REPRODUCTION AND DISPERSAL 



837 



as protandry; both protandry 

 and protogyny are seen in 

 maize. The most special- 

 ized means of preventing 

 close pollination, namely, 

 that in which the pollen is 

 impotent on the stigma of 

 the same (lower, is illustrated 

 in rye, though in wheat and 

 barley, and probably in most 

 monoclinous species, close 

 pollination is not necessarily 

 excluded. 



Miscellaneous features of -wind- 

 pollinated flowers. Wind-polli- 

 nated flowers usually contrast with 

 those that are insect-pollinated 

 in their lack of showiness, odor, 

 and nectar, though some of them 

 are conspicuously colored (as in 

 the cottonwood and field sorrel). 

 The perianth mostly is inconspicu- 

 ous (either through its greenish 

 or brownish color or its small size) 

 and often it is absent; when pres- 

 ent, it consists commonly of a 

 calyx, the corolla being rarely in 

 evidence. None of these features 

 would occasion comment, but for 

 the corresponding presence of 

 showiness, odor, and nectar in 

 insect-pollinated flowers and for 

 the consequent assumption that 

 in the latter these features prob- 

 ably are advantageous. The dis- 

 tribution of species with wind-pol- 

 linated flowers has been thought 

 to differ somewhat from that of 

 other seed plants. For example, 

 the percentage of the former is 

 greater in windy habitats than 

 elsewhere (as on small islands and 

 along shores), while the flowers 



b c f 



* u-VW7~ / 



FIG. 1165. Pollination in the tape grass 

 (Vallisneria spiralis); s, staminate plant; p, pis-, 

 dilate plant; the staminate flowers are borne in a 

 spike (&) ; upon detachment they rise (a) to the 

 surface, open out (b), and float on the water; the 

 pistillate flowers (/) are borne in spathes (d) on 

 long scapes (e), just reaching the water surface, 

 where the floating staminate flowers may come in 

 contact with them (c) ; note also the vertical ribbon- 

 like leaves (/) and the stolon (r), o representing a 

 new potential plant or offset. After KERNER. 



