REPRODUCTION AND DISPERSAL 



835 



In many species with wind pollination, and especially in those without 

 catkins that move readily in the wind, the stamens have long and slender 

 filaments, which so expose the anthers that they are shaken in the 

 gentlest air movements (as in the grasses and the box elder, figs. 1159, 

 1162, 1163). In the nettles the pollen is discharged 

 into the air by a sudden move- 

 ment of the filaments. In many 

 plants pollen that falls in quiet 

 weather accumulates in pockets 

 of one sort or another, whence 

 it is scattered readily by the 

 first breeze. In most wind- 

 pollinated species (not, how- 

 ever, in most grasses and 

 sedges) the pollen is produced 

 in great abundance; this is a 

 matter of much advantage in 

 view of the great waste. The 

 abundance of pine pollen re- 

 sults sometimes in the so-called 

 sulfur showers, and the abun- 

 dance of ragweed pollen in the 

 air is thought to be a factor in 

 causing hay fever. 



Wind-scattered pollen com- 

 monly is smooth, light, and dry, and hence easily 

 blown about (fig. 

 1161), and in the 

 pines, dispersal is 

 facilitated further 

 by the presence 

 of a wing on each 

 side of the grain 

 (fig. 1164). In 

 wind-pollinated 

 species the pollen 

 grains are not 



easily wetted, thus further resembling the spores of fungi and ferns; 

 this is highly advantageous, since moistening might prevent wind 



FIG. 1162. A 



panicle branch of the 

 meadow fescue (Fcs- 

 tuca elatior), a plant 

 with monoclinous 

 wind-pollinated flow- 

 ers ; note the unopened 

 spikelets above with 

 their imbricated 

 scales; below to the 

 right is a spikelet in 

 which two of the lower 

 flowers have opened, 

 each disclosing two 

 plumose stigmas and 

 three stamens whose 

 long and slender fila- 

 ments expose the an- 

 thers to the wind. 



FIG. 1163. The 

 upper part of a plan- 

 tain spike (Plantago), 

 illustrating protogyny 

 in monoclinous wind- 

 pollinated flowers; 

 note that the conspic- 

 uous plumose stigmas 

 (g) appear before the 

 stamens are evident; 

 in the older flowers 

 note the long and 

 slender filaments (/) 

 and the triangular an- 

 thers (a); c, calyx; cf t 

 corolla. 



FIG. 1164. A pollen grain of a 

 pine (Pinus), showing the two wings 

 which aid in its dispersal by wind; 

 highly magnified. From COULTER 

 and CHAMBERLAIN. 



