WIND POLLINATION 377 



finest dust, in the air, and consequently the extent and 

 thoroughness of their dispersal. The sufferer from hay 

 fever knows too well how ubiquitous is grass pollen in the 

 air in early summer. The pollen grains of the pine have 

 two little flotation bladders formed by the inflation of the 

 extine at the opposite ends of the grain. 



The way in which the pollen flies into a cloud, especially 

 when the plant is shaken, draws attention to the free exposure 

 of the stamens. The floral envelopes being absent, the 

 stamens stand or hang freely in the air. This may be 

 emphasised by unusual length in the filaments. The 

 attachment of the anthers to the filament is often very slight, 

 so that the faintest breath of wind sets them shaking, an 

 arrangement admirably seen in the grasses, where the attach- 

 ment to the middle of the anther makes this still less stable. 

 The same thing is achieved in a different way in the birch 

 and the hazel, where the male inflorescence, the catkin, 

 is a pendent and easily swung tassel. In the hazel the 

 pollen falls from the stamens into the slightly hollowed 

 back of the bract next below ; out of this it is shaken only 

 when the wind sets the catkin swaying ; that is, it passes 

 into the air only when conditions are such as to secure its 

 dispersal. A more advanced type of mechanism is shown 

 by those flowers already mentioned in which the stamens, 

 at some point of the flower's opening, are flung violently 

 back and the pollen explodes, as it were, into the air, e.g. in 

 the wall pellitory. 



The stigmas, like the stamens, are freely exposed. The 

 conspicuous and beautiful crimson stigmas of the hazel 

 project in groups of three from the bud-like female catkins. 

 Frequently the stigmas are long and feathery, as in the 

 grasses. 



In trees and shrubs the access of pollen is frequently 

 facilitated by early flowering, so that pollination takes place 

 before the leaves can act as a screen to catch the drifting 

 grains. The hazel, the ash, the elm, are examples. In the 

 grasses the need for this does not arise, as the foliage is less 

 obstructive and the flower spikes rise above its general level. 



