PBEFACE. 



IN presenting the following memoir on the Fresh-water Alg*e of 

 the United States, we are keenly sensible of the great task we 

 have undertaken. The subject-matter is in an unsettled state, pass- 

 ing through a period of transition. There is an Old School and a 

 New School. The former accepts all forms as distinct plants, and 

 the latter rejects a large portion of the forms, mostly unicellular, as 

 only conditions of development. Suggestions for a new nomen- 

 clature have been made. The latest by Dr. Anton Hansgirg, Pro- 

 fessor in the Royal University of Prague ; but they seem premature. 

 There are too many questions of life-history remaining undeter- 

 mined. 



The most complete work representing the Old School, is Raben- 

 horsfs Flora Europaea Algarum (1864-1868). An admirable work in 

 its day, and indispensable for reference at the present time : but a 

 very large number of forms therein described, making up many 

 genera, some of which contain fifty or more species, are nothing 

 more nor less than conditions of development. 



Thuret of France has claims for later suggestions. He does not 

 reject the unicellular forms as mere conditions, but proposes to 

 reverse the order of arrangement by placing those more highly 

 developed, or those most nearly allied to the larger marine algse, 

 first in rank ; and then gradually descend to the lower forms. The 

 idea meets with favor. Dr. O. Kirchner in preparing a prodomus 

 of the Fresh-water Algse of Silesia (1878) adopted the proposition. 

 His work being imbued with the more modern ideas of the value, 

 (or valuelessness) of many forms, and being quite in accord with my 

 own observations and convictions, his volume has been a valued 

 companion, and I am largely indebted to Dr. Kirchner for the order 

 of arrangement followed in the preparation of the body of the 



