l centimetres in diameter.

[Illustration: FIG. 7.--Forms of _Cyanophyceae_. _A_, _Nostoc_. _B_,
_Gloeotrichia_, x 1. _C_, individual of _Gloeotrichia_. _D_,
Chrooecoccus. _E_, _Nostoc_. _F_, Oscillaria. _G_, _H_, _Tolypothrix_.
All x 300. _y_, heterocyst. _sp._ spore.]

In the higher forms special cells called heterocysts are found. They
are colorless, or light yellowish, regularly disposed; but their
function is not known. Besides these, certain cells become
thick-walled, and form resting cells (spores) for the propagation of
the plant (Fig. 7, C. _sp._). In species where the sheath of the
filament is well marked (Fig. 7, _H_), groups of cells slip out of the
sheath, and develop a new one, thus giving rise to a new plant.

The bacteria (_Schizomycetes_), although among the commonest of
organisms, owing to their excessive minuteness, are difficult to
study, especially for the beginner. They resemble, in their general
structure and methods of reproduction, the blue-green slimes, but are,
with very few exceptions, destitute of chlorophyll, although often
possessing bright pigments,--blue, violet, red, etc. It is one of
these that sometimes forms blood-red spots in flour paste or bits of
bread that have been kept very moist and warm. They are universally
present where decomposition is going on, and are themselves the
principal agents of decay, which is the result of their feeding upon
the substance, as, like all plants without chlorophyll, they require
organic matter for food. Most of the species are very tenacious of
life, and may be completely dried up for a long time without dying,
and on being placed in water will quickly revive. Being so extremely
small, they are readily carried about in the air in their dried-up
condition, and thus fall upon exposed bodies, setting up decomposition
if the conditions are favorable.

A simple experiment to show this may be performed by taking two test
tubes and partly filling them with an infusion of almost any organic
substance (dried leaves or hay, or a bit of meat will answer). The
fluid should now be boiled so as to kill any germs that may be in it;
and while hot, one of the vessels should be securely stopped up with a
plug of cotton wool, and the other left open. The cotton prevents
access of all solid particles, but allows the air to enter. If proper
care has been taken, the infusion in the closed vessel will remain
unchanged indefinitely; but the other will soon become turb