265 



serrulate leaves of P. lncens-%vow\) perhaps may be used as the 

 distinctive mark. 



7. P. ainplifolius-group. With both floating leaves and 

 not linear submerged leaves. Stem with bundles in the 

 C- or 0-endodermis. Vascular bundles of the axial cylinder 

 P. amplifoUus Tuck. P. stenosfachijs K. Sch. 



- Claytonii Tuck. - Cheesemanni A. Benn. 



- Thunbergii Cham, e Schlecht. - linguatiis Hagstr. ^) 



- illinoensis Morong. 



thin, 



bark. 



free. 



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

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

 cular bundles; the black parts = mechanical tissue; Ig. channels formed l)y 

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

 verse section of the stem; h, epidermis; s/i. 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. flnitans (300:1); epidermis cells with ridge- 

 formed stripes on the outer walls. 



8, P. natans-groiip. 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. C-endodermis. Vas- 

 cular binidles of the axial cylinder free (fig. 3 C). 



P. natans L. P. Oakesianus Robbins. 



') Referring to Hagstrom in: P. Dusen, Zur Kenntnis der Gefiisspflanzen des 

 siidlichen Patagonien. Ofvers. af Kongl. Vetenskaps-Akademiens Forhand- 

 lingar, 1901. 



