Sometimes, owing to external conditions, such as lack of food certain bacteria produce spores. The power of spore formation is not possessed by all bacteria and those which are able to bring it about are difficult to kill for the spores, which contain the living material of the bacterium are surrounded with walls which will resist boiling, drying, freezing and all manner of ill treatment. The spore formation of bacteria is very simple, all or part of the living contents of the bacterium becomes surrounded by a tough wall and remains so surrounded till circumstances are favourable, when the wall bursts, its contents escapes and becomes a bacterium, capable of founding a new colony by the method of splitting we have already described. Now let us try to find out what sort of plants we are to look for, when we are searching for bacteria, under our microscope. They exist in many forms, to which special names have been applied, and it is unfortunate that, very often, their external form varies according to their state, thus a bacterium may be spherical when young and rod shaped when older. Some bacteria are spherical and are known as _Cocci_ or _Micrococci_, from Greek words meaning a berry or a little berry respectively; sometimes these spherical bacteria occur in pairs, then they are called _Diplococci_ (double berries); or in chains, _Streptococci_ (chain berries); or in bunches, _Staphylococci_ (grape berries). They may resemble short rods, when they are called _Bacteria_, a name, by the way, which is also applied generally to all microbes; they may, on the other hand, have the appearance of longer rods and then they are called _Bacilli_. Some of these longer rods may be curved or even corkscrew shaped when they are known by the name of _Spirilla_. Rather fearsome names some of these we fear and we wished to avoid long names, but they appear over and over again in books and papers relating to bacteria so we are compelled to introduce them to our pages. Many bacteria possess no power of movement, others swim rapidly, by the aid of the lashing movement of little whip-like structures with which they are furnished. After all this preamble, which we hope has cleared up certain misconceptions regarding bacteria and has given the reader some insight into their habits, we may proceed to the examination of some of the plants themselves. At the outset we have a confession to make. Bacteria can only be studied seriously, by those who possess very expensive and elaborate apparatus; considerable technical skill is required to prepare the plants for examination--many of them indeed can only be seen after they have been stained and lastly, to trifle with the disease-causing members of the family may lead to dangerous if not fatal results.