Method of Study 137 



identify them, as far as possible, with previously described species, 

 paying especial attention to the historical interpretation of the 

 latter. 



One great lack in the study of such forms as those under con- 

 sideration, is the want of correlation of observations. Here, more 

 than in higher forms, different phases of the same plant are likely 

 to be identified as different species ; again, because of a supposed 

 resemblance between different forms, or, because two or more 

 really diverse filaments are found growing together, they are asso- 

 ciated under one name, without any proof of connection. 



In order to obviate this difficulty, and to place species defini- 

 tion upon a firmer basis, we have made as thorough a study as 

 possible, considering the large number of forms treated, of life 

 histories. Our rule has been to make collections repeatedly from 

 the same station, or rather from several stations, as well as to 

 follow such collections by laboratory observation. 



Extended culture is a difficult matter in the case of algae like 

 these which generally grow in rapidly running water. Such 

 plants usually develop differently in a stagnant culture. To fit up 

 a laboratory with a sufficient number of tanks supplied with fresh 

 water would be expensive and not always feasible. A method, 

 which has proved to be of value in certain cases, might profitably 

 be employed further. We have sometimes placed fresh specimens 

 in glass cylinders (e. g., large bottles with the bottoms knocked 

 out), covered both ends with thin muslin, and anchored them 

 near the surface in a running brook. Frequent examinations of 

 such cultures may easily be made. Some plants do not take 

 kindly to even this sort of captivity, perhaps because the change 

 in the composition of the water is not beneficial ; others may be 

 successfully kept until after important observations have been 

 made. Many species have refused to disclose their reproductive 

 processes even though carefully watched, but some advance in 

 this direction has been made. 



As a rule, new species have not been proposed except in cases 

 where repeated collections from the same stations have admitted 

 of no doubt regarding the validity of the form in question. In one 

 or two instances very marked forms have been described as new 

 upon the strength of only one collection. Many specimens ap- 



