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



numerous channels, and in the margins, at least in their lower half, with several 

 layers of mesophyllum. Thus we here have a real transitional form to the leaf cha- 

 racteristic to P. natans. The marginal strand is very 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 Javanici and Natantes as P. limosellifolius by its involucral leaves between the 

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

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



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

 Barnet, Vt, U. S. A., 82, Blanchard (hb. Stockholm.), East Mass., U. S. A., Morong 

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

 P. diversifolius Raf. 



Subsectio 15. Hybrid! (Graebner) Hagstr. 



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

 P. Nuttallii. 



Caulis teres vel compressus. Folia biformia, natantia evaginata petiolata 

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

 cinervia valde reticulata vaginata et ligulata vel evaginata ligulis vaginantibus. 

 Ligtilce (vaginae) membranacese truncatse vel subretusse, connatse vel fissse, in spatio 

 antico inter nervos laterales paucinervise vel subenervias. Pistillum stylo nullo vel 

 subnuUo, stigmate parvo humili. Frvctus compressus rotundatus lateribus impressus 

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



Neither cauline nor rhizomatic turios are found in the specimens examined 

 by me. 



The shape of the fruit and embryo etc. make us refer P. Nuttallii to this 

 group. Prof. Graebner on the contrary ranks it among his Fluitantes. 



The anatomic state of the first four species is noticeable. The crosscut form 

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

 ones. In the lower stem-internodes the lateral bundles of each side are fused into a 

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

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

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

 stem-prolongation, being a little thicker, both the median and the lateral bundles 

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

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

 bundle. In P. gramineus this trio-bundle also dissolves into a bundle-trio in the 

 branches of the highest rank (cf. the fig. 102!). Besides, the anatomic diagram of 

 the upper branches serving as stem- prolongation, differs also from that of the basal 

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

 whereas the subepidermal bast-bundles of the species, furnished with such strands 



