V] 



LEAVES OF THE PONDWEEDS 



6i 



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 an 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^. 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 o{ Potamogeton 

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



Another species of Potamogeton^ 

 P. pu/c/ier, 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 

 laminae of variable breadth. Examples 

 of this group are P. lucens^ P.perfoliatus 

 and P. crispus. In these and related 

 species the blade is exceedingly thin, 

 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 {s 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. 



Fig. 38. Potamogeton zosteri- 

 f alius, Schum. Upper paxt 

 of leaf; mn, sn^, sw., , in, vas- 

 cular bundles; s, bast bun- 

 dles; rs, bast bundle along 

 margin, (x 12 circa.) [Raun- 

 kiaer, C. (1903).] 



