This process of self-fertilization is the usual method, and no doubt keeps the variety true or unmixed; but the occasional presence of varieties in a wheat-field shows that cross-fertilization is sometimes secured.
Only where there is no such struggle for existence does self-fertilization often prove satisfactory for many generations."
Self-fertilization occurs when the pollen of a given flower affects the egg-cell of the same individual flower.
But even in plants with hermaphrodite flowers self-fertilization is often provided against by the structure of the parts or by the period of ripening of the organs.
They differ structurally from the normal form in being capable of self-fertilization only, and in the shape and details of their spermatozoa.