378 THE BIOLOGY OF FLOWERING PLANTS 



In windy situations the percentage of anemophilous 

 plants tends to increase. We have already quoted Wallace's 

 remarks about the Galapagos Islands. Knuth reckoned that 

 of the flora of Germany as a whole the anemophilous plants 

 make up 21 "5 per cent. ; in the wind-swept plains of 

 Schleswig-Holstein the figure is 27 per cent. ; in the North 

 Friesian Islands it is 36*25 per cent. ; and in the very low 

 Halligen Island it is 47 per cent. There is not here a 

 positive correlation between anemophily and wind, for 

 strong wind is not favourable to pollination, but a negative 

 correlation, due to the difficulty of life for flying insects in 

 such conditions. 



Some estimates have been made of the distance to which 

 pollen may be carried by the wind. It is said that pollen 

 of the pine may travel for over 500 miles, and " pollen rains " 

 have been observed from 30 to 40 miles out to sea. It 

 does not seem that transport to a distance is of much im- 

 portance, since wind-polHnated plants are habitually gre- 

 garious — forest trees and meadow grasses. 



V. Hydrophilous Flowers 



The number of plants with water-borne pollen is not 

 very large. In a great many aquatics the inflorescences 

 stand above the water, and the flowers are pollinated by 

 insects, as in the water lilies, water plantains, arrow-head ; 

 or by the wind, as in the bur-reeds, bulrushes, and pond- 

 weeds. This happens even with plants which are otherwise 

 completely submerged, as with the water milfoil and some 

 water buttercups. 



A famous case which stands between air-borne and 

 water-borne pollen is the much investigated Vallisneria 

 spiralis. A recent paper by Wyllie (19 17) clears up some 

 obscurities and corrects some errors of earlier descriptions. 

 The solitary female flowers are carried to the surface of the 

 water by an elongation of the peduncle, which may reach 

 a yard in length. They open, and lie, with their stigmas 

 recurved, in a little depression of the water. The male 



