These positions seem at first purely morphological, or of no physiological signification; but Dr.Hooker informs me that within the same ovarium the upper ovules alone in some cases, and in others the lower ones alone are fertilised; and he suggests that this probably depends on the direction in which the pollen-tubes enter the ovarium.

If so, the position of the ovules, even when one is erect and the other suspended within the same ovarium, would follow the selection of any slight deviations in position which favoured their fertilisation, and the production of seed.
Several plants belonging to distinct orders habitually produce flowers of two kinds--the one open, of the ordinary structure, the other closed and imperfect.

These two kinds of flowers sometimes differ wonderfully in structure, yet may be seen to graduate into each other on the same plant.

The ordinary and open flowers can be intercrossed; and the benefits which certainly are derived from this process are thus secured.
The closed and imperfect flowers are, however, manifestly of high importance, as they yield with the utmost safety a large stock of seed, with the expenditure of wonderfully little pollen.

The two kinds of flowers often differ much, as just stated, in structure.