Fig. 24.
Yeast cells growing under the microscope.
a, Single cells. b, Two cells forming by division. c, A group of cells where division is going on in all directions.
"The yeast plant has never been found wild. It is only known as a cultivated plant, growing on prepared liquor. The brewer has to sow it by taking some yeast from other beer, or by leaving the liquor exposed to air in which yeast spores are floating; or it will sow itself in the same way in a mixture of water, hops, sugar, and salt, to which a handful of flour is added. It increases at a marvellous rate, one cell budding out of another, while from time to time the living matter in a cell will break up into four parts instead of two, and so four new cells will start and bud. A drop of yeast will very soon cover a glass slide with this tiny plant, as you will see under the second microscope, where they are now at work (Fig. 24)."
"But perhaps the most curious of all the minute fungi are those which grow inside insects and destroy them. At this time of year you may often see a dead fly sticking to the window-pane with a cloudy white ring round it; this poor fly has been killed by a little fungus called Empusa muscæ. A spore from a former plant has fallen perhaps on the window-pane, or some other spot over which the fly has crawled, and being sticky has fixed itself under the fly's body. Once settled on a favourable spot it sends out a tube, and piercing the skin of the fly, begins to grow rapidly inside. There it forms little round cells one after the other, something like the yeast-cells, till it fills the whole body, feeding on its juices; then each cell sends a tube, like the upright tubes of the Mucor (Fig. 23) out again through the fly's skin, and this tube bursts at the end, and so new spores are set free. It is these tubes, and the spores thrown from them, which you see forming a kind of halo round the dead fly as it clings to the pane. Other fungi in the same way kill the silkworm and the caterpillars of the cabbage butterfly. Nor is it only the lower animals which suffer. When we once realise that fungus spores are floating everywhere in the air, we can understand how the terrible microscopic fungi called bacteria will settle on an open wound and cause it to fester if it is not properly dressed.
"Thus we see that these minute fungi are almost everywhere. The larger ones, on the contrary, are confined to the fields and forests, damp walls and hollow trees; or wherever rotting wood, leaves, or manure provide them with sufficient nourishment. Few people have any clear ideas about the growth of a mushroom, except that the part we pick springs up in a single night. The real fact is, that a whole mushroom plant is nothing more than a gigantic mould or mildew, only that it is formed of many different shaped cells, and spreads its tubes underground or through the trunks of trees instead of in paste or jam, as in the case of the mould."
Fig. 25.
Early stages of the mushroom. (After Sachs.)