INTRODUCTION. 



Although beset with difficulties in the outset, no branch of natural science 

 offers more attractions, when once the study is fairly entered upon, than the fresh- 

 water algce. The enthusiasm of the student will soon be kindled by the variety 

 and beauty of their forms and wonderful life processes, and be kept alive by their 

 abundance and accessibility at all seasons of the year ; for unlike other plants, the 

 winter with them is not a period of counterfeited death, but all seasons, spring, 

 summer, autumn, and winter alike, have their own peculiar species. They have 

 been found in healthy life in the middle of an icicle, and in the heated waters of 

 the boiling spring ; they are the last of life alike in the eternal snow of the moun- 

 tain summit and the superheated basin of the lowland geyser. 



In their investigation, too, the physiologist can come nearer than in almost any 

 other study to life in its simplest forms, watching its processes, measuring its forces, 

 and approximating to its mysteries. Sometimes, when my microscope has revealed 

 a new world of restless activity and beauty, and some scene of especial interest, as 

 the impregnation of an oedogonium, has presented itself to me, I confess the 

 enthusiastic pleasure produced has been tempered with a feeling of awe. 



To any on whom through the want of a definite pursuit the hours hang heavy, 

 to the physiologist who desires to know cell-life, to any student of nature, I can 

 commend most heartily this study as one well worthy of any pains that may be 

 spent on it. 



An aquarium will often, in the winter time, give origin to numerous interesting 

 forms, but it is not a necessity to the fresh-water algologist; besides his microscope 

 and. its appliances, all that he absolutely needs is a few glass jars or bottles and 

 the fields and meadows of his neighborhood. 



The great drawback to the investigation of these plants has been the want of 

 accessible books upon them. In the English language there is no general work 

 of value, and the various original memoirs are separated so far and wide in the 

 Continental and English journals, as to be of but little use to most American 

 readers. The Flora Europceum Algarum Aquoi Dulcis et Suhmarince, of Prof. Ra- 

 benhorst, has done much to facilitate the study, and its cheapness brings it within 

 the reach of all. It merely gives, however, brief diagnoses of the various species, 

 but with the present memoir will, I trust, suffice for the American student, at least 

 until he is very far advanced in his researches. 



1 November, 1871. ^ 1 ^ 



