148 



DISPERSION OF POLLEN BY THE WIND. 



as soon as a wind sets them in motion the pollen is discharged through the gaps 



in little clouds. 



A temporary deposition of the pollen on the backs of the flowers is common to 

 all the numerous trees and shrubs which have their male flowers aggregated in 

 pendent catkins or spikes resembling tassels or fringes in appearance, as, for 

 instance, the Hazel (Corylus, see fig. 235), the Alder (Alnus, see fig. 228), the 

 Walnut (Juglans, see vol. i. p. 742), Birches, Poplars, and Hornbeams. The floral 

 spikes of all these plants are erect at first, and in the form of short, thick cones 



Fig. 236. Curled Pondweed Potamogeton crispua) in the act of pollination. 



and cylinders. A short time before the anthers burst the axis of the spike 

 elongates and becomes pendent, causing all the flowers seated upon it to assume an 

 inverted position with their originally upper faces turned to the ground and their 

 backs upwards. The back of each flower is so contrived as to catch the pollen 

 falling from the anthers of the flowers above it, and retain it until the tassels are 

 set swinging by a gust of wind, and the pollen is in consequence dissipated 

 (cf. vol. i. p. 741). 



Sometimes the hollow upper surfaces of sepals, petals, or bracts serve as 

 landing-stages for the pollen when it is discharged. This is the case, for example, 

 in various species of the Pondweed genus (Potamogeton), in the Arrow-grass 

 (Triglochin), and the Sea -Buckthorn (Hippophae). In the Curled Pondweed 

 (Potamogeton crispus), a plant which lives submerged in ponds and slow running 



