250 J. O. HAGSTEOM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



Subsectio 24. Praelongi. Hagstr. — P. prcelongus-gron-p Raunki^e. 



Caulis teres ramosus. Folia omnia submersa membranacea sessilia margine 

 integerrima apice cucullata, basi cordata semiamplexicaulia vel infima lanceolata; li- 

 gulse fibrosse ± persistentes ecostatse. Pistillum stylo conspicuc ± angusto. Fnichis 

 magnus valde carinatus. — Anatoviia caulis: ?t-endodermis, fasciculi librif. et vascu- 

 lares corticales numerosi. Fasciculi tubi centralis prototypice dispositi. — Prsefolia- 

 tio convoluta. Turiones rhizomatici desunt. 



On account of the peculiar cucullate form of the leaf-apex etc., see below, the 

 single hereto belonging species cannot be placed together with P. perfoliaitis, 

 though the pistils and stigmas of the two species are much the same. In his Ana- 

 tomical Potamogeton-Studies (1903) C. Ratjnki^r therefore is correct in putting 

 them in separate groups after having classed them together in his earlier work, De 

 danske blomsterplanters naturhist. (1896, 108 — 109). In the Synopsis der mitteleur. 

 Fl. 1912, P. GRiEBNER still unites them into a common group Perfoliati. 



P. praeloiigus Wulfen. 



Plantarum nov. descriptiones, in Roemer, Archiv fiir die Botanik, III, st. 3, 

 1805, 331. — P. jlexuosus Wredow, Mecklenb. Fl. 1807, ex Schleicher, Cat. pi. 

 Helv. 1815, 23. — P. flexicaulis Detharding, in Strelitzer Anzeig., 1809 no. 50. — 

 P. acwninatus Wahlenberg, Flora upsal., 1820, 58. — P. perfoliahis v. lacusiris Wall- 

 man, in S. Liljeblad, Utkast till en Sv. flora, 1816, 706. — P. gramineus a borea- 

 lis L^STADius, Beskrifning ofver nagra sallsyntare vaxter etc., in K. Sv. Vet. Ak. 

 Handl. 1824, 162. — P. salicijolius Wolfg. ex Fries, Summa veg. scand. I, 1846, 

 213, et ex Hartman, Handb. Skand. Fl., 1879, 432.' — Figs. 1, A, 116, 117, P, 

 118, C. 



With regard to the styles and stigmas this species does not essentially differ 

 from P. perfolialus ; the poUengrains are also much the same, of middle size, spheric 

 or spheric-ovoid in shape. Nor do the form and the nervation of the leaves present 

 any more considerable differences. The ligules are thicker and more persistent but 

 without the double ridges like P. perfoliatus and alpinus. What, nevertheless, necessi- 

 tates to put these habitually similar species into separate groups is above all the fruit- 

 form, the form of leaf-apex, the character of leaf-margin, and some anatomical proper- 

 ties. The lid of the putamen mostly quite rounded in P. perfoliatus, is in P. prm- 

 longus sharply keeled, the fruit therefore, both in fresh and dry state with a promi- 

 nent keel especially upwards, while the fruit of the former in fresh state is rounded 

 on the back and merely in dry state sometimes endowed with a faint false keel. 

 The cells of the endocarp, on the contrary, in both groups comparatively thin-walled 



' A fossil from Toppeladugard (Scania) described by me in Geol. Foren:s Forliandl. 1906 under the 

 name of Holstia sxjilendens has later proved to be P. praJonyus. 



