TJic Diatoms of Nebraska 23 



The simplest manner of auxospore formation is by the proto- 

 plasmic contents of a cell escaping from its siliceous shell and 

 growing to the normal size for the species and then secreting a 

 new shell about itself. Or in some cases two cells are formed in 

 this way from a single mother cell. Another method is for the 

 contents of two cells to escape and conjugate, forming in some 

 cases one, and in other cases two, auxospores. 



Since diatoms multiply with great rapidity, the diminution in 

 size ought to require the very frequent formation of auxospores. 

 But the actual condition is that they are of comparatively rare oc- 

 currence. The discrepancy arises from the fact that the foregoing 

 description of a diatom is only partly true. In some cases the 

 girdle of the old valve inside of which the new valve is formed 

 enlarges so that the new valve is as large as, or larger than, the 

 old one. In Lysigonium {Melosira), a genus in which the cells 

 adhere together, long filaments are formed which are practically 

 uniform in size. Otto Miiller, Die Zellhaut unci das Gescts dcr 

 Zellteilungs folge von Meloscira arenaria Moore, has shown that 

 in this species the girdle is enlarged according to a regular law, so 

 that the minimum size is very seldom reached. And according to 

 this law only one auxospore is formed, where, if the diminution in 

 size took place regularly, 1,052,100,000,000 would be formed. 

 This easily accounts for the infrequent occurrence of auxospores 

 in this species. 



In Fragilaria, another genus in which the cells adhere in bands, 

 the girdles can be seen meeting, but not overlapping, and in this 

 genus, too, there is no apparent diminution in size of the filaments. 



Diatomite, when pure, is a white or grayish substance so light 

 that it floats on water. It is often mixed with other substances. 

 In Nebraska it usually contains more or less calcium carbonate, the 

 amount varying from a slight trace to so much that it becomes a 

 hard limestone with comparatively few diatoms. The deposit 

 from Greeley County, however, is practically pure. In our region, 

 at least, it is not often mixed with other substances than calcium 

 carbonate. Conditions favorable to the growth of diatoms were 

 favorable to the growth of the organisms producing the carbonate, 

 and conditions under which inorganic sediment was formed were 



