26 Clarence J. Elmore 



The location of individual diatoms on the slide may be indi- 

 cated by drawing a circle around each specimen on the coverglass. 

 Sanford's Indelible Stamping ink has been found satisfactory for 

 this purpose. It may be applied with a pen and does not run on 

 the glass. When dry it is not much affected by the oil of an oil 

 immersion objective. If there are several specimens marked on 

 one slide, a diagram of the location of each may be drawn on the 

 label, indicating each species by a number. 



RELATIONSHIP 



The systematic place of diatoms is a matter about which there 

 has been much disagreement. They were formerly regarded as 

 animals and placed among the Infusoria. They are now uni- 

 versally regarded as plants, but there seems to be no general agree- 

 ment as to their relationship. The placing them among the Brown 

 Algae was based merely on their color, which is probably only an 

 accidental or perhaps a physiological resemblance and has no sys- 

 tematic significance. That they are a derived and not a primitive 

 group is indicated by the fact that they appeared comparatively 

 late in geological time, none being known earlier than the Devo- 

 nian. And since their appearance very little change has taken 

 place. It is possible that they have evolved through earlier periods 

 without their siliceous cell walls, and suddenly, as if by mutation, 

 their walls became silicified. Hence all of their earlier history is 

 lost, and at their first appearance in geological time they had 

 reached their present form. 



Forms like Lysigonmm (Mcloscira) seem closely related to the 

 unsilicified filamentous algae and are probably derived from them. 

 And the round diatoms in which the cells are separate may be con- 

 sidered as the cells of a filament which have become separated. 



In the filamentous diatoms that form ribbon-like bands, instead 

 of cylindrical filaments we have the same structure except that the 

 filaments have become flattened. And the single-celled long (flat) 

 diatoms may be considered as the cells of such a filament broken 

 apart. 



Whatever views various botanists have as to the relationships of 

 diatoms, they are nearly all agreed that diatoms and desmids are 



