SEXUAL Rl PRODUC'lION IN DIA lO.MS 83 



coniplcrc process has been observed only in a relativly few species, it 

 is parriallv known in a great many more. Because many species are 

 \ cr\- small and their chromosomes are numerous, detailed cytological 

 studies arc difficult. The best summaries of this work have been made 

 by Geitler (1932) and Fritsch (1935). 



The diatoms may be divided into tw^o distinct taxonomic groups. 

 Methods of auxospore formation characteristic of each taxonomic 

 group \\ill be treated under the appropriate heading. 



CENTRALES 



The Centrales are usually regarded as the more primitive group 

 of diatoms because they are found in earlier geological strata than the 

 Pennales. 



The most common type of reproduction is fission after mitosis 

 Microspores are also produced in this group, but as yet it has not been 

 proved that they are reproductive cells. They are formed by the 

 division of the protoplast. Each microspore has two flagella. 



Other types of reproduction are those which result in auxospore 

 formation. Little is as yet known concerning the nuclear reorganiza- 

 tion that takes place in their formation. 



The simplest type of auxospore formation is that which has been 

 reported in the genus Melosira. The two halves of the wall of the 

 cell are pushed apart by the protoplast. Over the protoplast is secreted 

 a slightly silicified pectic membrane called the perizonium. After a 

 lapse of time new valves and connecting bands are formed inside the 

 perizonium, and a new individual results. In Melosira mnmmdoides 

 the auxospore lies outside the theca of the parent. There is consider- 

 able difference of opinion about the nuclear phenomena that accom- 

 pany^ auxospore formation in this genus (Karsten, 1897; Geitler, 

 1932). 



In Biddidphia mobiliensis (Bergon, 1907) cell division imme- 

 diately precedes auxospore formation. The two daughter protoplasts 

 escape from the parent cell and form a pair of spores. Little is known 

 about the nuclear behavior during this process. 



In Chaetoceras cochlea (Fritsch, 1935) the auxospores arise 

 laterally on the parent cell by budding. A similar lateral formation 

 of auxospores also takes place in some species of Rhizosolenia (Schiitt, 

 1893). 



