DISPERSION OF POLLEN BY THE WIND. 



145 



(Ulmus, see fig. 232), the Japanese Boeconia {Bocconia Japonica), the Meadow-rues 

 with erect and divergent stamens (Thalictrum aquilegifolium, T. angustifoliuvi, 

 T. flaviim, &c.) and several species of Poterium and Sanguisorba. The filaments 

 of Elms are straight at all times, but they elongate shortly before the anthers open 

 to about double their original length, and the dehiscent anthers are then displayed 

 as pairs of gaping valves. In Bocconia the anther-cavities are in the form of long 

 narrow niches, and the erect, radiating, filiform filaments vibrate under tlie slightest 

 breath of wind like the aigrettes ladies sometimes wear on their heads. In tlie 

 species of Meadow- ^ 



A 4 



r-t' 



.^ 



A 



&vK^ 



^^ 



^ 



-X^^\)l 



^ 





rue which grows 

 commonly over the 

 lower Alps (Thalic- 

 trwrn aquilegifoli- 

 um), and in the Si- 

 berian Burnet (San- 

 guisorba alpina), 

 the stamens are cla- 

 vately thickened to- 

 wards the top, and 

 are organized like 

 those of Bocconia 

 in such a manner 

 as to be easily set 

 swinging even when 

 the air is only 

 slightly stirring. 

 The various species 

 of Plantago, Thalic- 

 trum, and Ulmus 

 are also remarkable 



for the fact that the fissures formed in their anther-lobes during dry weather close 

 up quickly in the event of rain, and remain shut until the rain has ceased and the 

 atmosphere has again become dry. 



In all the cases discussed hitherto the pollen escapes from the place where 

 it originates into the air direct. But there are also a large number of plants 

 whose pollen falls in the first instance on to some spot within the province of the 

 flowers, where it is protected from becoming wet, and in which it remains for a 

 period of varying duration; nor is it blown away until conditions favourable to its 

 dispersal obtain in the environment. Very diflferent parts of a flower are utilized 

 in this manner as temporary halting-places for the pollen. In Pines and Firs the 

 backs of polliniferous scales subserve this purpose in a curious way. The upper 

 1 surface of each anther-scale in the Mountain or Dwarf Pine (Pinus Pumilio) is 

 I slightly excavated owing to the lateral edges being reflexed and the broad 



Fig. 234.— Male Flowers of Yew (Taxvs baccata). 

 Anther with closed pollen-sacs. 2 Anther with its pollen-sacs open and empty, 

 whose lower flowers are discharging their pollen. 1, 2 x 2; » x 7. 



