THE GREEN ALGAE OF NORTH AMERICA 83 



that we are fully acquainted with any region ; even on the New 

 England coast, which has been studied so long, species new to 

 the region are continually turning up. 



Not only in comparing different regions, but in comparing 

 different families and genera, the treatment must be unequal. 

 For the family of the Oedogoniaceae, for instance, we have 

 Hirn's admirable monograph, based on an ample supply of 

 material, American as well as foreign; from this a very satis- 

 factory account of this family can here be given, but with the 

 proviso that species described by Him from extra-American 

 localities will probably some time be found here. Of the 

 genus Chlamydomonas Wille has published a thorough re- 

 vision, but with no reference to American material or locali- 

 ties ; little attention has been given to the genus here ; only 

 fresh material can be used for study, so that practically the 

 genus will remain unrepresented. A case different from both 

 these is presented by the genus Cladophora ; probably more 

 species have been described in this genus than in any other 

 genus of algae, except possibly some of the minute diatoms and 

 desmids. Cladophoras abound everywhere, in fresh and salt 

 water ; herbarium specimens are practically as good for pur- 

 poses of determination as the living plants ; and yet this genus 

 presents more difficulties than any other treated in this work. 

 Species have been described recklessly, from imperfect material, 

 often with little or no consideration of what had before been de- 

 scribed ; the plants vary much in response to environment, and 

 the individual plant may vary much at different stages of 

 growth, but except in a very few instances we have no investi- 

 gations to show just what forms belong in the same life cycle. 

 These three instances show the inequality that exists, and must 

 long exist, in our knowledge of the different types about to be 

 considered. 



Under each genus will be found a key, for convenience in 

 identifying plants under consideration. These keys are based 

 as far as possible on the more obvious characters, which are 

 not necessarily the more important systematically, but the sys- 

 tematic characters will be found in the appropriate place under 

 each species. The characters used will apply only to the 



