834 



ECOLOGY 



follows the inner wall, and it may pursue a tortuous course, or it may 

 grow directly toward a micropyle; pollen tubes have been shown to 

 exhibit prochemotropic reactions toward certain carbohydrates and pro- 

 teins, including those that are secreted by stigmas. 



Wind pollination. — Features that favor the scattering of pollen. — 

 The simplest form of pollination and the one most closely related 



to spore dispersal in the lower 

 plants is wind pollination,^ 

 and wind-pollinated plants 

 have many features which re- 

 semble those of the fungi, 

 bryophytes, and pteridophytes 

 rather more than they do 

 those of the insect-pollinated 

 seed plants. In many cases 

 the staminate flowers are 

 arranged in catkins, which 

 usually are slender, pendu- 

 lous inflorescences that yield 

 gracefully to breezes (fig. 

 1161). Catkins suited for 

 wind pollination are especially 

 characteristic of many trees 

 and shrubs (notably the pop- 

 lars, oaks, birches, and other 

 Amentiferae, and also most of 

 the conifers), which perhaps 

 is advantageous in view of 

 the relative exposure of such 

 plants to wind; in most of 

 these plants, also, the flowers 

 develop before the leaves, 

 thus further facilitating ex- 



FlG. 1 161. — A flowering twig of a hazel 

 {Corylus americana), a slirub which has monoe- 

 cious wind-pollinated flowers; note that the stam- 

 inate flowers are lowermost and are in catkins 

 (c) which sway in the breeze, the pollen grains 

 (/>) often appearing in clouds; s, scale leaves which 

 protect the flower buds in winter; the pistillate 

 flowers develop from scaly buds (6), and at anthe- 

 sis the stigmas (f ) are exserted. 



posure to wind. The pistil- 

 late flowers sometimes are in catkins (as in poplars and birches), but 

 often they are not (as in oaks and hickories) ; such arrangement, appar- 

 ently, is of no particular advantage. 



' Species with wind pollination often are called aneviophilous, a term that should be 

 discarded, together with other humanistic words as applied to plants. 



