MOIlE MAUItlAOE CUS¥OUS. 



117 



of separate sexes. This is the coinmon airow- 

 head, a plant that grows in watery ditches, and 

 a capital example of the threefold type in its 

 simpler development. Each llower, whether 

 male or female, has a green calyx of three small 

 sepals, and a white corolla of three much larger 

 and somewhat papery petals (Fig. 20). But the 

 male flowers have in their centre an indefinite 

 number of clustering stamens ; while the female 

 flowers have an equally numerous set of tiny 

 carpels. The blossoms grow in whorls on the 



Fia. 20. — I. MALE, AND II, FEMALE FLOWERS ■ 

 OF ABBOWHEAD. 



same stem, the males above, the females beneath 

 them. At first sight you would think this a bad 

 arrangement, because you might fancy pollen 

 •from the males would certainly fall or blow out 

 upon the females beneath them. But the plant 

 prevents that catastrophe by a very simple 

 dodge, which we shall have occasion to notice 

 in many other parallel cases. The flowers open 

 from below upward ; thus the females mature 

 first, and are fertilised by insects which bring to 

 them pollen from other plants already rifled ; 

 later on the males follow suit, and their pollen 



