ANIMALS AND PLANTS 255 



this time the style is still short, but in older flowers 

 (B) the style elongates and bends down, so that the 

 receptive stigma (si) occupies the same position as 

 did the anther in the younger flower; and when a bee 

 enters, with its back dusted with pollen, some of this 

 is certain to adhere to the stigma. 



The milkweed family exhibits another peculiar method 

 of cross-fertilization. In the common milkweed (As- 

 clepias) the very peculiar flowers (Fig. 56, C) are 

 characterized by having the pollen in little packets 

 (poUinia) (Fig. 57, E), which are contained in closed 

 receptacles and can be dislodged only through insect 

 agency and by using considerable force. Indeed, it is 

 not unusual for the butterflies, which are the common 

 agents in pollination here, to have their tongues or legs 

 caught so firmly in the clefts through which the pollen- 

 masses are ordinarily extracted, that they are held fast, 

 and perish. The poUinia are provided with adhesive 

 disks by which they become firmly attached to the head 

 or legs of the insect, and are carried thus to other 

 flowers. 



Most remarkable of all flowers, however, are some 

 of the orchids, among which, perhaps, are found the 

 most specialized of all floral structures. The flowers of 

 some orchids are of great size and wonderful beauty, 

 and some of them exhibit most marvellous contrivances 

 for insuring cross-fertilization. One of the simpler 

 types is shown in the figure (Fig. 57, F-J), and will 

 illustrate the character of these mechanical arrange- 

 ments. As in all orchids, one of the petals is modified 

 into the " lip " {I), which is prolonged backward into 

 a long hollow spur (sp), which forms the nectary. 



