254 



EVOLUTION OF PLANTS 



of sage (Salvia) the flower is shaped much as in La- 

 mium, but the stamens are reduced to two, and the 

 pistil does not mature until after the pollen is shed, so 

 that self-pollination is quite impossible. The anther 

 is very peculiar in form and balanced upon the short 

 filament, so that an insect striking against the lower 

 end of the elongated anther pushes the upper end, 

 with the pollen, down upon its back (Fig. 57, A). At 



Fig. 57 (Cross-fertilization). — A, a flower of a sage, Salvia pratensis, 

 allowing the way in which a bee, visiting the flower, forces down the 

 stamens so that tlie anthers, ati, strike its body ; the stigma, st, is not 

 in a position to be hit by the insect ; B, an older flower of the same ; 

 the style has elongated so that the stigma will be pollinated by a bee 

 which has already visited another flower ; the position of the undis- 

 turbed stamens is indicated by the dotted lines ; C, flower of a milk- 

 weed ( Asclepias) ; p, the cleft through which the pollinia are extracted ; 

 D, median section of the milkweed flower ; st, stigma ; p, pollen-mass, 

 orpoUinium; an, the base of the stamen; E, a pair of pollinia with- 

 drawn from the anther ; F, the flower of an orchid (Or.chix spectabilLs) ; 

 the upper perianth leaves are bent back to expose the column, or gyno- 

 stemium, gy ; I, the lip prolonged backward into the spur, sp ; o, the 

 ovary ; G, the column of F, seen from in front ; an, the anther, con- 

 sisting of the two receptacles, each containing a poUinium terminating 

 in the disk, d; st, one of the stigmatic surfaces on each side of the 

 opening of the spur ; H-J, the pollinia removed from the anther, show- 

 ing the change of position on exposure to the air. (Figs. A, B after 

 Noll.) 



