iv] POLLINATION AND LEAF FORM 57 



the water, and surrounds the stigmas of the female flower. It 

 has been suggested that perhaps the pollen may be attracted 

 to the receptive surfaces by currents due to some secretion from 

 the stigmas. It has been shown in E. canadensis that the spines 

 on the outer coat of the pollen-grain hold back the surface-film 

 from contact with the body of the spore and thus imprison 

 enough air to keep it afloat 1 . 



A somewhat different method, in which water also plays 

 a part, is found in Vallisneria' 2 ', while in Hydromystria the 

 pollination is sometimes effected by wind and sometimes by 

 water 3 . In Elodea densa 4 , the large white flowers contain nectar, 

 and insect pollination occurs; this genus thus shows transitions 

 between the entomophilous members of the family, such as 

 Hydrocharis, and the hydrophilous and anemophilous genera. 



Among vegetative characters, perhaps the most notable 

 feature of the Hydrocharitaceae is the great variation in the 

 form and mode of life of the leaf in the different genera. To 

 illustrate this we may briefly enumerate the leaf characters of 

 a few genera selected entirely from the fresh-water members 

 of the family. 



Hydrocharis. In certain species, heart-shaped floating leaves alone. 



Stratiotes. Stiff, serrated, linear leaves, sometimes entirely submerged, 

 sometimes partially aerial. 



Boottia. Lower leaves short-stalked and submerged; upper leaves long- 

 stalked and often aerial. 



Ottelia. Leaves differentiated into submerged leaves, with a narrow 

 blade, and stalked leaves with broader blades, which may be submerged, 

 floating or aerial. 



Vallisneria. Leaves entirely submerged, ribbon-like, growing in rosettes. 



Hydrilla and Elodea. Leaves entirely submerged, short and linear, 

 growing on elongated axes. 



Three genera of the Hydrocharitaceae, Enhalus, Halophila 

 and Thalassia, live in salt water; these we shall consider in 

 Chapter x. 



1 Wylie, R. B. (1904). 2 See p. 235. 



3 Hauman, L. (1915). * Hauman-Merck, L. 



