PREFACE. 



Of all the various branches of Natural History, none has been more enthusias- 

 tically and more successfully prosecuted in the United States than Botany. The 

 whole field has been most thoroughly occupied, save only as regards certain of the 

 lower cryptogams, and amongst the latter, it is the fresh-water Algre Avhich alone 

 can be said to have been almost totally neglected. In this fact lies my apology for 

 offering to the scientific public the following memoir. 



In doing this, so far from thinking that the work contains no error, I hasten to 

 disarm criticism, and to ask with solicitude for a favorable reception, in view 

 of the difficulties of the investigation, which I have conducted alone, and almost 

 unaided. 



The investigation was first undertaken in connection with my elementary studies 

 of Materia Medica and Therapeutics, and has since been prosecuted at intervals 

 amidst the distractions of medical teachings and practice, and in some cases Avith- 

 out immediate access to authorities. The field covered is so wide that it is almost 

 impossible to exhaust it, and, if it were not for rapidly increasing professional 

 engagements, I would gladly devote more time to the subject ; but, as it is, I must 

 leave to others to carry on the work thus begun. 



While saying this, it is but just to state that nothing here published has been 

 done hastily, but that all is the result of arduous and conscientious investigation. 



A very large part of my material has been of my oAvn gathering, and was 

 studied whilst fresh; but I am indebted to several persons fol' aid by collections. 



First of all, I desire to offer my thanks to Dr. J. S. Billings, U. S. A., and to 

 Professor Kavencl, of South Carolina; to the former for assistance in various 

 ways, and for collections made nrar AVashington City ; to the latter for very large 

 collections made in Texas, South Carolina, and Georgia. I am also indebted to 

 Mr. C. F. Austin for a large collection gathered in Northern New Jersey, to Mr. 

 William Canby for some beautiful specimens obtained in Florida, to Professor 

 Sereno Watson for Rocky Mountain plants, and to Dr. Frank Lewis for a number 

 of White Mountain desmids. 



These various collections were partly dried and partly preserved in a watery 

 solution of carbolic acid or of acetate of alumina, both of which I have found more 

 or less satisfactory preservatives. 



The present investigations embrace all families of the fresh-water algae except 

 the Diatomacece, which, as every one knows, are so numerous as to constitute in 



(V) 



