Make Your Own Yeast With Potatoes

From one of my favorite vintage cook books, Mrs. Wilson's Cook Book, 1920:

HOME-MADE YEAST
Wash four potatoes and then cut in slices, without peeling, and place in saucepan, and add three pints of water. Cook until the potatoes are soft and then add
One-half cupful of hops[whole food stores or home brewing stores online].
Cook slowly for one-half hour. Rub the mixture through a fine sieve and then pour hot mixture on
One and one-half cupfuls of flour,
One tablespoonful of salt,
One-quarter cupful of brown sugar.

Stir until well mixed, beating free from lumps. Cool to 80 degrees Fahrenheit. Now add
One yeast cake dissolved in one cupful of water, 80 degrees Fahrenheit
Stir well to mix and then let ferment in a warm place for ten hours. Now pour into jar or crock and store in a cool place.
TO USE
Use one and one-half cups of this mixture in place of the yeast cake. Always stir well before using and take care that the mixture does not freeze. This potato ferment must be made fresh every eighteen days in winter and every twelve days in summer.
[pg 9]

Reportedly, these starters can last for a very long time, often passed from mother to daughter before the days of packaged yeast.