— 265 - 



serrulate leaves of P. lucens-group perhaps may be used as the 

 distinctive mark. 



7. P. amplifolius-group. With both floating leaves and thin, 

 not linear submerged leaves. Stem with bundles in the bark. 

 G- or O-endodermis. Vascular bundles of the axial cylinder free. 

 P. amplifolius Tack. P. stenostachys K. Sch. 



- Claytonii Tuck. - Cheesemanni A. Benn. 



- Thunbergii Cham, e Schlecht. - linguatus Hagstr. l ) 



- illinoensis Morong. 



Fig. 4. Transverse sections of the axial cylinder of: A, P. Drummondii (70: 1); 

 B, P. sclerocarjms (70:1); the hatched parts = pith; the white parts = vas- 

 cular bundles; the black parts = mechanical tissue; lg, channels formed by 

 obliteration of vessels. — C, P. perfoliatus (95 : 1): a small portion of the trans- 

 verse section of the stem; h, epidermis; sh, stratum of cells between the epi- 

 dermis and the outmost lacunae; d, wall between two lacunae; s, bast bundle 

 formed by one cell only. — D, P. fluitans (300:1); epidermis cells with ridge- 

 formed stripes on the outer walls. 



8. P. natans-group. With linear, thick submerged leaves and 

 with numerous floating leaves, the blades of the latter at last 

 dropping caused by a special construction of the uppermost part 

 of the petioles. Stem with bundles in the bark; next to epidermis 

 bundles (bast bundles) are also to be found. G-endodermis. Vas- 

 cular bundles of the axial cylinder free (fig. 3 0). 



P. natans L. P. Oakesianns Robbins. 



J ) Referring to Hagström in: P. Düsen, Zur Kenntnis der Gefässpflanzen des 

 südlichen Patagonien. Öfvers. af Kongl. Vetenskaps-Akademiens Förhand- 

 lingar, 1901. 



