harper: nature of types in pediastrum 103 



historical types the nomenclatorial grouping might remain perma- 

 nently at variance with a natural classification. It is to be re- 

 membered, however, that the careful student of to-day always 

 endeavors to arrange all available material in groups, based on 

 conceptions of phylogenetic types. So far as is possible he selects 

 as typical those specimens which illustrate most perfectly the 

 fundamental characters of the group for which they are to stand 

 in the type relation. The aim of any student of a group of genera 

 or species is to determine what are the natural types and his work 

 is likely to be lasting or ephemeral in direct proportion to his 

 success judged by this standard. 



The possibility of discovering biological form types for the 

 bewilderingly complex individuals and groups found among the 

 higher plants may well seem too remote a possibility to have any 

 practical significance, even if such types can be conceived as 

 having any specific reality. The statement that every individual 

 is equally normal in the sense that it as much as any other is the 

 product of its environment plus its inherited constitutional 

 characteristics is even taken to mean that every individual is 

 equally typical of the "interbreeding group of blood relatives" 

 to which it belongs. 



The types set up by breeders of domesticated animals are arti- 

 ficially determined and may or may not coincide with what is natural 

 for the race. Types determined by anthropometrical measurements 

 give very exact pictures of the dimensional characteristics of a pop- 

 ulation but are not, directly at least, referable to biological prin- 

 ciples of organization further than as they illustrate general tend- 

 encies to harmonies and adaptations in structure and functions. 



What I have suggested as to the possibility of recognizing the 

 form type of a semi-coenobe like Pediastrum is much more ob- 

 viously true in the case of such simple filamentous coenobes as we 

 find in the free floating species of Spirogyra and other representa- 

 tives of the Conjugatae. Here the principle of organization of 

 the colony is of the simplest. We have merely a sort of cellular 

 metamerism, the exactly similar units being repeated in an 

 indeterminate series. The oblong cylindrical cell form is trans- 

 mitted directly in cell division and the axis of cell growth regularly 

 persists through sexual cell fusion, zygospore formation, ^ and 

 germination. The arrangement of the cells is determined simply 



