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 (st) 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 
(pollinia) (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 pollinia 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” (2), which is prolonged backward into 
a long hollow spur (sp), which forms the nectary. 
