390 THE BIOLOGY OF FLOWERING PLANTS 



Only on the closed stigma does the pollen germinate unless 

 the air is very moist. 



Sensitive stamens are more common. A familiar 

 example is afforded by the barberries. A touch on the inside 

 of the filament causes it to move towards the ovary. In 

 many Compositae (Small, 19 17), and particularly in the 

 Cynareae, a rapid contraction of the filaments, by as much as 

 20 per cent., draws the staminal tube down over the style, 

 and brushes out the pollen. In the Cistaceae the bunches 

 of stamens move outwards when the filaments are bent. 

 In all cases the movements are reversed after a short rest 

 period. The use of the contraction of the filaments in the 

 Compositae is evident. As the same result is achieved by 

 the majority of species by the growth of the style, and 

 without sensitive filaments, it cannot be said that the 

 contractile filament adds much to the efficiency of the floral 

 mechanism. 



The movement of the barberry stamens is described by 

 Knuth as ensuring that the visiting insect, which stimulates 

 the filament, shall receive pollen. As the larger insects, 

 such as bees, to which alone the explanation could apply, 

 would certainly receive pollen without any movement, the 

 advantage is again questionable. Still more doubtful is the 

 case of the Cistaceae. We may take as an example Cisliis 

 salvifolius, recently described in detail by Knoll (19 146). 

 The numerous stamens surround the stigma and pollen falls 

 from them on it. The stamens, if vigorously bent inwards, 

 move, after the lapse of about a second, rapidly outwards 

 till they lie against the petals ; after a few minutes they 

 resume their former position. Knoll maintains that by 

 this means the flower presents alternately " male " and 

 " female " conditions to the visiting insect ; in the former 

 the visitor is more likely to receive, in the latter to deposit, 

 pollen. It is not proved that the chances of cross-polhna- 

 tion by a large insect rambling about the flower are materially 

 improved (Fig. 60). 



