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, 1 

 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 calkins, 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- 

 I " 



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- 



FiG. 1161. A flowering twig of a hazel 

 (Corylus americana), a shrub 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 

 (p) 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 (g) 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. 



1 Species with wind pollination often are called anemophilous, a term that should be 

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



