222 J. O. HAGSTKOM, CRITICAL EESEAECHBS ON THE POTAMOGETONS. 



convinced that the forms in question are a hybrid plant, which, like P. Zizii, in some 

 crosses is able to produce some fruit or other. For the distribution and the subsist- 

 ence of the plant, however, this faculty, no doubt, plays a very subordinate part. 

 What I have said above of P. Zizii is true about P. nitens also. The list below displays 

 its abundant occurrence in the Scandinavian peninsula, but there is no independent 

 species P. nitens there and I have seen no such a plant from any other place either. 



By cooperation of P. perfoliatus with cordate clasping leaf-base and gramineus, 

 the submersed leaves of which are lanceolate, the base of those leaves of the bastard 

 usually grows rounded-cordate, half clasping. Most rarely we meet with forms, the 

 stem-leaves of which present a lanceolate base, in which case the branch-leaves, 

 nevertheless, have a more or less rounded base. In doubtful cases the nitens- and 

 gramineus-iorms are distinguished by the bases of the branch-leaves. The apex of 

 the submersed leaves is in general cuspidate, extremely seldom rounded. In the 

 former case often sharp as in P. gramineus, but often a little broader (influence from 

 P. perfoliatus), in perfect analogy with the hybrids P. decipiens and Bahingtonii. 

 The upper leaves of the spike-region turn usually, by cooperation of the two species, 

 short and broadly petioled with sloping lamina. When gramineus in this part predo- 

 minates the petioles grow longer, nearly so long as in P. gramineus, but always a 

 little broader especially upward. If P. perfoliatus, again, has gained ascendency, these 

 leaves also turn sessile with the base half clasping or ovate-lanceolate. As regards 

 the consistency and structure, the top-leaves are an intermediate thing between 

 floating leaves and submersed leaves, typically furnished with numerous stomata 

 (influence from P. gramineus). As the difference between the efficients is most 

 evident in the spike-region, the hybrid forms also grow most different there, and 

 consequently I have tried to group the numerous forms of the hybrid according to 

 those conditions. Cf. O. Hagstrom in Neuman, Sveriges Flora 1901, 797! This 

 is so much the rather suitable, as the hybrid, like the parents, generally in an early 

 state produces spikes, whereupon it prolongs itself by branches of as far as the fifth 

 rank or more, by which a great abundance of spikes exists and consequently plenty 

 of involucral leaves mostly are present. The submersed leaves are always 

 endowed with small, most frequently very fugacious denticles. Nerve-spaces usually 

 more elongated than in P. gramineus (influence from P. perfol.). Ligules decaying 

 sooner than in P. gramineus, likewise an influence from the other parent-species. 

 From the same cause the peduncles are typically shorter than in P. gramineus, but 

 often a little thickened at the middle or upwards. The spikes of the specimens now 

 examined are to the greatest part quite sterile. According to our experience fruiting 

 nitens, of which so much is spoken, is extremely rare, but in different collections, 

 true enough, I have now and then met with fruiting specimens determined as P. 

 nitens, which in reality have been pure P. gramineus. 



As regards the stem-anatomy you will always find one or two cortical circles 

 of vascular and bast bundles beside the subepidermal strands. The endodermis-cells 

 and the diagram of the stele vary from pure, oblong gramineus-ty^e to pure trio-type 

 of P. perfoliatus, but an amalgamation of both, a hybrid-diagram, is the commonest. 



