190 J. O. HAGSTEOM, CRITICAL EESEARCHES ON THE POTAMOGETONS. 



original habitat in the inner Asia, from where it has spread westward to Europe 

 and eastward to N. America. And it seems also to have arrived at the South 

 American continent. — The northern limit in N. America may be the 50th lat. and 

 in the Old and the New World, as a whole, the year-isotherm of about 8° C. 



In Africa it has crossed the equator and spread to the western islands, but if 

 it were an original African plant it ought to have early spread to South America 

 and should now be met with there more abundantly than is the case. 



P. nodosiis PoiR. X Nntttallii Cham. & Schl. (P. siibsessilis n. hybr.). 



Caulis teres (= P. nodosus), anatomia ut in P. Nuttallii. Folia submersa 140 

 — 200 X 5 — 7 mm subsessilia in petiolum attenuata, nervatura ut in P. Nuttallii. 

 Folia natantia ligulseque ut in P. nodosa. Sterilis. — Fig. 96. 



In habit this form is like P. nodosus, but its characters are inter- 

 mediate. Especially it is characterized by the submersed leaves being 

 a little broader than has P. Nuttallii and tapering into a stalk-like base, 

 but as to the nervation etc. reminding of P. Nuttallii. 



Distribution. N. America. Eastern Mass., U. S. A., Morong 

 Fi 96 p stib ^^^^- Uppsal.), by MoR. looked upon as a P. loncliitis. — In the Journ. 

 sessins haoste. of Botauy 1908, p. 250, Ar. Bennett states that he is the owner of two 

 ed leaves (n little spccimcns (from Ncw Yoiiv, and East Mass.) collected by Morong and 

 "" "^'^ ' supposed to be this hybrid, which he names P. americayms X P. pensylvanicus. 



Of P. nodosus X Richardsonii is spoken above, p. 148. The denticulation and 

 the form of the apexes of the submersed leaves beside the presence of a pseudo- 

 hypoderma in the stem are here decisive. 



Subsectio 22. Natantes Grabbner. 



CaiiUs teres ± ramosus. Folia natantia longe petiolata; submersa linearia crassa 

 semiteretia ± canaliculata; ligulce fibrosse persistentes. Pistillum et jr actus ut in 

 Am/plijoliis. — Anatomia caulis: M-endodermis, fasciculi librif. et vase, corticales 

 numerosi. — Turiones rhizomatici desunt. 



The species here concerned are obviously connected with the Am plifoUus-gr ou]) 

 by P. amplifolius with its strong ligules and cortical bundles in the stem. They 

 are all rich in sclerenchyma and, besides, provided with an internodal ?t-endodermis. 

 Hereby they form a bridge to the coriaceous-leaved species of the next group especi- 

 ally those in which the central vascular bundles are free as for instance P. illinoensis. 



Another characteristic uniting this subsection with the Lucentes appears in the 

 submersed leaves, reduced to petioles and midribs, forming so-called phyllopodia, fre- 

 quently seen in P. lucens. 



Finally the pistils with their short stjdes are common to both groups. 



Rhizomatic turios are not known in this group. 



