THE AXGIOSPERMAE 1179 



as we shall show later, or they may have disappeared without trace. The 

 latter condition is sometimes constant in a whole genus, as in Veronica, 

 where only two stamens are present of the five which make up the full 

 complement in Scrophulariaceae, or in Centranthus, where only one out of 

 five is present. It may, however, affect only certain individuals in a species, 

 as in Glechoma hederacea, where some plants have hermaphrodite flowers and 

 others have carpellate flowers only, a condition known as gynodioecism. 



The suppression of stamens is indeed a tendency which we can see in 

 various stages of fulfilment. Its beginning may, perhaps, be traced in 

 contabescent or impotent stamens which, though normal in appearance, 

 produce nothing but abortive pollen. 



Where a parallel tendency to the reduction of the gynoecium is also 

 operative, the extreme condition of dicliny may be reached, where all the 

 flowers are either only staminate or only carpellate, or even dioecism, 

 where the sexes are segregated into different plants. 



The ideal in floral efliciency would be an equivalence between the num- 

 ber of pollen grains produced and the number ®f ovules to be fertilized and 

 the complete elimination of pollen wastage. While this ideal is nowhere 

 completely realized, we can see, by comparing advanced and primitive 

 types of flowers, that progress towards it has been one of the keynotes in 

 floral evolution. 



The question whether the unisexual condition in the flower is a 

 primitive feature in Angiosperms, or a secondary condition, is one that has 

 been differently answered by systematists. Putting on one side however, 

 for the present, all arguments based on the supposed ancestry or evolution 

 of the Angiosperms, we can only conclude that the many evidences of the 

 transformation of hermaphrodite into unisexual flowers show that this is a 

 real event, while the opposite transformation remains purely hypothetical. 

 An important part of the evidence to which we refer consists in the traces 

 of organs of the opposite sex found in many functionally unisexual flowers. 

 Examples of the persistence of rudimentary stamens in female flowers are 

 fairly common ; for instance in Ilex, Akebia {Fig. 1 1 52), Pilea, Liquidambar and 

 Crozophora among Dicotyledons and in Freycinetia and Geonoma among 

 Monocotyledons.* Even where no visible rudiment of the stamens persists 

 the space for them on the receptacle may be present, but blank, as in 

 Menispermmti. The opposite condition, namely the persistence of abortive 

 carpels in male flowers, is less common, for, the carpels being situated 

 apically, it seems to be relatively easier to cut short the growth of the 

 receptacle and so eliminate the gynoecium altogether. Nevertheless 

 examples occur, as in Ilex, Akebia and Freycinetia among those mentioned 

 above, while in Crozophora, the three carpels are replaced in the male flower 

 by three large, additional stamens. What is " normal " in this genus has also 

 been found as an abnormality in Oakesia (Liliaceae), the styles of the 

 abortive gynoecium forming perfect anthers. 



* Stamen-like structures are induced to appear in female flowers of MeJandrium dioicum, 

 where they are usually absent, by the attacks of Ustilago nntherarum. 



