— 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. jyoli/f/oiiifoliKs-grou^p. 



2. 4 bundles in the axial cylin- 

 der P. sclerocarpus-gvou\). 



**. Axial cylinder's bundles united 

 in three groups or in only one. 



1. With subepidermal bast bund- 

 les; leaves without sheaths 



P. javanicus-gi'on\) 



2. Without subepidermal bast 

 bundles; several leaves with 



sheaths P. /iubriclus-gvou\:). 



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. zosferifoHus-gvoup. 



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. pusil I its-group. 



2. Without subepidermal bast bundles in 



the stem P. co)ifervoides-gvoiip. 



1. P. <leiisiis-groii|i. 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. praeloiigus-groiip. Leaves all thin, broad sessile and sub- 

 merged with a broad base semiamplexicaul, and cuculate at the 



