The World Too Small to See

Microorganism is the one grouping in this subject that deliberately breaks the rule the previous track spent five tutorials establishing. It is a statement about size and nothing else, so following it back into the classification lands in Monera, Fungi and Protista at once, and then in viruses, which belong to no kingdom at all. The track starts by taking the word apart, then turns to the arithmetic that makes microbes so hard to keep up with: binary fission leaves two cells where there was one, with no parent and no juvenile, so a count doubles rather than adding, and twenty-four doublings turn one cell into sixteen million. Bread mould then supplies a microbe you can watch, with every part answering a job — threads that digest the loaf from outside and absorb what dissolves, and a stalk whose only purpose is to lift spores clear of the food. The last two tutorials take the practical consequences in both directions: the four microbial jobs the living world could not continue without, and the two ways of stopping microbial growth, where killing is permanent and slowing lasts only as long as the conditions do.