836 ECOLOGY 



dispersal and lead to premature germination. Even among the 

 submersed aquatics there are some species {e.g. many pondweeds) that 

 at anthesis develop aerial flowering shoots, which produce light pollen 

 that is not easily wetted and that is scattered by wind.^ The fact that 

 anthers dehisce chiefly when dry is of much significance in the pro- 

 tection of pollen from moisture. 



Features that favor pollen reception. — In wind-pollinated species the 

 stigmas commonly are large and conspicuously exserted (figs. 11 60, 

 1161), and sometimes they are feathery plumose (as in the grasses, figs. 

 1162, 1163), the "silk" of corn being a familiar and conspicuous 

 example of these characters. In the conifers, where there is no stigma, 

 the pollen may be caught in a drop of mucilaginous liquid exuded 

 from the ovule. 



Features that favoY cross pollination. — While wind-pollinated flowers 

 structurally are relatively simple, they are on the whole as well fitted 

 for cross pollination as are insect-pollinated species, and they exhibit 

 even many of the specialized features which are regarded as more 

 characteristic of the latter. In the first place, many and perhaps most 

 wind-pollinated plants are diclinous, and in these, of course, there can 

 be no autogamy. A large number of the diclinous forms are dioecious 

 {e.g. the poplar, ash, box elder, juniper, date palm, and meadow rue), 

 and their pollination necessarily is xenogamous (figs. 1159, 1160). 

 Among common monoecious forms are the oaks, hickories, birches, 

 alders, pines, nettles, and most of the sedges (fig. 1161); while geito- 

 nogamy as well as xenogamy might occur in such plants, the chance of 

 it is minimized in the many cases in which the pistillate flowers are 

 higher than the staminate (as in the hazel, the pine, and in many 

 sedges). Furthermore, in monoecious species the pistillate flowers of 

 a given individual blossom before the staminate, and sometimes several 

 days before, as in some alders and cattails. Even in dioecious plants 

 the pistillate flowers commonly mature before the staminate. 



In monoclinous wind-pollinated flowers (as in the grasses and plan- 

 tains, figs. 1162, 1163), cross pollination commonly is favored by the 

 consecutive maturity or dichogamy of the anthers and stigmas. In the 

 plantain (fig. 1163) the stigmas mature first, exhibiting a phenomenon 

 known as protogyny, while the earlier maturation of the anther is known 



• This phenomenon is especially striking in Myriophytlum, since the hitherto flaccid 

 and submersed main stem axis becomes at the tip rigidly erect and emersed just before 

 anthesis. 



