— 263 — 



*. Axial cylinder with 4 — 12 free 

 vascular bundles. Submerged 

 leaves generally broad, not quite 

 linear. 



1. 6—12 bundles in the axial 

 cylinder P. polygonifolius-group. 



2. 4 bundles in the axial cylin- 

 der P. sclerocar pus-group. 



**. Axial cylinder's bundles united 

 in three groups or in only one. 



1. With subepidermal bast bund- 

 les; leaves without sheaths 



P. javanicus-group 



2. Without subepidermal bast 

 bundles; several leaves with 



sheaths P. hybridus-group. 



B. Leaves all submerged and linear, all without 

 sheaths. Bundles of the axial cylinder united 

 in three groups or most generally in only one. 



a. Leaves with 3 — 5 principal nerves from vas- 

 cular bundles between which several fine 



nerves formed of bast bundles only P. zosterifolius-gvowp. 



b. Leaves with 1 — 5 of vascular bundles formed 

 nerves, but no bast bundles between them, 

 with exception of the bast bundle always 

 lying in every leave-edge. 



1. With a circle of subepidermal bast bund- 

 les in the stem P. pusillus-group. 



2. Without subepidermal bast bundles in 



the stem P. confervoides-gronp. 



1. P. densus-group. With only thin, subopposite, submerged 

 leaves meeting around the stem, without sheaths except at the 

 involucral leaves. Stem without bundles in the bark. Endodermis- 

 cells thin-walled; vascular bundles of the axial-cylinder united into 

 one bundle or but slightly separated in three groups (Fig. 3 L). 



P. densus L. 



2. P. praelongus-group. Leaves all thin, broad sessile and sub- 

 merged with a broad base semiamplexicaul, and cuculate at the 



