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 l 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, A), are borne in clusters. The general 

 structure of the flower can be understood from figure 134, 

 A and B. 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 parr of anthers is 

 1 Asclepias and Acerates. 



FIG. 133. The common milkweed 



(Asclepias syriaca) 

 Photograph by Jesse L. Smith 



