REPRODUCTION BY SEED 177 



flower is said to be protandrous (Gr. protos, first ; andros, 

 male) ; when the pistil, protogynous (Gr. gyne, female). 

 The flowers, in these cases, may be regarded biologically 

 as unisexual, being male when the stamens are ripe, and 

 female when the pistils. The two periods, however, 

 generally overlap, and during this time self-pollination 

 is possible. In most flowers the stamens ripen first, but 

 a few are protogynous e.g., plantain, figwort, Christmas- 

 rose, most grasses, and all fly-traps. The pellitory 

 (Parietaria offlcinalis) exhibits very pronounced pro- 

 togyny. The style protrudes from the flower before the 

 bud opens, and by the time the stamens are ripe the 

 stigmas have fallen off. 



3. The Mechanical Structure of the flower is such that 

 the pollen cannot, naturally, get deposited upon the 

 stigmas of the same flower -e.g., orchid, iris. These 

 flowers, therefore, must be cross-pollinated, and if they 

 are entomophilous, the insects touch and pollinate the 

 stigmas on entering the flower, and on leaving it move 

 the floral parts in such a way that the pollen they bear 

 away is not deposited on the stigmas. 



4. Dimorphic and Trimorphic Flowers. The primrose 

 is dimorphic i.e., it has two kinds of flowers (Gr. di, 

 two ; morphe, form). In one the anthers are situated 

 upon the corolla-tube at a higher level than the stignla ; 

 in the other the positions are reversed. Charles Darwin 

 showed that effective fertilization is only brought about 

 when the stigma of a long-styled or pin-eyed form is 

 pollinated by pollen from the stamens of the short-styled 

 or thrum-eyed flower, and vice versa that is to say, 

 pollination with the best results takes place between 

 organs at the same level. In the purple loosestrife 

 (Ly 'thrum Salicaria), a marsh and river-side plant, three 

 kinds of flowers occur, three levels being interchangeable 

 among two whorls of stamens and the stigma. Here, 

 again, pollination between organs at the same level is 

 best, and these must be in different flowers. 



In all bisexual flowers, however efficient the devices 

 may be to prevent self-pollination, it is always possible 

 for it to occur. In some cases it regularly takes place 

 if cross-pollination fails. The flowers of the Composite 

 family, for example, always set a full head of seeds, and 

 this could not happen if self-pollination were impossible. 



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