DISPERSION OF POLLEN BY THE WIND. 149 



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 large, 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 ofi" from the spikes of the Pondweed as they project above the water; but 

 if a calm prevails a certain amount i i- 



v\.V.I,l/ll/ 



of the pollen drops into the cavity 



of the particular perianth -leaf 



immediately below the anthers. , '* S' 



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 |^ tj' 



stage of development, the four Fig. 237. -Arrow-grass (rnpZocAui paZ«stre). 



radiating stigmatic lobes being in 1 a flower with brush-like stigma already mature ; all the anthers 



,. f,- r, i. i.r still closed. 2 A flower with the stigma withered whilst the three 



a receptive condition, but the an- ^^^^^^^^ ^^^-^^^^ have opened and are depositing their poUeu in 



thers yet indehiscent and the peri- ^^e concave perianth-leav-es at their bases. lu both flowers the 



•^ ^ lower front penanth-leaf has been cut off. x 8. 



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 two 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 (c/. figs. 237^ and 237 2). 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 opens 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 



