Abstract
ABSTRACT The effects of embryo envelopes dealt with here are: impermeability to water, impermeability to oxygen, and mechanical resistance to radicle protrusion. To prevent water penetration, one or more layers of the embryo envelopes, as well as all possible openings in it such as the hilum or chalaza of the seed coat, should be watertight. This layer usually consists of compactly arranged cells, with various hydrophobic compounds impregnated into the cellulose network of the walls and/or deposited upon them. Restriction of oxygen without hindrance of water penetration may be due to a long path through which the dissolved oxygen has to diffuse in water, to a continuous layer of mucilage covering the embryo, to oxygen-consuming compounds in the envelopes, and to selective permeability of living cells. Resistance to radicle penetration is caused by any type of mechanical tissue that has a higher mechanical restraining force than the growth potential of the embryo.
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