— 266 — 



9. P. polygonifolius-group. With thin, not linear submerged 

 leaves and with floating leaves. Stem without bundles in the bark 

 (or but very exceptionally with a single bundle). G- or O-endo- 

 dermis. Axial cylinder with 6—11 free vascular bundles (fig. 9). 



P. polygonifolius Pourr. P. color atus Hörnern. 



- alpinus Balb. - pulcher Tuck. 



- fluitans Roth. - Tepperi A. Benn. 



- mexicanus A. Benn. 



10. P. sclerocarpus-group. With thin submerged leaves and with 

 floating leaves. Stem without bundles in the bark (or but excep- 

 tionally with a few ones). G- or O-endodermis. Axial cylinder 

 with 4 free but closely standing vascular bundles (fig. 4 A — B). 



P. scleroearpus K. Sch. P. Drummondii Benth. 



11. P. javanicus-group. With floating leaves and with linear 

 submerged leaves, all without sheath. Stem with a circle of sub- 

 epidermal bast-bundles but otherwise without bundles in the bark. 

 Axial cylinder's vascular bundles united in three or in only one group. 



P. javanicus Hassk. P. Vaseyi Robbins 



- Miduhikimo Makino. - lateralis Morong. 



12. P. hybridus-group. With floating leaves and with linear 

 submerged leaves, which are generally endowed with sheath. Stem 

 without bundles in the bark. Axial cylinder's vascular bundles 

 united in three or in only one bundle. 



P. hybridus Mich. P. spirillus Tuck. 



13. P. Robbinsii-group. Leaves all linear, submerged, with 

 sheaths, with (3— )5 principal nerves of vascular bundles, between 

 which several fine nerves of only bast-bundles occur; leaves 

 minutely serrulate. The stem terete, with bast bundles in the 

 bark especially in the outer part of it; the chief ones are lying 

 next to the epidermis. Axial cylinder's vascular bundles united in 

 three groups. 



P. Bobbinsii Oakes. 



14. P. zosterifolius-group. Leaves all linear, thin, sheathless, 

 submerged, with 3 — 5 principal nerves of vascular bundles between 

 which several fine nerves of bast bundles only (fig. 5). Stem com- 

 pressed, with a circle of subepidermal bast bundles but otherwise 

 without bundles in the bark or with but a small bundle in each 

 end of the transverse section. Axial cylinder's vascular bundles 

 united in three or in only one group (Fig. 3 M). 



