190 J. O. HAGSTRÖM, CRITICAL RESEARCHES ON THE TOTAMOGETONS. 



original habitat in the inner Asia, from where it haa spread westward to Europé 

 and eastward to N. America. And it seems also to ha ve 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. nodosus Pom. x Nuttallii Oham. & Sghl. (P. snbsessilis n. faybn,). 



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 ligulaeque ut in P. nodoso. 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., Mobong 

 ii- io / Li ( n ^* Uppsal.), by Mor. looked upon as a P. lonchitis. — In the Journ. 

 etaui» baostb. f Botany 1908, p. 250, Ar. Bennett states that he is the owner of two 



Top.i oj snbmers- . 



ed Leaves (»little specimens (from New York, and East Mass.) collected by Morong and 



supposed to be this hybrid, which he names P. am&ricanus P. pensylvanicus. 



Of P. nodosus x Richardsonii is spöken 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 Graebner. 



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

 semiten-tia ± canalioulata; liguke fibrosw persistentes. Pistillum et frurhis ut m 

 Arn/plifoliia. — Anatomia caulis: u-endodermis, fasciculi librif. et vase. cortieales 

 aumerosi. Tariones rhizomatici desunt. 



The Bpecies here concerned are obviously connectcd with the Aniplif<>Iins-zvou\) 

 by /'. amplifoliua with its strong ligules and oortical bundles in the st om. They 

 are all rich in sclerenchyma and, besides, provided with an intornodal //-endodermis. 

 Ilen-by they form a bridge to the ooriaccous-leaved speoies of the next group especi- 

 ally those in which the eentral vascular bundles are tfee as for instanec /'. illitun nsis. 



Another characlerist ie iiniting this subseet ion with the Lucentes appears in the 

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

 quently Been in P. lucens. 



1'iually the pistils with their short styles are common to both groupa, 



Rhizomatio turios are not knowu in this group. 



