1073. Seeds will not germinate so well in light as in darkness ; because light decomposes the carbonic acid gas, expels the oxygen, and fixes the carbon; thus hardening all the parts of the seed, and preventing vegetation. Seeds sown as soon as they are gathered, generally vegetate, at the latest, in the ensuing spring; but when they are first dried they will frequently lie a whole year in the soil without vegetating. Many seeds, when buried beyond a certain depth, lose their vegetative powers; others, as clover, retain them, and when brought up to the surface, will germinate, after having been buried for many years. The conditions which are necessary for the germination of seeds, are heat, moisture, and atmospheric air ; and the reverse of these are the most favourable for conveying seeds to a distance. After much experience it has been found, that seeds packed loosely in coarse canvass bags, and hung to the ceiling of the cabin of a ship, where they are perfectly dry and cool, will retain their vegetative powers much better than when enveloped in wax or tallow, or mixed with sugar or charcoal. No material will preserve seeds so long as coarse brown paper, made from old tarred rope, in which a large quantity of tar is incorporated. Cartridge paper offers seeds no protection whatever. Surrounding seeds with moist earth rammed very hard, so as to exclude the air, is said to prevent germination, and at the same time retain the vital principle. In general, the most difficult seeds to preserve are those which contain much oil ; but there are many exceptions in the case of the seeds of the Brassica family, and of mustard, and other cruciferous plants.
