This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
I noticed, some time ago, an inquiry by a lady as to the most successful mode of germinating canna seed. Having subsequently received seed of the new canna Marechal Valliant (it having the reputation of being the most ornamental variety of that family), we thought we would germinate it with more success than we usually had. Having previously tried it in the various ways approved by scientific seed growers, I found the following method to be the most reliable: Prepare the soil in boxes, pans, etc.; sow therein your seed a half inch deep, and place in a low temperature. Keep very wet with cold water, and in this situation remain for three days. Keep them cool and wet all the time. The seed being now thoroughly saturated with cold water and other substances contained in soil, remove the box or pans to a dark, warm place, where the temperature will average 70 or 75 degrees, and keep the soil humid and moist. The sudden change from cold to heat will have a great germinating influence on the seed; the hard shell will soften, and the seed will sprout rapidly.
You ask, perhaps, why not soak the seed in cold water for a period before sowing? Wet soil, that has been exposed to the weather, is to be preferred, as the ammoniac substances of the soil have a softening influence.