148 INTRODUCTION TO BOTANY 
then reach the mature pistillate flowers which they pollinate. 
On attempting to fly out of the flower chamber they find it 
impossible to get through the fringe of hairs. After a time 
(often a few days) the stigmas wither, and in place of each a 
drop of nectar appears, on which the insects feed. At length 
the staminate flowers mature and allow a considerable quan- 
tity of pollen to fall to the 
bottom of the chamber. 
The insects crawl about 
in this, become thoroughly 
dusted with it, and finally, 
as the palisade hairs wither 
and droop, escape and fly 
away to another blossoming 
arum plant, and cross-polli- 
nate its flowers in turn. The 
number of insect visitors to 
a single flower cluster is 
enormous, about four thou- 
sand midges being found 
in one flower chamber. 
140. The milkweed; a 
pinch-trap flower. The milk- 
weeds! are admirable in- 
stances of what are called 
pinch-trap flowers. There 
are more than twenty kinds of milkweed in the central and 
northeastern states, the commonest in many portions of the 
country being the one shown in figure 133. The flowers, of 
peculiar form (fig. 134, 4), are borne in clusters. The general 
structure of the flower can be understood from figure 134, 
A and &. The detail of its structure that is of most inter- 
est in the study of modes of pollination is the way in which 
the pollen is borne. Each of the five anthers produces two 
rather large pollen masses. Between each pair of anthers is 
Fig, 188. The common milkweed 
(Asclepias syriaca) 
Photograph by Jesse L. Smith 
1 Asclepias and <Accrates. 
