This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
The Yeast Plant (Saccharomyces Cerevisioe), such as is used by the brewer, if put in any clear liquid containing suitable food (malt, for instance), the liquid, if stood in a warm place for some hours, will become cloudy or muddy, this being due to the rapid multiplication of the Yeast Plant. The temperature of the fermenting liquid is raised as a result of the chemical changes being brought about in the constitution of the liquid by the Yeast Plant. If a drop is examined under the microscope, the plant is seen to be oval, smaller than the Protococcus, but without the green colouring pigment of that, showing that it belongs to the great group of Fungi. It is also rapidly multiplying by budding at one end. The tiny protuberance or offset grows to nearly the size of its parent, and drops away as a new individual.
The "clubbing" of turnips, cabbages, cauliflower, and other members of the Crucifer family is due to another fungus, which lives within the roots during summer and other mild periods, causing great swellings to arise. At this period each individual multiplies rapidly, and rests enclosed in a cell wall of its own during winter; but with a rise of temperature in spring the protoplasm quits the cells and unites in a jelly-like mass, which moves through or over damp soil in quest of fresh plants to attack.
From these three plants it will be seen that the protoplasm possesses certain properties. They can absorb food materials, manufacture the food, breathe, give off certain ingredients as waste products, reproduce themselves, and in two cases are possessed of motion, the capability of which resides in the protoplasm itself (fig. 2). The Protococcus can manufacture its own food from raw materials, by reason of the presence of green colouring matter under the influence of light. The Yeast Plant must be supplied with malt, grapes (in wine-making), or some other food already in an organized form. As the club-root fungus (Plasmodiophora brassicae) is also colourless, it must have organized food, and, as it feeds upon living plants, it is a parasite. All of them absorb oxygen to give them energy, and as it combines with some of their substance, carbon dioxide is given off. The process is equivalent to breathing or respiration, as in animals, and is absolutely essential to all living things.