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 essentials to germination are air, moisture, and a suitable temperature. The oxygen of the air is necessary for the purpose of respiration, to keep the embryo alive even while it is resting in the dry seed; but before germination can take place it must respire more freely to induce the chemical changes necessary for the resumption of growth. Moisture is required to soften the testa and other hard parts with which it may be surrounded, as well as to swell up the tissues of the embryo and endosperm, where such is present. A certain degree of heat is necessary before any growth can take place, and this has to be determined in each particular case. The seed of the Sycamore may germinate at freezing-point, but many other plants require a considerably higher temperature. On the other hand, very few species of plants will germinate in a higher temperature than 104° to 108° F. The best temperature, therefore, for germinating seed lies somewhere between freezing-point and the last-named figures. The temperature at which seeds will germinate most quickly is the best in all cases; hence the value of ascertaining this approximately. As soon as germination is completed most plants will thrive best with less heat, more air and light, but particularly those seedlings whose cotyledons rise above-ground and become green.
Fig. 60. - Seeds with the Testa or Skin winged, or provided with a tuft of Hairs, the object in each case being to secure the dispersal of the Seed.
1, Lepigonum. 2, 3, Aspen. 4, 5, Cinchona.
Seeds which contain no endosperm usually germinate very quickly because the embryo already contains all the reserve food within itself, as in the Cabbage, Turnip, Mustard, and Willow. The Almond, Plum, and Cherry take a long time to soften the hard shell in which the seed is enclosed, while endosperm is absent. The seeds of many trees fail to germinate at all if kept dry over the winter before being sown. If allowed to get dry, the seeds of Canna often require filing and steeping in warm water before they will absorb sufficient water to induce germination. Carrots, Parsnips, Parsley, and Onions take a long time to germinate, because the small embryo has to feed on the endosperm and grow to some size before it can leave the seed. Grass seeds have a starchy endosperm, but germinate quickly because the embryo is situated on the outside of the mass and remains attached to it by the cotyledon while the first leaf rises above-ground. The stored materials are converted from insoluble into soluble matters, which the embryo can absorb. Starch is changed into liquid sugar or glucose by a kind of ferment set up in the endosperm or in the tissues of the embryo itself. Many seeds contain a large quantity of oil, which gets changed into starch and finally into sugar, before being used up by the embryo. Light is unnecessary for these processes, and is detrimental chiefly by drying up the moisture and causing great fluctuations of temperature. After the seed leaves are expanded light is of the greatest importance. Oily seeds soon lose the power of germination; starchy seeds, like wheat and barley, and the seeds of the Pea family, which contain no endosperm and little or no oil, may live for ten to forty years, but the process of breathing alone would, in the course of a relatively short period, consume the live substance of any seed. [J. f.]