THE MONOCOTYLEDONES i973 



Pondweed, is often cultivated in ponds in this country. A. fenestrale, which 

 used to be placed in a separate genus Ouvirandra, has submerged lattice- 

 like leaves with no lamina in the meshes between the veins. It occurs in 

 ponds in dense shade in parts of Madagascar and is sometimes cultivated 

 in greenhouses. (See Vol. I, p. 95^, Fig. 944.) • , , • 



The Hydrocharitaceae include a small group of aquatic herbs m 

 which the plants may be either completely submerged, as in Vallisneria 

 in which only the flowers come to the surface, or the whole plants may flcat 

 as in Hydrocharis. The family includes some fifty species grouped m fifteen 

 genera' Three genera are found in the sea. Among those which occur in 

 Britain are Hydrocharis morsus-ranae (Fig. 1903) (Frogbit), Stratiotes 

 abides (Water' Soldier) and Elodea canadensis (Water Thyme) which 



Pj(, igo2.— Hydrocharis morsus-ranae. Frogbit. Floating 



plant. 



was introduced into this country from America in 1841. It is interesting 

 to note that almost without exception only female plants of Elodea occur 

 and reproduction is entirely vegetative, by means of turions, condensed 

 apical shoots which become detached. Elodea like Valhsnerm is polli- 

 nated by water. In the former the female flowers are raised to the 

 surface of the water and receive floating pollen which has been shed by 

 detached male flowers. In Vallisneria spiralis the female flowers are also 

 elevated to the water surface but the detached male flowers do not shed their 

 pollen but float as a whole on the surface and pollination is eflfected by the 

 anthers rubbing against the stigmas, bursting and then discharging their 

 pollen (see also p. 1296). In Hydrocharis pollination is by insects and in the 

 female flowers nectar-secreting scales are formed at the base of the petals, 

 their structure being remarkably similar to those in the genus Ranunculus. 

 In Stratiotes the dioecious flowers come above the water only for pollination 

 and submerge again while the fruits ripen. 



The Scheuchzeriaceae are a small family containing five genera and 

 about twenty species which include herbaceous plants found mainly in 

 marshy places. They bear rushlike leaves often sheathing at the base 



