134 DISPERSION OF POLLEN BY THE WIND. 



As has been above implied, however, it is not every aerial current that is 

 adapted to serve as an agent for transferring pollen to stigmas. The least 

 favourable winds ai-e those which are combined with atmospheric deposits. 

 Besides the fact that the pollen-dust would be washed away from its resting- 

 places by the rain and carried to the ground, it must perish in consequence of 

 the soaking. Storms of wind without rain are also anything but beneficial, for 

 they forcibly whirl aw^ay any pollen that they encounter and carry it in one 

 direction only, and, as but a small proportion, if any, of the stigmas requiring to 

 be fertilized lie in the path of the wind, the greater part of the pollen is wasted. 



The result aimed at is best achieved when the pollen-dust, after being 

 removed from the spot where it has been produced or deposited, is distributed 

 uniformly over an ever-extending area, becoming, in a manner of speaking, diluted 

 and forming a cloud of gradually increasing dimensions but diminishing density, 

 so that the thousands of loose pollen-cells which have up to that time been 

 crowded together within the province of the flower and contained in a space about 

 the size of a pin's head are scattered over an area many million times as great. 

 A gradual dispersion of the kind is only occasioned by a gentle wind. The 

 light breezes which sweep through valleys shortly after sunrise, ascending air- 

 currents such as one perceives quivering over heated plains at noon, the alter- 

 nating land and sea breezes of the coast-winds which, in passing over cornfields, 

 set the corn in gentle waving motion, and in woods cause a scarcely audible 

 rustle — such are the most propitious agents of pollination. It is easy to observe 

 how, at the proper season, under the influence of a gentle wind of the kind one 

 little cloud of dust after another detaches itself from the flowers of the plants in 

 question and slowly soars away. Owing to the fact that the motion of aerial 

 currents is undulatory and undergoes at short intervals alternate augmentation 

 and diminution, the first motion of the pollen as it dissipates itself is also in 

 waves; but the little cloud is soon withdrawn from observation as it proceeds 

 on its way, and the only thing we can clearly discern is that p)ollen, like dust 

 raised on a road, ascends in an oblique direction. 



The form and distribution of the stigmas to be covered with dust-pollen are 

 also in harmony with these conditions. Most plants, whose pollen is in the 

 form of dust, and transported entirely by currents of air, have dioecious or 

 moncecious flowers, and those which develop hermaphrodite flowers exhibit 

 complete dichogamy, that is to say, the androecium and gynoeeium ripen at 

 difierent times, so that when mature pollen is being discharged from the anthers 

 of a flower the stigmas of the same flower are already withered, and therefore 

 no longer in a condition to receive the pollen-cells, or else they are still so 

 immatui'e that they cannot be covered with pollen. Any possibility of the 

 transference of pollen from the anthers to the stigmas situated close to them in 

 the same flower being attended with success is as effectually excluded in dicho- 

 gamous plants as it is in monoecious and dioecious species, and the pollen has to 

 be blown to other flowers in the neighbourhood whose stigmas happen to be in 



