FLOWERS AND INSECTS. 



127 



usually two pollen-sacs, and stretched between them is the 

 stigmatic surface. In this case, however, the pollen grains 

 are not dry and powdery, but cling together in a mass, and 

 cannot escape from the sac without being pulled out (see 

 Fig. 133). The same sort of pollen is developed by the 

 milkweeds. 



(2) Consecutive maturity. — In these cases the pollen and 



Fig. 133. A flower of an orchid (Habenaria). At 1 the complete flower is shown, 

 with three sepals behind, and three petals in front, the lowest one of which has 

 developed a long strap-shaped portion, and a still longer spur portion, the opening 

 to which is seen at the base of the strap. At the bottom of this long spur is the 

 nectar, which is reached by the long proboscis of a moth. The two pollen sacs of 

 the single stamen are seen in the centre of the flower, diverging downwards, and 

 between them stretches the stigma surface. The relation between pollen sacs and 

 Btigma surface is more clearly shown in 2. Within each pollen sac is a mass of 

 sticky pollen, ending below in a sticky disk, which may be seen in 1 and 'J. When 

 the moth thrusts his proboscis into the nectar tube, his head is againsl the Btig- 

 matic surface and also againsl the disks. When be removes his head the disks 

 stick fast and the pollen masses are dragged out. Tn :} a pollen mass (a) is 

 shown sticking to each eye of a moth. Upon visiting another Sower these pollen 

 masses are thrust against the stigmatic surface and pollination is effected.— After 

 Graf. 



