REPRODUCTION AND DISPERSAL 



851 



are exposed so conspicuously and the pollen is so abundant that visiting 



insects scarcely can avoid getting more or less pollen on their bodies, 



even if they are searching only for nectar; of course, much pollen must 



adhere to all pollen-gathering insects. In the composites the dense 



massing of flowers into heads greatly facilitates pollen removal, since the 



visiting insects necessarily crawl over numerous flowers with their ex- 



serted stamens. In many flowers, especially in 



those which are zygomorphic or which contain 



but little pollen, there often are specialized features 



that facilitate pollen removal. Certain parts of 



the body (chiefly about the head) may receive the 



pollen somewhat locally; in certain flowers exserted 



stamens often are grasped by the alighting insect in 



such a way that the under parts of the body receive 



the pollen. 



a 



Some flowers with introrse anthers (i.e. opening in- 

 wards), as in the gentians, have nectar to the interior of 

 the stamens, while some flowers with extrorse anthers (i.e. 

 opening outwards), as in 7m, have nectar to the exterior of 

 the stamens. Often the stamens grow rapidly just before 

 dehiscence (as in Parnassid), assuming a position corre- 

 sponding to that of the stigma. In a number of instances 

 the insect occasions the release of the pollen, as in the 

 legumes, where the alighting of a bee causes the anthers to 

 protrude suddenly from the enclosing petal and to sprinkle 

 pollen over the visitor. In Pyrola and Kalmia the anthers 

 are held in unstable equilibrium, and the sudden release 

 coming with the insect visit causes the pollen to be shaken 

 out. In various ericads with pendulous flowers the sta- 

 mens have appendages, which are likely to be struck by 

 visiting insects in such a way as to result in the scattering 

 of the pollen. Sensitive mechanisms occur also in Lopezia, 

 where a petal-like structure holds the single stamen in un- 

 stable equilibrium, in Berberis, where the stamen itself is 

 sensitive to contact (fig. 1176), in Galeopsis, where contact 

 causes the anther lids to fly open, and in Crucianella, 

 where the style is held in unstable equilibrium until the 

 flower is touched, whereupon the style is suddenly released, bringing out with it a 

 shower of pollen. In Salvia, in which there is a swinging anther, an entering bee so 

 presses against the lower arm of the lever as to dust himself with pollen from the 

 upper arm (figs. 11661169). In orchids the pollen masses (pollinia) have an ex- 

 posed adhesive disk, which sticks to the head parts of a visiting insect. The some- 

 what similar pollinia of milkweeds have clips that fasten about the feet of the insect. 



FIG. 1176. A bar- 

 berry flower (Berberis 

 Thunbergii) with the 

 calyx and corolla re- 

 moved, so as to show 

 the pistil (p) and the 

 hypogynous stamens, 

 which at maturity lie 

 back upon the inner 

 surface of the petals; 

 when an insect comes 

 in contact with the base 

 (b) of the filament (/), 

 the latter flies forward, 

 assuming the position 

 of the stamen at the 

 right, and pollen is 

 dusted on the insect and 

 on the stigma (g) ; note 

 that the filaments (/) 

 broaden toward the 

 apex, and that the an- 

 ther valves (a) open 

 upwards, being hinged 

 at the filament apex. 



