SOME WIND-POLLINATED FLOWERS 67 



for cross pollination as are the more complex insect-pollinated 

 flowers. 



Plants that have their stamens and pistils in the same flowers 

 (perfect flowers) are said to be monoclinous, while those that have 

 these essential organs in separate flowers (imperfect flowers) are 

 diclinous. Diclinous plants are further subdivided into monoecious, 

 those which have the stamens and pistils in separate flowers on the 

 same plant, and dioecious, those that have their stamens and pistils 

 on separate plants. 



Very many, probably a majority, of wind-pollinated plants are 

 diclinous and many of these are dioecious. In the dioecious forms, 

 of course, nothing but xenogamy is possible. In monoecious forms 

 either xenogamy or geitonogamy is possible but in many cases the 

 pistillate flowers are placed higher on the plant than the staminate 

 and this makes geitonogamy very unlikely. The .likelihood of 

 geitonogamy is still further minimized in most cases by the fact that 

 the pistillate flowers on any individual plant bloom earlier, some- 

 times several days earlier, than the staminate flowers of the same 

 individual. 



When the stamens and pistils of a monoclinous flower mature at 

 different times the phenomenon is known as dichogamy. Dicho- 

 gamy is subdivided into protogyny, the maturing of the stigmas 

 before the anthers, and protandry, the maturing of the anthers before 

 the stigmas. Both protandry and protogyny are found among 

 monoclinous wind-pollinated plants and make cross pollination 

 rather certain among those plants that exhibit it. 



Still another, and even more specialized, means of insuring cross 

 pollination is found in a few anemophilous plants as, for example, 

 in rye. In this plant the pollen is impotent, that is it will not 

 germinate, on the stigma of the same flower in which it was produced 

 but will germinate readily on the stigma of any other plant of the 

 species. 



44. Some Wind-pollinated Flowers.— ^Nlany of our common trees 

 and shrubs which ])r()du(e their flowers, at least the staminate ones, 

 in catkins, and which bloom early in spring, often before the leaves 

 have de\eloped, are wind-pollinated. Among these are the poplars, 

 birches, oaks, hickories, walnuts, hazels and alders. The poplars 

 are dioecious and it is necessary, therefore, that pollen be blown from 

 one tree to another. The stamens of these plants are elastic and 

 when the weather is clear and dry the pollen is discharged forcibly 



