250 



EVOLUTION OF PLANTS 



While at first sight it would seem that flowers having 

 stamens and pistil together would usually be self -pol- 

 linated, such is rarely the case, at least in showy flowers. 

 An examination of these reveals many effective arrange- 

 ments by which this is prevented and cross-fertilization 

 made necessary. One of the commonest and simplest 

 means is the maturing of the stamens and pistil at dif- 

 ferent times. Usually it is the stamens which are ripe 



FIG. 56 (Cross-fertilization). A, flower of Erodium, one of the Geranium 

 family ; the flower is inconspicuous and capable of self-pollination ; B, 

 stamens and carpels of Erodium ; the stigmas, st, are mature when the 

 pollen is shed ; C, young flower of Pelargonium ; the pistil, p, is im- 

 mature ; D, the pistil of an older flower which has shed the anthers ; 

 the stigmatic lobes, st, are now ready for pollination ; E, young flower 

 of a nasturtium (Tropoeolum) ; three of the stamens are shedding their 

 pollen and occupy the space in front of the opening of the spur ; the 

 other stamens are still closed, and with the immature pistil, st, are 

 bent down ; F, stamens and pistil of an old flower ; the stamens have 

 all shed their pollen, and the receptive stigma, st, now occupies the 

 position in front of the opening of the spur ; G, flower of broom 

 (Sarothamnus) ; the stamens and pistil are included within the keel, 

 k; H, a flower which has had the keel forced down, liberating the 

 stamens and pistil. 



first (proterandry), but proterogyry, or the earlier 

 maturing of the pistil, may also occur, e.g, the common 



