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



with SO narrow leaves, about 3—4 mm, may be an extremely rare thing. Among 

 many hundreds of specimens I at least never saw such a narrow-leaved Zizii-ioTm. 

 The German auther G. Fischer regards P. Zizii as an independent species chiefly, 

 as it seems, on account of the fruit-spikes of some forms, or (1914) at least some 

 forms from Switzerland, as a subspecies under P. gramineus L. and proposes to make 

 out the hybridity by » Bastardierungsversuch zwischen P. lucens und gramineus " 

 (Bayer. Potamog. 1907)-. This way, hov/ever, is not performable, unfortunately. It 

 is much easier to examine the germinative ability of the seeds. I am persuaded 

 that P. Zizii very rarely, if ever, propagates itself by seeds. And if even seeds able 

 to germinate were to be found, this fact, however, would not be an instance against 

 the hybrid origin, since hybrids now and then produce fertile seeds. P. Graebner 

 in Potamogetonaceae, 1907, and Synopsis, 1913, ranges P. Zizii as an independent 

 species at the side of P. lucens. The Finlander C. W. Fontell leaves the question 

 open (Beitrage Kenntn. anat. Potam., 1909, 63). 



All the specimens of P. Zizii I have had an opportunity to examine lack 

 independent, constant, specific characters and possess no other properties than those 

 found in P. gramineus and P. lucens, or which constitute a modification of the 

 characters of both, in perfect correspondence with the so-called intermediate characters 

 of the other hybrids of this genus. 



Thus it is characterized 1) by the mode of prolonging the stem (intermediate) 

 with often two (1 — 3) spike-bearing branches of the first and 2d (3d) rank; and by 

 the way, in which the branches below the primary spike branch again (often gramineus- 

 like). 2) Involucral leaves often subcoriaceous, most frequently with conspicuous 

 petioles, nearly without exceptions longer than the cusp. 3) Stem-leaves very short- 

 petioled (intermediate). 4) Branch-leaves always comparatively small if even the 

 stem-leaves are of lucens-size. 5) The venation and structure of the submersed leaves 

 obviously intermediate. 6) Ligules of an intermediate size and shape. 7) Fruit, if 

 present, also intermediate and most frequently infertile. 8) Stem- anatomy intermediate, 

 and so are also 9) the rhizomatic turios. 



As to habit P. Zizii appears in all conceivable transitions from the most gra- 

 mineus-\i\s.e to the most lucens-\\\ie forms. 



Its fruiting faculty appears, as it seems, in part of the crosses nearly quite 

 undiminished or normal, in most forms again considerably reduced, and in many 

 cases, finally, it is quite lacking. 



Very explanatory is the occurrence of P. Zizii in Sweden. It is met with 

 locally together with P. gramineus and P. lucens, spreading to the parallel which is 

 the northern boundary of P. lucens but not beyond, though P. gramineus continues 

 to more northern latitudes. In other words, P. Zizii lacks a distribution area of 

 its own, being dependent, for its spreading, on P. gramineus and P. lucens. Sweden 

 maj' be the country of Europe where P. Zizii is most frequently met with, but 

 there is no »independent species P. Zizii" in Sweden, and as far as my experience 

 goes in no other contries either. 



As to the stem-anatom.y P. Zizii holds the medium between the two efficient 



