v] THE HORNED PONDWEED 71 



the flowers are distant, and when mature they are lifted, one by 

 one, a little above the water-surface. In other cases the inflo- 

 rescences are much reduced only four flowers being developed 

 in Potamogeton pusillus while in P. trichoides the individual 

 flowers are modified, the number of carpels being reduced to 

 one. In Zannichelliapolycarpa the flowers areunisexual (Fig. 45), 

 a male and female flower (or inflorescence) being found together 

 in one leaf-axil; the male flowers are generally reduced to a 

 single stamen (sf Fig. 45 B\ while several carpels with funnel- 

 shaped stigmas (g) are grouped together, and enclosed in a 

 membranous cup (m.c.*). This cup has been interpreted as a 

 spathe enclosing a group of female flowers, each reduced to one 

 carpel. The filament is at first very short, but elongates so as 

 to rise above the pistils at anthesis. The anther dehisces and the 

 pollen grains fall into the open mouths of the cornucopia- 

 shaped stigma, and slide down the stylar canal, whose diameter 

 is almost double that of the pollen grains. The descent of the 

 pollen grains through the water is due to the fact that when 

 they become ripe they are weighted with starch grains 1 . 



Owing to the air spaces in the pericarp wall, the achenes of 

 some of the Potamogetons float for a time, before becoming 

 waterlogged and sinking. The air-containing tissue of the peri- 

 carp in P.perfoliatus, and the cuticularised epidermal layer (0..), 

 are shown in Fig. 46, p. 72. 



The fruits of the Pondweeds, after becoming to all appear- 

 ance ripe, often rest for a considerable period before germina- 

 tion 2 , except in the case of P. densus, in which the achenes 

 sprout a few days after they fall. But this species is rather 

 remote from the rest of the genus in other respects, such as the 

 opposite arrangement of the leaves, and the absence of the 

 ligule. Sauvageau 3 has shown by experiment that in P. crispus 

 it is the hard integument which delays germination; when the 

 embryo is laid bare by the removal of part of the seed coat, 

 sprouting rapidly occurs. The same author observed that when 



1 Roze, E. (1887). 



2 The delayed germination of aquatics in general is considered in 

 Chapter xix, p. 243. 3 Sauvageau, C. (1894). 



