, since they
multiply by unicellular organs (spores), and the Phanerogams in
contradistinction are called Seed-plants (Spermaphyta), since
they multiply by seeds, multicellular bodies, the most important
part of which is the embryo (a plant in its infancy). Mosses,
Ferns, and Gymnosperms are together known as Archegoniatæ, since
they possess in common a female organ of distinct structure, the
Archegonium.

DIVISION I.

THALLOPHYTA.

The thallus in the simplest forms is unicellular; in the majority, however, it is built up of many cells, which in a few instances are exactly similar; but generally there is a division of labour, so that certain cells undertake certain functions and are constructed accordingly, while others have different work and corresponding structure. Vessels or similar high anatomical structures are seldom formed, and the markings on the cell-wall are with few exceptions very simple. The Myxomycetes occupy quite an isolated position; their organs of nourishment are naked masses of protoplasm (plasmodia).

As regards the external form, the thallus may be entirely without special prominences (such as branches, members), but when such are present they are all essentially alike in their origin and growth, that is, disregarding the hair-structures which may be developed. A shoot of a Seaweed or of a Lichen, etc., is essentially the same as any other part of the plant; only among the highest Algæ (Characeæ, certain Siphoneæ, _Sargassum_, and certain Red Seaweeds) do we find the same differences between the various external organs of the plant body as between stem and leaf, so that they must be distinguished by these names.

_Roots_ of the same structure and development as in the Seed-plants are not found, but _organs of attachment_ (rhizoids and haptera) serve partly the biological functions of the root.

SYSTEMATIC DIVISION OF THE THALLOPHYTES. To the Thallophytes belong three sub-divisions--Slime-Fungi, Algæ, and Fungi. Formerly the Thallophytes were divided into Algæ, Fungi, and Lichens. But this last group must be placed among the Fungi, since they are really Fungi, which live symbiotically with Algæ. The _Slime-Fungi_ must be separated from the true Fungi as a distinct subdivision. The _Algæ_ possess a colouring substance, which is generally green, brown, or red, and by means of which they are able to build up organic compounds from carbonic acid and water. The Bacteria, especially, form an exception to the Algæ in this respect; like the Fungi and Slime-Fungi they have as a rule no such colouring material, but must have organic carbonaceous food; these plants form no starch, and need no light for their vegetation (most Fungi require light for fructification). The Myxomycetes, Bacteria, and Fungi derive their nourishment either as _saprophytes_ from dead animal or vegetable matter, or as _parasites_ from living animals or plants (hosts), in which they very often cause disease.

A remark, however, must be made with regard to this division.
Among the higher plants so much stress is not laid upon the
biological relations as to divide them into “green” and
“non-green”; _Cuscuta_ (Dodder), a parasite, is placed among
the Convolvulaceæ, _Neottia_ and _Corallorhiza_, saprophytes,
belong to the Orchidacere, although they live like Fungi,
yet their relations live as Algæ. In the same manner there
are some colourless parasitic or saprophytic forms among the
Algæ, and stress must be laid upon the fact that not only the
Blue-green Algæ, but also the Bacteria, which cannot assimilate
carbonic-acid, belong to the Algæ group, Schizophyceæ. The
reason for this is that systematic classifications must be based
upon the relationship of form, development, and reproduction,
and from this point of view we must regard the Bacteria as
being the nearer relatives of the Blue-green Algæ. All the
Thallophytes, which are designated Fungi (when the entire
group of Slime-Fungi is left out), form in some measure a
connected series of development which only in the lower forms
(Phycomycetes) is related to the Algæ, and probably through them
has taken its origin from the Algæ; the higher Fungi have then
developed independently from this beginning. The distinction
of colour referred to is therefore not the only one which
separates the Algæ from the Fungi, but it is almost the only
characteristic mark by which we can at once distinguish the two
great sub-divisions of the Thallophytes.

The first forms of life on earth were probably “Protistæ,”
which had assimilating colour material, or in other words, they
were Algæ because they could assimilate purely inorganic food
substances, and there are some among these which belong to the
simplest forms of all plants. Fungi and Slime-Fungi must have
appeared later, because they are dependent on other plants which
assimilate carbon.[2]

_Sub-Division I._--=MYXOMYCETES, SLIME-FUNGI.=

The Slime-Fungi occupy quite an isolated position in the Vegetable Kingdom, and are perhaps the most nearly related to the group of Rhizopods in the Animal Kingdom. They live in and on organic remains, especially rotten wood or leaves, etc., on the surface of which their sporangia may be found.

They are organisms without chlorophyll, and in their vegetative condition are masses of protoplasm without cell-wall (_plasmodia_). They multiply by means of _spores_, which in the true Slime-Fungi[3] are produced in sporangia, but in some others[4] free. The spores are round cells (Fig. 1 _a_) which in all the true Slime-Fungi are surrounded by a cell-wall. The wall bursts on germination, and the contents float out in the water which is necessary for germination. They move about with swimming and hopping motions like swarmspores (_e_, _f_), having a cilia at the front end and provided with a cell-nucleus and a pulsating vacuole. Later on they become a little less active, and creep about more slowly, while they continue to alter their form, shooting out arms in various places and drawing them in again (_g_, _h_, _i_, _k_, _l_, _m_); in this stage they are called _Myxamœbæ_.

The Myxamœba grows whilst taking up nourishment from the material in which it lives, and multiplies by division. At a later stage a larger or smaller number of Myxamœbæ may be seen to coalesce and form large masses of protoplasm, _plasmodia_, which in the “Flowers of Tan” may attain the size of the palm of a hand, or even larger, but in most others are smaller. The plasmodia are independent, cream-like masses of protoplasm, often containing grains of carbonate of lime and colouring matter (the latter yellow in the Flowers of Tan). They creep about in the decaying matter in which they live, by means of amœboid movements, internal streamings of the protoplasm continually taking place; finally they creep out to the surface, and very often attach themselves to other objects, such as Mosses, and form sporangia (Fig. 2). These are stalked or sessile and are generally cylindrical (Fig. 3), spherical or pear-shaped (Fig. 4); they rarely attain a larger size than that of a pin’s head, and are red, brown, white, blue, yellow, etc., with a very delicate wall. In some genera may be found a “Capillitium” (Fig. 4 _cp_), or network of branched fine strands between the spores. Flowers of Tan (_Fuligo septica_) has a fruit-body composed of many sporangia (an Æthalium), which has the appearance of flat, irregular, brown cakes, inside the fragile external layer of which a loose powder, the spores, is found. It generally occurs on heaps of tanners’ bark, and appears sometimes in hot-beds in which that material is used, and is destructive by spreading itself over the young plants and choking them.

All the motile stages may pass into _resting stages_, the small forms only surrounding themselves with a wall, but the large ones at the same time divide in addition into polyhedral cells. When favourable conditions arise, the walls dissolve and the whole appears again as a naked (free-moving) mass of protoplasm.

To the genuine Slime-Fungi belong: _Arcyria_, _Trichia_, _Didymium_, _Physarum_, _Stemonitis_, _Lycogala_, _Fuligo_, _Spumaria_, _Reticularia_.

Some genera wanting a sporangium-wall belong to the Slime-Fungi: _Ceratiomyxa_, whose fruit-body consists of polygonal plates, each bearing stalked spores; _Dictyostelium_, in which the swarm-stage is wanting and which has stalked spores. _Plasmodiophora brassicæ_ preys upon the roots of cabbages and other cruciferous plants, causing large swellings. _Pl. alni_ causes coral-shaped outgrowths on the roots of the Alder (_Alnus_). _Phytomyxa leguminosarum_ may be found in small knobs (tubercles) on the roots of leguminous plants. It is still uncertain whether it is this Fungus or Bacteria which is the cause of the formation of these tubercles.

_Sub-Division_ II.--=ALGÆ=.

=Mode of Life.= The Algæ (except most of the Bacteria) are themselves able to form their organic material by the splitting up of the carbonic acid contained in the water, or air in some cases, and for this purpose need light. The majority live in water, fresh or salt, but many are present on damp soil, stones, bark of trees, etc.

With the exception of the Bacteria, no saprophytes have actually been determined to belong to this group, and only very few true parasites (for instance, _Phyllosiphon arisari_, _Mycoidea_, etc.), but a good many are found epiphytic or endophytic on other Algæ, or water plants, and on animals (for instance, certain _Schizophyceæ_ and _Protococcoideæ_; _Trichophilus welckeri_ in the hairs of _Bradypus_, the Sloth), and several species in symbiotic relation to various Fungi (species of Lichen), to Sponges (_e.g. Trentepohlia spongiophila_, _Struvea delicatula_), and to sundry Infusoria and other lower animals as Radiolarias, _Hydra_, etc. (the so-called _Zoochlorella_ and _Zooxantella_, which are perhaps partly stages in development of various Green and Brown Algæ).

=Vegetative Organs.= The cells in all the Algæ (excepting certain reproductive cells) are surrounded by a membrane which (with the exception of the Bacteria) consists of pure or altered cellulose, sometimes forming a gelatinous covering, at other times a harder one, with deposits of chalk or silica formed in it. The cell-nucleus, which in the Schizophyta is less differentiated, may be one or more (_e.g. Hydrodictyon_, _Siphoneæ_) in each cell. Excepting in the majority of the Bacteria, _colour materials_ (of which _chlorophyll_, or modifications of it, always seems to be found) occur, which either permeate the whole cytoplasm surrounding the cell-nucleus, as in most of the coloured Schizophyta, or are contained in certain specially formed small portions of protoplasm (chromatophores).

The individual at a certain stage of development consists nearly always of only one cell; by its division multicellular individuals may arise, or, if the daughter-cells separate immediately after the division, as in many of the simplest forms, the individual will, during the whole course of its existence, consist of only a single cell (unicellular Algæ). In multicellular individuals the cells may be more or less firmly connected, and all the cells of the individual may be exactly alike, or a division of labour may take place, so that certain cells undertake certain functions, and are constructed accordingly; this may also occur in parts of the cell in the large unicellular and multinuclear Algæ (Siphoneæ, p. 62).

The cells in most of the Algæ belong to the _parenchymatous_ form; these, however, in the course of their growth, may very often become somewhat oblong; in many Algæ (particularly Fucoideæ and Florideæ) occur, moreover, _hyphæ-like threads_, which are very long, often branched, and are either formed of a single cell, or, more frequently, of a row of cells, having a well-pronounced apical growth. The parenchymatous as well as the hyphæ-like cells may, in the higher Algæ (especially in certain Fucoideæ and Florideæ), be further differentiated, so that they form well-defined anatomico-physiological systems of tissue, _i.e._ assimilating, conducting, storing, and mechanical.

With regard to _the external form_, the thallus may present no differentiation, as in many unicellular Algæ, or in multicellular Algæ of the lower order, which are then either equally developed in all directions (_e.g. Pleurococcus_, Fig. 47), or form flat cell-plates (_Merismopedium_) or threads (_Oscillaria_, Fig. 21). The first step in the way of differentiation appears as a difference between apex and base (_Rivularia_, _Porphyra_); but the division of labour may proceed so that differences may arise between vegetative and reproductive cells (_Œdogonium_, Fig. 54); hairs and organs of attachment (rhizoids and haptera), which biologically serve as roots, are developed, and even leaves in certain forms of high order, belonging to different classes (_e.g. Caulerpa_, Fig. 59; _Characeæ_, Fig. 61; _Sargassum_, Fig. 72; and many Florideæ).

=The non-sexual reproduction= takes place _vegetatively_, in many instances, simply by division into two, and more or less complete separation of the divisional products (Diatomaceæ, Desmidiaceæ (Fig. 36), many Fission-plants, etc.), or by detached portions of the thallus (_e.g. Caulerpa_, _Ulva lactuca_, etc.; among many Schizophyceæ, small filaments known as _hormogonia_ are set free), or _asexually_ by special reproductive cells (_spores_) set free from the thallus; these may be either _stationary_ or _motile_. The stationary reproductive cells (spores) may either be devoid of cell-wall (tetraspores of the Florideæ), or may possess a cell-wall; in the latter case they may be formed directly from the vegetative cells, generally by the thickening of the walls (_akinetes_), or only after a process of re-juvenescence (_aplanospores_). Aplanospores, as well as akinetes, may either germinate immediately or may become resting-cells, which germinate only after a period of rest.

THE MOTILE ASEXUAL REPRODUCTIVE CELLS are spherical, egg- or pear-shaped, naked, _swarmspores_ (_zoospores_), which have arisen in other cells (_zoosporangia_), and propel themselves through the water by means of cilia; or they are _Phyto-Amœbæ_, which have no cilia and creep on a substratum by means of pseudopodia. The cilia, which are formed from the protoplasm (in the Bacteria, however, from the membrane), are mostly situated at the pointed and colourless end, which is directed forwards when in motion, and are 1, 2 (Fig. 5 _B_), 4 or more. Both the cilia in the Brown Algæ are attached to one side (Fig. 65); they are occasionally situated in a circle round the front end (_Œdogonium_, Fig. 6 _a_, and _Derbesia_), or are very numerous and situated in pairs distributed over a large part or nearly the whole of the zoospore (_Vaucheria_). Besides being provided with one or more nuclei (_Vaucheria_), they may also have a red “eye spot” and vacuoles, which are sometimes pulsating, _i.e._ they appear and reappear at certain intervals. The swarmspores move about in the water in irregular paths, and apparently quite voluntarily, revolving round their longer axes; but they come to the surface of the water in great numbers either because of their dependence on light, or driven by warm currents in the water, or attracted by some passing mass of food material. The swarmspores germinate, each forming a new plant, as their movement ceases they surround themselves with a cell-wall, grow, and then divide; in Fig. 6 _b_, two may be seen in the condition of germination, and about to attach themselves by means of the front end, which has been developed into haptera (see also Fig. 5 _B_, lowest figure).

=The sexual reproduction= here, probably in all cases, consists in the coalescence of two masses of protoplasm, that is, in the fusion of their nuclei.

The simplest and lowest form is termed =conjugation=, or =isogamous= fertilisation, and is characterized by the fact that the two coalescing cells (termed gametes) are equal, or almost equal, in shape and size (the female gamete in the _Cutleriaceæ_, _e.g. Zanardinia collaris_, Fig. 7, is considerably larger than the male gamete). The cell in which the _gametes_ are developed is called a _gametaugium_, and the reproductive cell formed by their union--which generally has a thick wall and only germinates after a short period of rest--is termed a _zygote_ or _zygospore_. The conjugation takes place in two ways:--

(_a_) In the one way the gametes are motile cells (_planogametes_, _zoogametes_, Fig. 8), which unite in pairs during their swarming hither and thither in the water; during this process they lie side by side (Fig. 8 _d_), generally at first touching at the clear anterior end, and after a time they coalesce and become a motionless _zygote_, which surrounds itself with a cell-wall (Fig. 8 _e_). This form of conjugation is found in _Ulothrix_ (Fig. 8 _d_), _Acetabularia_, and other Algæ (Figs. 45, 56, 66).

(_b_) Among other Algæ (_e.g. Diatomaceæ_ and _Conjugatæ_), the conjugating cells continue to be surrounded by the cell-wall of the mother-cell (_aplanogametes_ in an _aplanogametangium_); the aplanogametangia generally grow out into short branches, which lie close together and touch one another, the wall at the point of contact is then dissolved (Fig. 39). Through the aperture thus formed, the aplanogametes unite, as in the first instance, and form a rounded zygote, which immediately surrounds itself with a cell-wall. Various modifications occur; compare Figs. 37, 39, 41, 43.

The highest form of the sexual reproduction is the =Egg- or Oogamous= fertilisation. The two coalescing cells are in the main unlike each other in form as well as size. The one which is considered as the male, and is known as the _spermatozoid_ (_antherozoid_), developes as a rule in large numbers in each mother-cell (_antheridium_); they are often self-motile (except in the Florideæ, where they are named _spermatia_), and are many times smaller than the other kind, the female, which is known as the _egg-cell_, (_oosphere_). The egg-cell is always a motionless, spherical, primordial cell which can either float about freely in the water, as in the Fucaceæ (Fig. 9), or is surrounded by a cell-wall (_oogonium_); generally only one oosphere is to be found in each oogonium, but several occur in _Sphæroplea_ (Fig. 10). The result of the spermatozoid coalescing with the egg-cell is, as in this case, the formation of a oospore, which generally undergoes a period of rest before germination (the Florideæ are an exception, a fruit-body, _cystocarp_, being produced as the result of coalescence).

An example of fertilisation is afforded by the Alga, _Sphæroplea
annulina_ (Fig. 10). The filamentous thallus is formed of
cylindrical cells with many vacuoles (_r_ in _A_); some cells
develope egg-cells (_B_), others spermatozoids (_C_), the latter
in a particularly large number. The egg-cells are spherical,
the spermatozoids of a club- or elongated pear-shape with two
cilia at the front end (_G_; _E_ is however a swarmspore).
The spermatozoids escape from their cells through apertures
in the wall (_o_ in _C_) and enter through similar apertures
(_o_ in _B_) to the egg-cells. The colourless front end of the
spermatozoid is united at first with the “receptive spot” of the
egg-cell (see _F_), and afterwards completely coalesces with
it. The result is the formation of a oospore with wart-like
excrescences (_D_).

The female (_parthenogenesis_) or male (_androgenesis_) sexual cell may, sometimes without any preceding fertilisation, form a new individual (_e.g. Ulothrix zonata_, _Cylindrocapsa_, etc.).

=Systematic division of the Algæ.= The Algæ are divided into the following ten classes:

1. SYNGENETICÆ; 2. DINOFLAGELLATA, or PERIDINEA; 3. DIATOMACEÆ;
4. SCHIZOPHYTA, FISSION-ALGÆ; 5. CONJUGATÆ; 6. CHLOROPHYCEÆ,
GREEN-ALGÆ; 7. CHARACEÆ, STONE-WORTS; 8. PHÆOPHYCEÆ; 9.
DICTYOTALES; 10. RHODOPHYCEÆ.

Among the lowest forms of the Algæ, the Syngeneticæ, the Dinoflagellata, and the unicellular Volvocaceæ (Chlamydomoneæ), distinct transitional forms are found approaching the animal kingdom, which can be grouped as animals or plants according to their method of taking food or other characteristics. Only an artificial boundary can therefore be drawn between the animal and vegetable kingdoms. In the following pages only those forms which possess _chromatophores_, and have _no mouth_, will be considered as Algæ.

Class 1. =Syngeneticæ.=

The individuals are uni- or multicellular, free-swimming or motionless. The cells (which in the multicellular forms are loosely connected together, often only by mucilaginous envelopes) are naked or surrounded by a mucilaginous cell-wall, in which silica is never embedded. They contain one cell-nucleus, one or more pulsating vacuoles, and one to two band- or plate-like chromatophores with a brown or yellow colour, and sometimes a pyrenoid.

Reproduction takes place by vegetative division, or asexually by zoospores, akinetes (or aplanospores?). Sexual reproduction is unknown. They are all fresh water forms.

To this class may perhaps be assigned the recently arranged and
very little known orders of _Calcocytaceæ_, _Murracytaceæ_,
_Xanthellaceæ_, and _Dictyochaceæ_, which partly occur in the
free condition in the sea, in the so-called “Plankton,” and
partly symbiotic in various lower marine animals.

The _Syngeneticæ_ are closely related to certain forms in the animal kingdom, as the Flagellatæ.

Order 1. =Chrysomonadinaceæ.= Individuals, uni- or multicellular, swimming in free condition, naked or surrounded by a mucilaginous covering. The cells are generally oval or elongated, with 2 (rarely only 1) cilia, almost of the same length, and generally with a red “eye-spot” at their base, and with 2 (rarely 1 only) band-shaped chromatophores. Reproduction by the longitudinal division of the individual cells either during the swarming, or during a resting stage; in the multicellular forms also by the liberation of one or more cells, which in the latter case are connected together.

A. Unicellular: _Chromulina_, _Cryptoglena_, _Microglena_,
_Nephroselmis_.

B. Multicellular: _Uroglena_, _Syncrypta_ (Fig. 11), _Synura_.

Among the unicellular Chrysomonadinaceæ are probably classed
some forms which are only stages in the development of the
multicellular, or of other _Syngeneticæ_.

Order 2. =Chrysopyxaceæ= are unicellular, and differ mainly from the preceding in being attached either on a slime-thread (_Stylochrysalis_), or enclosed in an envelope (_Chrysopyxis_, Fig. 12). They have two cilia, and multiply by longitudinal (_Chrysopyxis_) or transverse division, and the swarming of one of the daughter-individuals (zoospore). Division may also take place in a motionless stage (_palmella-stage_).

Order 3. =Dinobryinaceæ.= The individuals are originally attached, uni- or multicellular; each individual cell is distinctly contractile, and fixed at the bottom of a cup-shaped, open envelope. Cilia 2, but of unequal length. Asexual reproduction by zoospores, which are formed by straight or oblique longitudinal division of the mother-cell, during a palmella-stage which is produced in the winter aplanospores. _Epipyxis_, _Dinobryon_.

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 (?).

[=i]_ (lateral views).]

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.

The condition termed “Zooglœa,” which reminds us of _Nostoc_, is produced by the cells becoming strongly mucilaginous. A number of individuals in active division are found embedded in a mass of mucilage, which either contains only one, or sometimes more, of the above-named forms. The individuals may eventually swarm out and continue their development in an isolated condition. Such mucilaginous masses occur especially upon moist vegetables (potatoes, etc.), on the surface of fluids with decaying raw or cooked materials, etc. The mucilaginous envelope is thrown into folds when the Bacteria, with their mucilaginous cell-walls, multiply so rapidly that there is no more room on the surface of the fluid.

The cells of the Bacteria are constructed like other plant-cells in so far as their diminutive size has allowed us to observe them. The cell-wall only exceptionally shows the reactions of cellulose (in _Sarcina_, _Leuconostoc_; also in a Vinegar-bacterium, _Bacterium xylinum_); a mucilaginous external layer is always present. The body of the cell mostly appears to be an uniform or finely granulated protoplasm. Very few species (_e.g. Bacillus virens_) contain chlorophyll; others are coloured red (purple sulphur Bacteria); the majority are colourless. _Bacillus amylobacter_ shows a reaction of a starch-like material when treated with iodine before the spore-formation. Some Bacteria contain sulphur (see p. 37). The body, which has been described as a _cell-nucleus_, is still of a doubtful nature.

Artificial colourings with aniline dyes (especially methyl-violet, gentian-violet, methylene-blue, fuchsin, Bismarck-brown and Vesuvin) play an important part in the investigations of Bacteria.

MOVEMENT. Many Bacteria are self-motile; the long filaments of _Beggiatoa_ exhibit movements resembling those of _Oscillaria_. In many motile forms the presence of cilia or flagella has been proved by the use of stains; many forms have one, others several cilia attached at one or both ends (Fig. 23) or distributed irregularly over the whole body; the cilia are apparently elongations of the mucilaginous covering and not, as in the other Algæ of the protoplasm. In _Spirochæte_ the movement is produced by the flexibility of the cell itself. Generally speaking, the motion resembles that of swarm-cells (_i.e._ rotation round the long axis and movement in irregular paths); but either end has an equal power of proceeding forwards.

The swarming motion must not be confounded with the hopping
motion of the very minute particles under the microscope
(Brownian movement).

VEGETATIVE REPRODUCTION takes place by continued transverse division; hence the name “Fission-Fungi” or “Fission-Algæ,” has been applied to the Bacteria.

SPORES. The spores are probably developed in two ways. In the ENDOSPOROUS species (Figs. 28, 29), the spore arises as a new cell inside the mother-cell. The spores are strongly refractile, smaller than the mother-cell, and may be compared to the aplanospores of other Algæ. In addition to these there are the ARTHROSPOROUS species in which the cells, just as in _Nostoc_ and other Blue-green Algæ, assume the properties of spores without previously undergoing an endogenous new construction, and are able to germinate and form new vegetative generations (Fig. 27). The formation of spores very often commences when the vegetative development begins to be restricted.

The spores germinate as in _Nostoc_ by the bursting of the external layer of the cell-wall, either by a transverse or longitudinal cleft, but always in the same way, in the same species (Fig. 28, example of transverse cleft).

DISTRIBUTION. Bacteria and their germs capable of development, are found everywhere, in the air (dust), in surface water, and in the superficial layers of the soil. The number varies very much in accordance with the nature of the place, season, etc. They enter, together with air and food, into healthy animals and occur always in their alimentary tract.

GROWTH AND REPRODUCTION depend upon the conditions of temperature. There is a certain minimum, optimum and maximum for each species; for instance (in degrees Centigrade)--

Minim. Opt. Maxim.
_Bacillus subtilis_ + 6 c. 30 + 50
_B. anthracis_ 15 20–25 43
_Spirillum choleræ asiaticæ_ 8 37 40 (but grows
only feebly
if under
16°).
_Bacterium tuberculosis_ 28 37–38 42

The functions of life cease on a slight excess of the maximum or minimum temperature, numbness setting in when either of these limits is passed. _Crenothrix_-threads provided with mucilaginous envelopes may, according to Zopf, sustain a temperature of-10°. Some Bacteria are said to be able to resist the exposure to as low a temperature as-110° for a short time. It is not known at what degree of cold the death of the Bacteria occurs: the greatest degree of heat which the vegetative cells can withstand is about the same as that for other vegetative plant-cells, namely, about 50–60° C. Certain Bacteria, _e.g. B. thermophilus_, grow and thrive vigorously at 70° C. Many spores, on the contrary, are able to bear far higher temperatures (in several species a temperature for some duration of above 100°, those of _Bacillus subtilis_, for instance, can withstand for hours a temperature of 100° in nutrient solutions; the spores remain capable of development after exposure to a dry heat of 123° C.).

The _Desiccation_ of the air, if prolonged, kills many forms when in the vegetative condition. The spores however can bear a much longer period of dryness, some even several years.

OXYGEN. Some species cannot live without a supply of free oxygen (_Aerobic_), _e.g._ the Vinegar-bacteria, the Hay-bacilli, the Anthrax-bacilli, the Cholera-_Microspira_. Other species again thrive vigorously without supply of free oxygen, and are even checked in their development by the admission of air (_Anaerobic_), _e.g._ the butyric acid Bacterium (_Clostridium butyricium_ = _Bacillus amylobacter_). A distinction may be drawn between obligate and facultative aerobics and obligate and facultative anaerobics. Several Bacteria, producing fermentation, may grow without the aid of oxygen when they are living in a solution in which they can produce fermentation; but, if this is not the case, they can only grow when a supply of oxygen is available. A great number of the pathogenic Bacteria belong to the facultative anaerobics.

A luminous Bacterium (_Bacillus phosphorescens_) which in the presence of a supply of oxygen gives a bluish-white light, has been found in sea-water. Phosphorescent Bacteria have frequently been observed upon decaying sea-fish, as well as on the flesh of other animals; by transferring the Bacteria from cod fish to beef, etc., the latter may be made luminous.

_Organic carbon compounds_ are indispensable for all Bacteria, (except, as it appears, for the nitrifying organisms), as they can only obtain the necessary supplies of _carbon_ from this source. The supplies of _nitrogen_, which also they cannot do without, can be obtained equally as well from organic compounds as from inorganic salts, such as saltpetre or ammonia-compounds. The various “ash-constituents” are also essential for their nourishment.

While Moulds and Yeast-Fungi grow best in an acid substratum, the _Bacteria_, on the other hand, generally thrive _best_ in a _neutral_ or slightly _alkaline_ one.

In _sterilization_, _disinfection_, and _antisepsis_, means are employed by which the Bacteria are killed, or checked in their development, for instance, by heat (ignition, cooking, hot vapours, hot air, etc.), or poisons (acids, corrosive sublimate). The process of preserving articles of food, in which they are boiled and then hermetically sealed, aims at destroying the Bacteria, or the spores of those which already may be present in them, and excluding all others.

As the Bacteria are unable to assimilate carbon from the carbonic acid of the air, but must obtain it from the carbon-compounds already in existence in the organic world, they are either _saprophytes_ or _parasites_. Some are exclusively either the one or the other, _obligate_ saprophytes or parasites. But there are transitional forms among them, some of which are at ordinary times saprophytes, but may, when occasion offers, complete their development wholly or partly as parasites--_facultative parasites_; others are generally parasitic, but may also pass certain stages of development as saprophytes--_facultative saprophytes_.