15 
the flowering season. Peduncles 1-3 inches in length, commonly 
much thicker than the stem, mostly solitary. Spikes cylindri- 
cal, %-1 inch long, usually not fruiting freely. Stipules hardly 
keeled, acute. Fruit - obovate, about 1% lines long and 1 
line broad, nearly. straight on the face, 3-keeled, middle keel 
sharp; the style apical or often sub-apical ; sides of the nutlet not 
pitted, but sometimes slightly impressed; embryo circle incomplete, 
the apex pointing towards the base. 
This species may readily be distinguished from Р. natans, 
which it resembles, by its much smaller parts, its almost uniformly 
elliptical floating leaves, its delicate phyllodia, thickened peduncles, 
tricarinate fruit, and even-sided nutlet. 
My friend, Arthur Bennett (Jour. Bot. 1890, р. 301), regards 
this species as the P. JVuZ/a//zi of Ch. and Sch. (Linnza, ii. p. 226), 
which would give that name the priority, having been published 
in 1827. - With this judgment I am unable to agree, since it ap- 
pears to me that the fruit of P. Oakesianus does not correspond to 
the description or the figures given in that work. It is there said 
to be ‘oblique lenticulari-suborbicularis," but in shape it is 
rarely otherwise than narrowly obovate. The sides of the nutlet 
are said to be impressed in the middle, whereas in this species 
they are even. Our plant fails especially to agree with the de- 
scription and figure of these authors in the embryonic curve, their 
seed being said to be “ cochleato-convolutum, unico et paululum 
quod supersit anfractu,” but in this case the seed is not cochleate- 
convolute, nor is the embryo coiled upon itself, the apex simply 
pointing to the base. The figure of the embryo in Linnza as 
compared with the embryo of P. Oakesianus is quite conclusive. 
The fruit figured by Cham. and Schlecht. seems to me to agree 
so exactly with that of P. Clayton, Tuck., that I have not hesitated 
to adopt their name for that species, as will be seen below. 
The name here used was given by Dr. Robbins in honor of 
his old and intimate friend, William Oakes, of Ipswich, Mass. 
A rather rare species, occurring in still waters, Anticosti, Canada, 
(Macoun), N. H. to N. J. and westward to the Adirondacks, N. Y. 
A doubtful form is sent from Nebraska by H. J. Webber. It is 
exceedingly abundant in some of the small ponds of Nantucket, 
where it fruits very freely. June-Aug. (Plate XXVI.) 
