134 



J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



numerous channels, and in the margins, a t least in their lower halt', with several 

 Layers of mesophyllum. Thus \ve here have a real transitional form to the leaf cha- 

 racteristic to P. natans. The marginal strand is verv strong. The nervation and 

 form are exhibited in the fig. 59. Ligules 5-nerved in the front-field, open, convolute. 

 By its submersed leaves P, Vaseyi holds quite a similar intermediate place between 

 the Javanioi and Natantes as P. HmoseUifolius by its involucral leaves between the 

 Pusilli and Javanici. Stem-anatomy with verv thin subepidermal strands, otherwise 

 like that of P. pusillus and the Corean javanicus. Peduncle, see the fig. above! 



Distribution. N. America, Ottawa, Oanada, 82, Fletcher (hb. Stockholm.). 

 Barnet, Vt, U. S. A., 82, Bla>tchari> (hb. Stockholm.), East Mass., U. S. A., Mordm: 

 (hb. Uppsal.), Chisago Lake, Minn., Tavlor (hb. Stockholm.), a little verging to 

 P. diversifolius Raf. 



Subsectio 15. Hybridi (Graebner) H igstr. 



Hybridi Graekner in A. Engler, Das Pflanzenr. IV, 11, 1907. 50. exclus. 

 P. Nuttallii. 



Caulis teres vel compressus. Folia biformia, natantia evaginafa petiolata 

 ovalia obtusa basi attenuata pauci- vel multinervia, submersa linearia obtusa pau- 

 cinervia valde reticulata vaginata et ligulata vel evaginata ligulis vaginantibus. 

 Ligulw (vaginae) membranacese truncata3 vel subretusa% connatic vel fiss;e, in spatio 

 antico inter nervös laterales paucinervia' vel subenervi;e. PisliUum stvlo nullo vel 

 subnullo, stigmate parvo humili. Fruetus compressus rotundatus lateribus impressus 

 carinatus vel tricarinatus; carina media saepe undulato-cristata ; embryo spiralis. 



Neither cauline nor rhizomatic turios are found in the specimens examined 

 by mc 



The shapc of the fruit and embryo etc. make us refer /'. NuttalHi to this 

 groii)). Prof. GRAEBNER on the contrary ranks it among his Fluilantcs. 



The anatomic state of the first four species is notkvahlc. The crosscut Innu 

 of the central stele is oval and the lateral bundles are separated from the median 

 oncs. In the lower st em-internodes the lateral bundles of each side are Eueed into a 

 common bundle, and the median bundles havc a eommon xylem part dissolving to 

 a common eavity (oblong diagram-type). Higher up in the stem, again, the fcwo 

 median bundles run separately (four-bundled diagram). In bhe internodes of the 

 stein-prolongation, being a little thieker, both bhe median and the lateral bundles 

 at last dissolve and the more original state with eight bundles return (eighl-bundled 

 diagram). The trio, however, remains united forming a compound bundle, a fcrio- 

 bundle. In /'. gramineus this tiio-bundlo also dissolves into a bundle-trio in the 

 branches of the highest rank (ef. the lig. 102!). Besides, the anatomic diagram of 

 bhe apper branches serring as stem-prolongation, dillers also from that of the basa) 

 or lower internodes of the stem by the epidermis possessing a one-oelled hypoderma, 



u hcreas the subepidermal bast-bundles of the speeies, furnished with sueh strands 



