There are a few plants whose seed will develop under water while others retain enough of the scant dews of the desert nights to waken the seed into growth.
Usually, however, a moderate water supply is essential, too much causing decay, and too little precluding growth altogether. As to temperature, a maple seedling will germinate on a cake of ice and many other seeds grow in extreme cold, while a smaller number tolerate high temperatures.
The majority, however, germinate most freely between 60° and 80° F. Air from some source is essential to growth, for seeds, like all living things, must breathe. Many can obtain the needed supply even from the air dissolved in the water in which they maybe submerged.
In addition to these external conditions, the embryo must also have a supply of stored food for immediate use while the roots .and leaves are developing. This food may be stored in the cotyledons, as in the bean and pea, or outside the embryo, as in the case of the endosperm of the corn and other grains.
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