LEAVES OF THE PONDWEEDS 



61 



The various species of Potamogeton show transitions between 

 plants with floating leaves, capable of producing a land form, 

 and plants with submerged leaves, living entirely beneath the 

 water-surface, except that they raise their flowers slightly into 

 the air. Potamogeton natans may be taken as a type of the Pond- 

 weeds with floating leaves; these consist of a sheathing base 

 with stipules, a long petiole and ah elliptical to lanceolate blade, 

 leathery in texture. The early leaves on each shoot, which do not 

 reach the water-surface, are phyllodic and represent only the 

 petioles of the perfect leaves. Intermediate leaf-forms also 

 occur, with small, spoon-like expansions of the apex 1 . The 

 relation between the narrow submerged 

 leaves and the broad floating leaves is 

 identical with that subsisting between 

 the two corresponding leaf-types in 

 Sagittaria. The land form of Potamogeton 

 natans is shown in Fig. 125, p. 196. 



Another species of Potamogeton^ 

 P. pulcher, Tuckerm., of N. America, 

 produces not only broad floating leaves 

 but broad submerged leaves, while 

 others, such as P. heterophyllus, Schreb., 

 have ovate or oblong floating leaves, 

 but their submerged leaves are of a 

 narrower type. 



The more completely aquatic species 

 form submerged leaves alone, with 



c -iii j i -r-i i FIG. 38. Potamogeton zosteri- 



lammae or variable breadth. Examples foiius, Schum. Upper part 

 of this group are P. lucens, P. perfoliatus of , leaf; ** *i. sn z- <. vas- 



& " J cular bundles; s, bast bun- 



and P. crispus. In these and related dies; rs, bast bundle along 

 species the blade is exceedingly thin, margi j r X ( ^^ . [ f aun ' 

 often with only one plate of cells be- 

 tween the two epidermal layers, but it is supported by fibrous 

 strands running the length of the leaf (j in Fig. 38). The 

 lamina is often crisped or undulated at the margin in a 

 1 Schenck, H. (1885); see also Fig. 168, p. 339. 



