Chapter 22

1865 page 161.)]

It is a curious fact that in all the foregoing polygamous, dioecious, and gyno- dioecious plants in which any difference has been observed in the size of the corolla in the two or three forms, it is rather larger in the females, which have their stamens more or less or quite rudimentary, than in the hermaphrodites or males. This holds good with Euonymus, Rhamnus catharticus, Ilex, Fragaria, all or at least most of the before-named Labiatae, Scabiosa atro-purpurea, and Echium vulgare. So it is, according to Von Mohl, with Cardamine amara, Geranium sylvaticum, Myosotis, and Salvia. On the other hand, as Von Mohl remarks, when a plant produces hermaphrodite flowers and others which are males owing to the more or less complete abortion of the female organs, the corollas of the males are not at all increased in size, or only exceptionally and in a slight degree, as in Acer. (7/23. ‘Botanische Zeitung’ 1863 page 326.) It seems therefore probable that the decreased size of the female corollas in the foregoing cases is due to a tendency to abortion spreading from the stamens to the petals. We see how intimately these organs are related in double flowers, in which the stamens are readily converted into petals. Indeed some botanists believe that petals do not consist of leaves directly metamorphosed, but of metamorphosed stamens. That the lessened size of the corolla in the above case is in some manner an indirect result of the modification of the reproductive organs is supported by the fact that in Rhamnus catharticus not only the petals but the green and inconspicuous sepals of the female have been reduced in size; and in the strawberry the flowers are largest in the males, mid-sized in the hermaphrodites, and smallest in the females. These latter cases,--the variability in the size of the corolla in some of the above species, for instance in the common thyme,--together with the fact that it never differs greatly in size in the two forms--make me doubt much whether natural selection has come into play;--that is whether, in accordance with H. Muller’s belief, the advantage derived from the polleniferous flowers being visited first by insects has been sufficient to lead to a gradual reduction of the corolla of the female. We should bear in mind that as the hermaphrodite is the normal form, its corolla has probably retained its original size. (7/24. It does not appear to me that Kerner’s view ‘Die Schutzmittel des Pollens’ 1873 page 56, can be accepted in the present cases, namely that the larger corolla in the hermaphrodites and males serves to protect their pollen from rain. In the genus Thymus, for instance, the aborted anthers of the female are much better protected than the perfect ones of the hermaphrodite.) An objection to the above view should not be passed over; namely, that the abortion of the stamens in the females ought to have added through the law of compensation to the size of the corolla; and this perhaps would have occurred, had not the expenditure saved by the abortion of the stamens been directed to the female reproductive organs, so as to give to this form increased fertility.