DISPERSION OF POLLEN BY THE WIND, 



149 



ifcA 



-^ 



brooks, and in the height of summer raises its flower-spikes above the surface of 

 the water (see fig. 236), the large, fleshy, reddish-brown stigmas are ah-eady ripe to 

 receive the pollen at a time when the anthers close beside them are still closed. 

 The perianth-leaves of the flowers concerned are indeed still folded together, and 

 may be seen underneath the four projecting stigmatic lobes which are arranged in 

 a cross, whilst the anthers are hidden beneath the perianth. The shortly-stalked, 

 concave perianth-leaves do not open back until the stigmas have begun to wither. 

 Almost at the same instant longitudinal slits are formed down the lar^e, white 

 anthers, and they are speedily converted into gaping fissures, out of which flows 

 a copious supply of yellow pollen of mealy consistency. If a fresh, dry wind is 

 blowing at the moment of the dehiscence of the anthers part of the pollen is at once 

 carried ofl' from the spikes of the Pondweed as they project above the water; but 

 if a calm prevails a certain amount 

 of the pollen drops into the cavity 

 of the particular perianth - leaf 

 immediately below the anthers. 

 Here the pollen may remain for 

 hours together if there is no wind. 

 It is only blown away by a strong 

 puff" of wind, and is then conveyed 

 directly to other spikes projecting 

 out of the water whose flowers 

 happen to be in a much earlier 

 stage of development, the four 

 radiating stigmatic lobes being in 

 a receptive condition, but the an- 

 thers yet indehiscent and the peri- 

 anth-leaves still closed (see fig. 236). 



A still more striking instance of the temporary storage of pollen in concave 

 perianth-leaves is found in the Arrow-grass (Triglochin). Here, too, the develop- 

 ment of the stigmas precedes that of the anthers by tw^o or three days. During 

 the whole period that the brush-like stigma at the top of the ovary is sound 

 and in a receptive condition the anthers are closed, and they only open when the 

 stigmas have faded and turned brown (cf. figs. 237^ and 237^). The stamens, six 

 in number, are in two whorls of three each, situated one above the other (cf. 

 vol. i. p. 646), and underneath each stamen there is a deeply-concave perianth- 

 leaf. As soon as the anther oi^ens the pollen rolls into the receptacle thus 

 prepared beneath it, whilst in the meantime the perianth-leaf has moved a little 

 away from the axis and somewhat loosened its connection with it. The pollen 

 rests in its hollow until a puff" of wind sets the slender floral spikes swaying 

 to and fro and blows away the pollen. It is a noteworthy circumstance that 

 all six anthers of a flower do not open at once, but that first the lower whorl 

 of stamens comes into play, and that after their pollen has been carried away 



Fig. 237. —Arrow-grass (Triglochin pahtbti e). 



1 A flower with brush-like stigma already mature; all the nntheis 

 still closed. -A flower with the stigma withered whilst tlie tluee 

 inferior anthers have opened and are depositing their pollen in 

 the concave perianth-leaves at their bases. In both flowers the 

 lower front perianth-leaf has been cut 08". x 8. 



