DISPERSION OF POLLEN BY THE WIND. 



135 



the receptive stage of development. In all these dichogamous plants the flowei's 

 with stigmas in the receptive condition are situated higher than the anthers 

 from whicii mature pollen is being committed to the wind. If you look at any 

 of the species of Plantain (Plantago) a few days after they have begun to 

 flower, you find that only the styles with their stigmas ready to receive the 

 pollen project from the uppermost flowers in each spike, whilst the flowers from 

 which pollen is being shaken by the wind occupy the lower parts of the spike. 



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Fig. 228. — The common Alder (Alntis gtutinosa). 



Branch with flowers that open before the leaves are unfolded ; the male flowers grouped in the foi'm of pendent catkins, 

 and above them the female flowers grouped iu the form of little spikes. 2 Leafy branch at the top of which are the 

 rudimentary inflorescences for the following spring. 



In these lower flowers the stigmas are already withered, in the upper ones the 

 anthers are still closed. Therefore, in order to reach the receptive stigmas, the 

 pollen must travel upwards. The same conditions are found in most species of 

 Sorrel (RuTHiex), in the Wall-Pellitory (Parietaria), in Saltwort (Salsola), in 

 Arrow-grass (Triglochin), and in Pondweeds (Potamogeton), and many other 

 plants with hermaphrodite but perfectly dichogamous flowers (c/. figs. 236 and 

 237). 



This phenomenon is still more strikingly exhibited by moncecious plants, 

 i.e. where male and female flowers occur on the same individual. In the Oak, 

 the Beech, the Alder, &c., the catkins of mature polliuiferous flowers hang down 



