THE FLOWER AND INSECT-POLLINATION 161 



that self-pollination must be very unlikely. The nectar is 

 in a little pit at the bottom of the pocket. As the insect 

 crowds its way into the nar- 

 rowing pocket, its body is 

 dusted by the pollen ; and 

 when it visits the next flower, 

 and pushes aside the stigmatic 

 shelf, it is likely to deposit 

 upon it some of the pollen 

 obtained from a previously 

 visited flower. 



The orchids are most re- 

 markable in their arrange- 

 ment for insect-pollination. 

 In fact, each kind of orchid 

 is usually so adjusted to some 

 particular kind of insect that 

 no other insect can secure the 

 nectar or carry off the pollen. 

 There are two pollen sacs, and 

 the pollen grains cling to- 

 gether in a mass, which is 

 pulled out of the sac bodily. 

 A common arrangement is as 

 follows (Fig. 128). Each of 

 the elongated pollen masses 

 terminates below in a stalk 

 that ends in a sticky disk or 

 button, and between these two 

 buttons there extends the 

 concave stigmatic surface, at 

 the bottom of which is the 

 opening into the long tube-like spur containing the nectar. 

 Such a flower is adjusted to the visits of a large moth, with 

 a long sucking-tube (" proboscis ") that can reach the bottom 



FIG. 128. Flower of an orchid (Habe- 

 naria) : A, complete flower, showing 

 three broad sepals, three narrower 

 petals (the lowest one forming the 

 long lip and the much longer spur 

 which extends to the bottom of the 

 figure), two pollen-sacs, between which 

 extends the concave stigmatic surface 

 (at the bottom of which the opening 

 to the spur is seen) ; B, more enlarged 

 view of pollen-sacs with their sticky 

 buttons and the stigmatic surface 

 stretching between ; C, a pollen-mass 

 removed ; D, a button enlarged. 

 After GRAY. 



