THE FLOWER 



237 



344 



345 



are dioecious, or "of two households.'' Draw a flowering twig 

 of oak, pine, or willow. Where are the fertile flowers situated ? 

 Notice how very much more numerous the staminate flowers 

 are than the fertile ones. Why is this necessary? (275.) 



269. Dichogamy is the name applied to a condition where 

 the stamens and 



pistils mature at 

 different times, 

 as in the evening 

 primrose, oxeye 

 daisy, and most 

 of the composite 



family. It is a FlGS 344) 345. Flower of fireweed (Epilobium an- 



VerV Common gustifolium) : 344, with mature stamens and immature 



, , . pistil ; 345, the same a few days older, with expanded 



method in nature pistil after the anthers have shed their pollen. (After 



for preventing 



self-pollination, and quite as effective as the monoecious 



arrangement, since it renders the flowers practically unisexual. 



270. Dimorphism denotes a condition in which the sta- 

 mens and pistils are of different relative lengths in different 

 flowers of the same species, the stamens being long and the 



pistils short in some, the pistils 

 long and the stamens short in 

 others. Flowers of this sort are 

 said to be dimorphous, or dimor- 

 phic, that is, of two forms ; and 

 some species are even trimor- 

 346 347 phic. having the two sets of 



FIGS. 346-347. Flower of pul- ' ,. 



monaria : 346, long styled ; 347, short Organs long, Short, and medium, 



styled - respectively, in different indi- 



viduals. Examples of dimorphic flowers are the pretty little 

 bluets (Houstonia coerulea), the partridge berry, the swamp 

 loosestrife, and the English cowslip. Of trimorphic flowers 

 we have examples in the wood sorrel and the spiked loosestrife 

 (Ly thrum salicaria) of the gardens. These flowers were a 

 great puzzle to botanists until the celebrated naturalist, 



