The Diatoms of Nebraska 27 



closely related. By their mode of forming zygospores, desmids 

 are closely related to the filamentous Conjugatae, e.g., Spirogyra. 

 And some diatoms in their formation of auxospores have a form 

 of conjugation very similar to that of desmids. For this reason it 

 seems best to place both diatoms and desmids with the filamentous 

 Conjugatae under the phylum Zygophyceae, making of the un- 

 silicified groups a separate class and of the siliceous ones a class 

 of equal rank, the Bacillarioideae. 



THE USE OF THE TERM VARIETY 



Diatoms vary so greatly in size and form between one auxospore 

 stage and the next that the early collectors who based their species 

 on size and form alone described as species many conditions that 

 are mere stages in the life history of a single species. The size 

 necessarily decreases with each cell division until an auxospore is 

 formed, except in those genera whose valves do not overlap, be- 

 cause each new valve is formed inside of an old one. If one part 

 of the girdle is thicker than the rest, the new valve formed inside 

 this valve will be slightly constricted at this point; and this con- 

 stricting process will continue, making the constriction deeper and 

 deeper with each division until the next auxospore stage is reached. 

 If the thickening is at the ends, the valves will grow proportionally 

 shorter with each division. So some species occur in many con- 

 ditions that have been described as distinct species. The earlier 

 works recognized few varieties, making a separate species for each 

 condition in which a species was found. Later writers have 

 brought together many of these so-called species and called them 

 varieties, or in some cases, forms. But the term variety can 

 scarcely be applied to diatoms. The species of diatoms present 

 the same varieties now that they did in Tertiary times; so if these 

 varieties are thus fixed, they are not varieties, but species. And 

 those that merely represent conditions between two auxospore 

 stages are not entitled to a separate name any more than an oak 

 tree two feet high deserves a name separate from the name given 

 to an older tree of the same species. 



It is only by a most thorough and careful study of any species 

 that all of the forms in which it may appear can be determined. 



