20 J. O. HAQSTROM, CRITICAL RESEARCHES ON THE POTAMOQETONS. 



P. flliformis Pers. x pectinatns L. Ex Almquist, Om gruppen Ligulatte Fr. 

 af si. Pot, in Bot. Not. 1889, n. 2, 62. — P. suecicus Right., PI. Eur. I, 1890, 15 

 (nomen solum). 



Undeniably this hybrid is among the more difficult ones to determine cor- 

 rectly. In the sterility certainly we have a good clew yet a strict attention is 

 necessary, since P. pectinatus also sometimes appears more or less failing in the 

 evolution of fruits, dependent, as I think, on the often observed deformity of the 

 stigmata. On the contrary the pollen always seems well-shaped and richly deve- 

 loped in this species, whereas the hybrid always exhibits all grains or at least the 

 greatest part of them deformed. When sometimes one or a few fruits are brought 

 to maturity they hold the middle size between the two parents, but as far as 

 it is now explored, always with deformed embryo. The spikes consist sometimes 

 of 3 — 4 small-flowered often closely placed verticils, sometimes of 4 — 5 verticils, 

 usually with a little larger perianths, i. e. upon the whole more pectinatus-li'ke. 

 Stem commonly very thin and filiformis-\ike with 2 — 3 bundles in the middle inter- 

 nodes, but often toward the base a little thicker and with much stronger leaf- 

 sheaths, whereby its connexion with P. pectinatus conspicuously appears, more sel- 

 dom of tall and luxuriant growth reminding of a river-form of pectinatus and in 

 this case with more cortical strands, but also with more elongated leaves and more 

 intensive green colour than in P. pectinatus. The leaves ordinarily are more or 

 less obtuse {f Hi f. -like), but forms with subacute or cuspidate leaves now and then 

 occur. Sheaths more seldom long and pectinatus-Uke, often shorter and darkbordered, 

 reminding of P. filiformis. 



As many different forms occur, it seems necessary and convenient to group 

 them into series. According to the shape of the leaf-apex we shall have four 

 groups: 



a. Acutus: — Folia sensim attenuata ± acuta. 



Forms of this series are not by their leaf-apexes distinguishable from P. pectinatus. 



p. Intermedius: — Folia sensim attenuata ± obtusa. 



Forms belonging hereto are in every respect the most intermediate of all. The 

 apex is tapering but commonly rather obtuse. 



■[. Cuspidatus: — Folia ± cuspidata aut obtusa cum mucrone saltem pro 

 magna parte. 



In habit these forms may be more pectinatus-Uke, or approaching very much 

 to P. filiformis. 



5. Obtusus: — Folia obtusa ± lata, longa vaginis longis. 



Conspectus formarunn. 



a. Acutus Hagstr. 



1. f. holmiensis n. f. — Gaulis tenuissimus, internodiis elongatis (80—100 mm.); 

 folia angustissima (0,5—0,25 mm) elongata (ca. 100 mm) sensim attenuata, vaginis 



