DISPERSION OF POLLEN BY THE WIND. 151 



almost impossible. The way to the stigmas would be stopped by innumerable 

 barriers, and the pollen would inevitably be deposited upon these obstacles and 

 stranded. 



-^' As regards the stigmas, we find that in plants with dusty pollen they are 

 invariably fashioned so as to catch the dust. In one case they are fleshy and 

 swollen and have the surfaces which are exposed to the wind covered with a 

 velvety coating (see fig. 236), in another they are in the form of tufts of long 

 papillose or capillary filaments, as, for instance, in the Paper Mulberry -tree 

 (c/. figs. 229^ and 229*, p. 137); sometimes they assume the shape of delicate 

 feathers (c/. fig. 231, p. 139), sometimes of camel's -hair pencils and brushes 

 (fig. 237). At the time when pollination takes place they are always fully 

 exposed to the wind and so placed that when the pollen-cells are blown against 

 them they are caught like midges in a spider's web. Yet, in spite of all these 

 contrivances, it would remain very doubtful whether the stigmas would be 

 dusted with pollen through the action of wind were it not for the concurrence 

 of another circumstance. The wind is but an uncertain means of transport, 

 1 especially in the case of a passive object incapable of exercising any influence 

 / on the selection of a route. It is, therefore, important that the pollen should 

 be disseminated broadcast in as thorough a manner as possible, and this is only 

 possible if the number of pollen-cells is excessively large. Supposing that only 

 two thousand pollen-cells were produced in a Nettle-inflorescence and these 

 were surrendered to be the sport of the wind, it would be only by a lucky 

 chance that a single one of these cells would be caught by the stigmas of a 

 plant at a distance of 5 metres; but, inasmuch as the number of the cells 

 , constituting the pollen-dust of a Nettle amounts to millions, the probability 

 1 of successful pollination is increased to a proportionate extent. If the stami- 

 niferous flowers of Conifers, Hazels, Birches, Hemp, or Nettles be picked before 

 the dehiscence of their anthers and placed on a suitable substratum until the 

 anthers open, the mass of pollen-dust which is liberated is quite astonishing. 

 It seems scarcely credible that so large a quantity of pollen could have been 

 developed in anthers which are themselves so small, and the apparent 

 anomaly only becomes intelligible when one remembers that the cells were 

 packed closely together in the anthers, but afterwards lie simply in a loose 

 heap. In years peculiarly favourable to the flowering of Conifers vast clouds of 

 poUen are borne on gentle winds through the Pine-forests, and are often swept 

 right beyond them, so that not only the female flowers, needles, and branches 

 of the trees in question are powdered over with the yellow pollen, but also the 

 leaves of adjoining trees and even the grasses and herbs of the meadows 

 around. In the event of a thunder-shower at such a period the pollen may be 

 washed off the plants and run together by the water as it flows over the 

 ground, and then, after the water has run off", streaks and patches of a yellow 

 powder are left behind on the earth, a phenomenon which has given rise on 

 various occasions to the statement that a fall of sulphurous rain has taken place. 



