THE MILKWEED FAMILY. 231 



sentatives occur in the temperate regions of both hemi- 

 spheres. They are chiefly interesting for the extraordinary 

 structural modifications of their flowers, which " rival the 

 orchids, if not in the variety of their forms, at least in 

 their complexity and their perfect adaptation to insect 

 visitors." A study of the steps by which this gradually 

 increasing complexity of structure has been attained is 

 of the highest interest. The student should carefully com- 

 pare the flowers of other genera of Asclepiadacese, and 

 such representatives of related families as Apocynum, 

 Vinca, and others. 



SPECIAL STUDIES. 



I. It is found that only a very small proportion of the 

 flowers in an umbel set fruits. Why is this ? and 

 are those flowers which do not set fruits of any 

 value to the plant ? 

 II. Minute structure of poUinia and retinacula. 



III. Morphology of the cucuUi. 



IV. Development of the flower. 



V. Protective appliances in this family. 



