70 POTAMOGETONACEAE [CH. 



can be traced in the Potamogetonaceae. This series ranges 

 from forms such as Potamogeton natans, with an erect spike 

 of numerous flowers, through various intermediate types, to 

 the related genus Ruppia, in which the pollen floats, and the 

 stigmas are raised to the surface to receive it, and ultimately 

 to Zannichellia and various marine members of the family, in 

 which the pollination is entirely submerged. Even within the 

 genus Potamogeton itself, there are a number of gradations in 



m.c. 



A B 



FIG. 45. Zannichellia polycarpa, Nolte. A, shoot (nat. size) with flowers (/). 



B, flowers (enlarged) ; st, stamen; g, gynaeceum; m.c., membranous cup. May 25, 



1912. [A. A.] 



the direction of submerged life. The flowers possess, typically, 

 four stamens, and four free carpels. They appear, at first glance, 

 to possess also four perianth members, but more careful exami- 

 nation reveals that these are, in reality, leaf-like outgrowths 

 from the staminal connective 1 . The spike of P. natans is sup- 

 ported above the water by the two floating leaves immediately 

 below it. These are always opposite (cf. Fig. 37, p. 60), although 

 otherwise the leaves are alternate. In some species, e.g. P. pec- 

 tinatusy the spikes, instead of being stiff and erect, are thin and 

 flexible, and float horizontally on the water. In these forms 



1 Information as to the morphology and development of the flower and 

 fruit will be found in Hegelmaier, F. (1870), Schumann, K. (1892), etc. 



