178 SUPPOSED HYBRIDITY IN POTAMOGETON. 



careful examination to enable one to separate them from that 

 species with certainty. At the present time the lower leaves on the 

 young stems resemble those of a narrow-leaved form of perfoliatus, 

 while those on the upper part of the stem are nearly like those of 

 crispus. Mr. Billups informs me that he found only one patch of 

 the plant, a very large and dense one, of remarkably vigorous 

 growth, but producing no flower- spikes. 



Having carefully compared for some months the living plant 

 from the Dee with fresh examples of P. perfoliatus growing in the 

 Fens, I find it differs from that plant in not dying down in winter ; 

 but remains growing from late autumn to spring just as crispus 

 does ; which plant it also resembles in habit, and in the lower 

 leaves of the young stems being like ordinary leaves, but mounted on 

 the backs of the loiver stipules. Surely we may regard this Dee plant 

 as a hybrid between P. crispus and P. perfoliatus ? 



Dr. Tiselius sent me early in the present year an extensive 

 series of P. nitens. Amongst them was a remarkable form which 

 he labels 



" P. nitens Wil. 

 f. intermedia mihi. 



Antea P. intermedins mihi (ad interim) distributa." 

 I was much struck with the resemblance between this P . intermedins 

 and a series of plants collected in " Birsay Loch, Orkney " (Co. Ill) 

 by the late Dr. Boswell ; and also, in a less degree, by its resem- 

 blance to a Surrey form from the Woking Canal, collected by Mr. 



All these intermedvus-hke forms grow together with P. nitens and 

 P. heterophyllis, and suggest that they are the offspring of the two 

 segregates. This supposition presents some difficulties. One of the 

 supposed parents, P. nitens, seems not to be known with fertile spikes of 

 seed (possibly single drupelets may be produced in some instances, but 

 the proofs of this are doubtful to me), its pollen also seems to be 

 abortive. The supposed intermediates are produced in such abun- 

 dance that I think if P. nitens were the seed-bearing parent, some 

 examples of fruiting spikes of nitens would exist in herbariums ; but 

 I have met with none, nor can anyone tell me where one is to be 

 seen. All plants I have had sent to me as fruiting nitens are good 

 heterophyllus. 



On looking over collections of Potamogetons from localities 

 where P. heterophyllus grows with P. nitens, I have been frequently 

 surprised at the resemblance some specimens of the former plant 

 bear to the latter, so that again the idea of hybridity between the two 

 forms has been suggested. Some of the forms from these localities 

 are nitens-like, and yet not resembling the P. intermedins of Tiselius. 

 And the question naturally arises : whence the resemblance ? 



I have no facts to offer in solution of this difficult question, and 

 no hypothesis which does not break down almost at the outset. I 

 do not think, as has been suggested to me, that nitens is the seed- 

 bearing parent, but rather that the pollen of nitens is occasionally 

 fertile. In which case a single potent spike would fertilize very 

 many drupelets of any Potamogeton it could cross with. 



