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178 SUPPOSED HYBRIDITY IN POTAMOGETON.
careful examination to enable one to separate tliem 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 hacks 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. intermedium
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 resemblance
to a Surrey form from the Woking Canal, collected by Mr.
Beeby.
All these intermedius-Y\ke forms grow together with P. nitens and
P. hete7-ophyllis, 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 y^'xihfertile sjnkes 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 7iitens-like, and yet not resembling the P. intermedius 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 seedbearing
parent, but rather that the pollen of nitens is occasionally
fertile. In which case a smgle potent spike would fertilize
many drupelets of any Potamoyeton it could cross with.
very