243 
different are they in general appearance. Each of them has re- 
ceived a varietal name, but they can hardly be considered as any- 
thing more than forms or states, as they readily run into each 
other when the conditions change. 
The type of Rafinesque, commonly called by late authors var. 
limosum, is entirely terrestrial, rooting in the mud upon the 
shore, 2 to 4 cm. high, leaves much longer than the fruit, alter- 
nate, linear, and entire or more commonly pectinate, sometimes 
pinnatifid. Fruit minute, slightly more than 1 mm. long and | 
mm. or less in breadth. Carpels usually smooth, sometimes a 
little rough, the groove small. Petals four, purplish. Stamens 
four. This form appears to be the offspring of the floating plant 
which often roots in the mud where it is cast ashore, and sends 
up an erect flowering stem, frequently leaving traces of the old 
submerged leaves below the rooting node. 
The form called JZ. ambiguum var. natans by Dr. Gray (Man. 
Ed. 1, p. 140) and which he seems to have considered the type, is 
intermediate between the type and the so-called var. capillaceum. 
It usually occurs in still, shallow water, with a spike of flowers 
and the typical floral leaves above the surface, and subverticillate 
or, more commonly, scattered pinnatifid leaves beneath ; the divi- 
sions are few. On specimens of this from the herbarium of Mr. 
Oakes, marked by him WZ. capillaceum, Nutt., I find spikes nearly 
30 cm. long. When botanizing on Nantucket in the summer of 
1887, I found these aquatic forms together in some of the small 
ponds, growing in such profusion that they were literally in heaps. 
A note sent to Mrs. Owen, and published on page 26 of her in- » 
teresting catalogue of the plants of the island, will show how inti- 
mately associated they are. 
“In both these ponds the plant is at first var. capz/laceum 
while immersed, but very soon it gets its head above water and 
immediately forms pectinate leaves—that is becomes the type as 
described in Gray’s Manual. I have plenty of specimens in both 
conditions which grew together.” 
The form called 17. ambiguumby Nutt. (Gen. 2, 213) and M7. 
ambiguum, var. capillaceum by Torr. & Gray (Fl. N. A. 1, 543) and 
_ other authors, is commonly a deep water plant, entirely submerged. 
Stems long, widely branching, very plumose when growing, leaves 
