2 
would have been impossible, at least in its present form, except 
for the assistance of friendly European botanists to whom I am 
indebted for large suites of specimens and invaluable counsel. 
With Mr. Arthur Bennett, of Croydon, England, distinguished for 
his extensive acquaintance with Potamogeton forms in all parts of 
the world, and for his contributions to the literature of the subject, 
I have enjoyed a correspondence covering the last twelve years, 
and during the course of that time have received from him speci- 
mens of Natas, Zanichellia, Ruppia, Zostera and Potamogeton, which 
represent nearly all the species of these families known to Euro- 
pean Herbaria. His generous assistance has not ended here, but 
he has examined for me the materials at Kew, the London Mu- 
seum of Natural History, Berlin and the Herbarium of Linnzus, 
so that I feel as well acquainted with these great collections as if 
I had visited them in person. І am also much beholden to that 
acute botanist of Cambridgeshire, England, Mr. Alfred Fryer, for 
beautiful specimens of the various interesting forms of Potamo- 
geton which occur in the fens about Chatteris, and for many 
original and discriminating analyses. Elegant specimens of the 
Northern Scandinavian forms have been contributed by Dr. Gustaf 
Tiselius, of Stockholm, than whom no one is a better judge of the 
Continental species of Potamogeton. The late Prof. Caspary, of 
Konigsburg, Prussia, was another of my European correspon- 
dents, from whom was received many valuable specimens. The 
collections of the Harvard Herbarium, of Columbia College, the 
Academy of Sciences at Philadelphia, and at the National Her- 
barium at Washington have been freely submitted for examina- 
tion, as well as the large private collections of Mr. Canby, of 
Wilmington, Delaware, I. C. Martindale, of Camden, New Jersey, 
and others. Indeed, I may say that there is scarcely a collector of 
pond weeds in the country with whom I have not at one time or 
other held correspondence, and from whom I have not received 
specimens. And yet, notwithstanding these advantages for a wide 
comparison and a close study of this group of plants, so protean 
are their forms, so eccentric their action, constantly changing 
under changed conditions of season and water, that I put forth 
this treatise with great diffidence, and feel that the subject is very 
far from being exhausted 
