CONCERNING NAVICULA SOCIALIS. 



BY T. CHALKLEY PALMER. 



In the original notice* of Navicula socialis. Palmer, 

 emphasis was laid on the peculiar grouping of the frustules 

 into motile chains consisting, as a rule, of four individuals. 

 It was remarked that " a history of the life-cycle of this 

 diatom, which will necessarily include an account of the 

 formation of these groups, is much to be desired." Since 

 the date of that publication, many new occurrences of the 

 species have been noted ; and while the life-cycle is at this 

 time far from being completely understood, further study has 

 served but to confirm the first conclusion that the unprece- 

 dented grouping into blocks of four is not fortuitous, occa- 

 sional or temporary, but a constant and essential character- 

 istic. The origin of these groups is still to be explained, but 

 their persistence is now well authenticated. 



That the frustules continue in this grouping is shown by 

 the fact that groups have been collected during every month 

 of the year in a certain spring in the neighborhood of the 

 writer's home. They occur there but sparsely in association 

 with a strong and persistent growth of Navicula Iridis. In 

 that isolated and sheltered pool of living water the little 

 groups of four continue their placid existence, undisturbed 

 by the snows and fierce frosts of winter, the thaws and floods 

 of spring, the desiccating heats of summer. Immersed in a 

 great gulf (about ten gallons) of purest liquid of a nearly 

 uniform temperature, and well shaded from the direct sun- 

 rays, the trouble of the seasons passes them by. 



The essential nature of the grouping is shown first of all, 

 as it seems to me, by the fact that, while the length of the 

 group will vary from i20ju. down to 60/u., the frustules of a 

 givoi group are all of the same size. These groups must 

 therefore be composed of individuals of a common and siraul- 



*Proc. Acad. Nat. Sci., Pliila., 1910, p. 460. 



