DISPERSION OF POLLEN BY THE WIND. 



139 



seen to be still tightly closed (230^ and 230 -). The latter do not open till two 

 or three — often even as much as four — days later, and only then in the event 

 of the air being dry. Dehiscence is accomplished by longitudinal Assuring of the 

 anther-lobes. The edges of these fissures contract very speedily, so that each of 

 the pair of anther-lobes is converted into an open recess wherein the pollen lies 

 in the form of a floury or powdery mass (230^). Just before dehiscence the 



Fig. 231. — Avena elatior. 



I ' A closed anther, a An open anther. ^ Spikelets on a calm day with glumes distended and anthers pendulous. * Spikelets 

 ! in a wind. The pollen escaping from the pendulous anthers in the spikelets to the riglit ; in that to the left (and below> 

 I the anthers (two only remaining) have shed their pollen ; in a third flower (in the same spikelet as the last-mentioned) 

 I the anthers are still closed and in process of being exserted. i, 2 x 12 ; s, * x 5. 



I 



janthers place themselves in such a position as to ensure the fissure being turned 

 ■upwards, so that the recesses full of pollen are not emptied so long as tlie air 

 lis still. It is only when the flowering branches begin to sway to and fro that 

 ithe pollen falls out of the loculi and is blown away in the form of a cloud of 



dust by the same breeze as set the boughs in motion. 



In another group of plants the anthers are borne on long filaments, and are 



set oscillating and vibrating by the least breath of wind, the pollen being in 



