This would be beneficial under certain circumstances, such as a change in the nature of the insects which visited the flowers, or in their becoming more anemophilous, for such plants require an enormous quantity of pollen. The increased action of the male organs would tend to affect through compensation the female organs of the same flower; and the final result would be that the species would consist of males and hermaphrodites. But it is of no use considering this case and other analogous ones, for, as stated in the Introduction, the coexistence of male and hermaphrodite plants is excessively rare.
It is no valid objection to the foregoing views that changes of such a nature would be effected with extreme slowness, for we shall presently see good reason to believe that various hermaphrodite plants have become or are becoming dioecious by many and excessively small steps. In the case of polygamous species, which exist as males, females and hermaphrodites, the latter would have to be supplanted before the species could become strictly dioecious; but the extinction of the hermaphrodite form would probably not be difficult, as a complete separation of the sexes appears often to be in some way beneficial. The males and females would also have to be equalised in number, or produced in some fitting proportion for the effectual fertilisation of the females.
There are, no doubt, many unknown laws which govern the suppression of the male or female organs in hermaphrodite plants, quite independently of any tendency in them to become monoecious, dioecious, or polygamous. We see this in those hermaphrodites which from the rudiments still present manifestly once possessed more stamens or pistils than they now do,--even twice as many, as a whole verticil has often been suppressed. Robert Brown remarks that "the order of reduction or abortion of the stamina in any natural family may with some confidence be predicted," by observing in other members of the family, in which their number is complete, the order of the dehiscence of the anthers (7/3. 'Transactions of the Linnean Society' volume 12 page 98 or 'Miscellaneous Works' volume 2 pages 278-81.); for the lesser permanence of an organ is generally connected with its lesser perfection, and he judges of perfection by priority of development. He also states that whenever there is a separation of the sexes in an hermaphrodite plant, which bears flowers on a simple spike, it is the females which expand first; and this he likewise attributes to the female sex being the more perfect of the two, but why the female should be thus valued he does not explain.
Plants under cultivation or changed conditions of life frequently become sterile; and the male organs are much oftener affected than the female, though the latter alone are sometimes affected. The sterility of the stamens is generally accompanied by a reduction in their size; and we may feel sure, from a wide-spread analogy, that both the male and female organs would become rudimentary in the course of many generations if they failed altogether to perform their proper functions. According to Gartner, if the anthers on a plant are contabescent (and when this occurs it is always at a very early period of growth) the female organs are sometimes precociously developed. (7/4. 'Beitrage zur Kenntniss' etc. page 117 et seq. The whole subject of the sterility of plants from various causes has been discussed in my 'Variation of Animals and Plants under Domestication' chapter 18 2nd edition volume 2 pages 146-56.) I mention this case as it appears to be one of compensation. So again is the well- known fact, that plants which increase largely by stolons or other such means are often utterly barren, with a large proportion of their pollen-grains in a worthless condition.
Hildebrand has shown that with hermaphrodite plants which are strongly proterandrous, the stamens in the flowers which open first sometimes abort; and this seems to follow from their being useless, as no pistils are then ready to be fertilised.