DISPERSION OF POLLEN BY THE WIND. 151 



almost impossible. The way to the stii^iiias 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 ileshy 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 



(cf. figs. 229' and 229 ^ p. 137); sometimes they assume the shape of delicate 



feathers (cf. 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, 



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 



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 libei-ated is quite astonishing. 



It seems scarcely credible that so large a quantity of pollen could ha\e 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 



pollen 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 bi-anches 



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. 



