~Saccharomyces~ (Fig. 81).--Note the round or oval cells of granular protoplasm (a) containing solid particles and vacuoles (c), and surrounded by a definite envelope.

~Reproduction.~--Budding; ascospores--as.e.xual.

Note the as.e.xual _reproduction_.

1. "Gemmation"--that is, the budding out of daughter cells (b) from various parts of the gradually enlarging mother cell. These are eventually cast off and in turn become mother cells and form fresh groups of buds.

[Ill.u.s.tration: FIG. 81.--Saccharomyces with ascospores.]

[Ill.u.s.tration: FIG. 82.--Torula.]

2. Spore formation--"ascospores" (e). These are formed at definite temperatures and within well-defined periods; e. g., Saccharomyces cerevisiae, thirty hours at 25 to 37C., or ten days at 12C.

~Torulae~ (Fig. 82).--Torulae, whilst resembling yeasts in almost every other respect, never form endo-spores. Note the elongated, sausage-shaped cells (a) the larger oval cells (b) and the globular cells (c) the former two often interlacing and growing as a film.

Note the absence of ascospore formation.

IX. SCHIZOMYCETES.

~Cla.s.sification and Morphology.~--Bacteria are often cla.s.sified, in general terms, according to their life functions, into--

_Saprogenic_, or putrefactive bacteria; _Zymogenic_, or fermentative bacteria; _Pathogenic_, or disease-producing bacteria;

or according to their food requirements into--

_Prototrophic_, requiring no organic food (e. g., nitrifying bacteria); _Metatrophic_, requiring organic food (e. g., saprophytes and facultative parasites); _Paratrophic_, requiring living food (obligate parasites);

or according to their metabolic products into--

_Chromogenic_, or pigment-producing bacteria; _Photogenic_, or light-producing bacteria; _Aerogenic_, or gas-producing bacteria;

and so on.

Such broad groupings as these have, however, but little practical value when applied to the systematic study of the fission fungi.

On the other hand, no really scientific cla.s.sification of the schizomycetes has yet been drawn up, and the varying morphological appearances of the members of the family are still utilised as a basis for cla.s.sification, as under--

~1. Cocci.~ (Fig. 83).--Rounded or oval cells, subdivided according to the arrangement of the individuals after fission, into--

_Diplococci_ and _Streptococci_, where division takes place in one plane only, and the individuals remain attached (a) in pairs or (b) in chains.

_Tetrads_, _Merismopedia_, or _Pediococci_, where division takes place alternately in two planes at right angles to each other, and the individuals remain attached in flat tablets of four, or its multiples.

[Ill.u.s.tration: FIG. 83.--Types of bacteria--cocci: 1, Diagram of sphere indicating planes of fission; 2, diplococci; 3, streptococci; 4, tetrads; 5, sarcinae; 6, staphylococci.]

_Sarcinae_, where division takes place in three planes successively, and the individuals remain attached in cubical packets of eight and its multiples.

[Ill.u.s.tration: FIG. 84.--Types of bacteria--bacilli, etc.: 1, Bacilli; 2, diplobacilli; 3 streptobacilli; 4, spirilla; 5, vibrios; 6, spirochaetae.]

_Micrococci_ or _Staphylococci_, where division takes place in three planes, but with no definite sequence; consequently the individuals remain attached in pairs, short chains, plates of four, cubical packets of eight, and irregular ma.s.ses containing numerous cocci.

~2. Bacilli~ (Fig. 84, 1 to 3).--Rod-shaped cells. A bacillus, however short, can usually be distinguished from a coccus in that two sides are parallel. Some bacilli after fission retain a characteristic arrangement and may be spoken of as _Diplobacilli_ or _Streptobacilli_.

Leptothrix is a term that in the past has been loosely used to signify a long thread, but is now restricted to such forms as belong to the leptothriciae (_vide infra_).

~3. Spirilla~ (Fig. 84, 4 to 6).--Curved and twisted filaments.

Cla.s.sified, according to shape, into--

Spirillum.

Vibrio (comma).

Spirochaeta.

Many Spirochaetes appear to belong to the animal kingdom and are grouped under protozoa; other organisms to which this name has been given are undoubtedly bacteria.

Higher forms of bacteria are also met with, which possess the following characteristics: They are attached, unbranched, filamentous forms, showing--

(a) Differentiation between base and apex;

(b) Growth apparently apical;

(c) Exaggerated pleomorphism;

(d) "Pseudo-branching" from apposition of cells; and are cla.s.sified into--

1. Beggiotoa. } Free swimming forms, which 2. Thiothrix. } contain sulphur granules.

3. Crenothrix. } 4. Cladothrix. } These forms do not contain 5. Leptothrix. } sulphur granules.

6. Streptothrix. A group which exhibits true but not dichotomous branching, and contains some pathogenic species.

The morphology of the same bacterium may vary greatly under different conditions.

For example, under one set of conditions the examination of a pure cultivation of a bacillus may show a short oval rod as the predominant form, whilst another culture of the same bacillus, but grown under different conditions, may consist almost entirely of long filaments or threads. This variation in morphology is known as "pleomorphism."

Some of the factors influencing pleomorphism are:

1. The composition, reaction, etc., of the _nutrient medium_ in which the organism is growing.

2. _The atmosphere_ in which it is cultivated.

3. _The temperature_ at which it is incubated.

4. Exposure to or protection from _light_.