27 
it growing vigorously. No other locality for it is known in this 
country. It is, however, common in Cambridgeshire, England, 
and Mr. Fryer has kindly sent me a fine suite of specimens col- 
lected in that locality. Here it has never been observed in fruit, 
but it fruits, though not very freely, in England. The fruit, which 
is characterized from British specimens, shows a near alliance to 
P. heterophyllus, from which, however, it differs considerably in 
other points. Indeed, while it resembles heterophyllus on the one 
side, it exhibits a decided likeness to P. angustifolius on the other, 
and accordingly it has sometimes been ranked with the one species 
and sometimes with the other. Mr. Fryer, whose opinion in any 
matter relating to the Potamogetons of his district deserves great 
weight, is inclined to regard this species as a hybrid between 
heterophyllus and angustifolius, and there are strong reasons for 
suspecting that it may be a hybrid, such as the fact that the 
anthers are usually either destitute of pollen, or possess only un- 
potential pollen, and hence their inability to fertilize the stigmas, 
and the limited area of its growth. But a weighty argument 
against this view is the fact that neither of the supposed parents 
occurs in Mystic Pond, and that it should be produced in localities 
separated by the Atlantic ocean. 
July-August. (Plate XXXV. The fruit is figured from British 
specimens supplied by Mr. Fryer.) 
12. POTAMOGETON ILLINOENSIS, Morong, Bot. Gaz. v. 50 (1880.) 
Stem from a thick running rootstock, stout, much-branched 
above. Floating leaves opposite, numerous, thick, coriaceous, 
414—514 inches long, 2-374 inches wide, the apex with a short, 
blunt point, rounded or sub-cordate or sometimes sloping at base, 
oval or broadly elliptical, 18-27-nerved. Petioles often broad and 
flattened, 3-4 inches long. Submerged leaves numerous, usually 
lanceolate, 4-8 inches long and 1-2 inches wide, 13-19-nerved, 
acuminate or the uppermost acute like the floating, occasionally 
sessile, but for the most part sloping at the base into a short, 
broad, flat petiole. These leaves are of a very dark green color, 
with a conspicuous midrib, rarely reduced to phyllodia or to leaves 
with a long, phyllodia-like apex and base, expanding into a narrow 
blade in the middle. 
