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o. Leaves all submerged, meeting around 

 the stem or semi-amplexicaul ; less fre- 

 quently sessile only, the stem then com- 

 pressed. 



A. Stem terete ; vascular-bundles of the 

 axial cylinder separated. 



1. With many bundles in the bark 



P. praelongus-growp. 



2. Without bundles in the bark, 

 or exceptionally with one or a 



very few bundles . . . . P. perfoMatus-gvowp. 

 A A. Stem compressed; without bundles 

 in the bark ; bundles of the axial cy- 

 linder united in three groups P. crispus-group 

 oo. Leaves not meeting around the stem or 

 semi-amplexicaul; the uppermost often 

 floating leaves; stem terete. 

 A. With many vascular and bast bund- 

 les in the bark, but never a circle 

 only of subepidermal bast bundles. 

 *. With free bundles in the axial 

 cylinder. 



J. Submerged leaves petiolate; 

 without marked floating lea- 

 ves P. malairif ts-gr oup. 



J J. With floating leaves and sub- 

 merged leaves. 



1. Submerged leaves broad, 

 thin and pellucid 



P. amplifolius-gvowp. 

 (Here sometimes also P. gra- 

 mineus of P. lucens-gvoup.) 



2. Submerged leaves linear, 

 semi-terete P. natans-growp. 



**. Bundles of the axial cylinder 



united in three groups ... P. lacens-gvowp. 

 A A. Without bundles in the bark or 

 exceptionally with a single or a very 

 few bundles ; some with a circle of 

 sub-epidermal bast-bundles. 



