3o8 



FLOWERS 



flowers and 



Fig. 

 of 



129. — Pollen grain 

 pine showing the 

 wings which aid in its 

 journey through the air. 



of many trees; poplars, oaks, birches, and pines are ex- 

 amples. In most wind-pollinated trees the flowers appear 

 before the leaves, another feature 

 which is evidently favorable to the 

 process. 



Many monoclinous 

 nearly all diclinous 

 ones are wind-pol- 

 linated ; insect-pol- 

 Unation, with a few 



exceptions, is confined to monoclinous 



flowers. (Why is monocliny more favor- 

 able to insect-pollination than dicliny is ?) 



As you have noted, close-polKnation is 



a physical impossibihty for diclinous 



flowers ; for them only geitonogamy and 



xenogamy are possible. Thus it is inter- 

 esting to note that many wind-pollinated 



plants have structures and habits which 



favor xenogamy and make geitonogamy 



difficult or impossible. Poplar, ash, box 



elder, juniper, and meadow rue insure 



xenogamy by being dioecious, while in the 



hazel (see Figure 104) and in the pine 



xenogamy is favored by the fact that the 



pistillate flowers are borne higher up than 



the staminate ones. Also in monoecious 



forms, the pistillate flowers usually ripen 



before the staminate ones of the same 



individual. In alders and in cat-tails 



this difference in the time of flower ripening may amount 



to several days. 



Fig. 130. — Part of a 

 panicle of a meadow 

 grass. Two of the 

 lower flowers have 

 opened. Note that 

 each of these has 

 two plumose stig- 

 mas and three sta- 

 mens whose long 

 and slender fila- 

 ments expose the 

 anthers to the wind. 



