SEXUAL RI PRODUCIION IN DIATOMS 91 



(Gcirlcr, 1935), in w hich one auxosporc is formed from a single cell. 

 The protoplast contracts, and two nuclei, two nucleoli, and tw^o chro- 

 matophores are formed. The valves of tlic cell are spread apart, and 

 the protoplast is transformed into an auxospore. Later there is found 

 only a single nucleus with a nucleolus. 



A modified type of autogamy occurs in AchnaJithes siibsessilis 

 and Govipboneina constrictiim var. capitata (Karsten, 1897, Geitler, 

 1952b). Within a single cell two gametes are formed which later fuse 

 to form a single protoplast which is transformed into an auxospore. 



Parthenogenesis is a method of auxospore formation in Cocconeis 

 plciceiituhi var. I'meata (Geitler, 1932). The nucleus of the parent cell 

 goes through two divisions, which correspond to the two meiotic 

 divisions except that reduction in chromosome number does not oc- 

 cur. Polar bodies are formed. The protoplast then becomes trans- 

 formed into an auxospore. Parthenogenesis is also known to occur in 

 one of the varieties of Cyinbella ventricosa. 



Asexual auxospore formation has been reported for Syiiedra 

 affinis (Karsten, 1897) and Rhabdonema arcuatwn. In these species 

 the mother cell divides by mitosis to form two daughter cells. These 

 protoplasts, instead of developing normal vegetative walls, become 

 auxospores. Further cytological investigation is needed to make sure 

 that this is truly asexual formation of auxospores (Fritsch, 1935). 



Sometimes two types of auxospore formation occur within a 

 single mass of copulating cells. For instance, in Goiiiphonevia par- 

 viihiin var. micropus three cells come together. One cell forms an 

 auxospore by automixis and the other two produce auxospores by 

 heteromixis (Geitler, 1932). 



The time interval for the fusion of gametes varies greatly. In 

 Navicula sevmiiihivj the fusion of gametes takes 2 to 3 minutes, 

 whereas in Ainphipleiira peUucida the process takes an hour (Geitler, 

 1932, 1952b). 



DEVELOPMENT OF ZYGOTE AND AUXOSPORE FORMATION 



On fusion of the gametes the zygote starts to develop. The fu- 

 sion of the nuclei is often delayed until the auxospore is developed. 

 In Navicula radiosa (Geitler, 195 2d), Navicula cryptocephala var. 

 veiieta (Geitler, 1952f) there are two pairs or four functional nuclei. 



