Auxospore formation 107 



(2) Two auxospores may be produced from a single cell by the division 

 of the protoplast. Each portion increases in size, emerges from the old wall, 

 and finally develops as in the first method. This type of auxospore-formation 

 has only been observed in Achnanthes longipes (fig. 77 F) and Rhabdonema 

 arcuatum. 



(3) An auxospore may be formed by true conjugation between two 



Fig. 77. A D, Cocconeis Placentula Ehrenb. ; A, vegetative cell showing chromatophore (ch) 

 and nucleus (n) ; B D, conjugation to form auxospore ; In, large nucleus ; sn, small 

 nucleus ; B, two cells of Cocconeis before conjugation but after the division of the nuclei ; 

 in C the large nuclei are lying side by side, the smaller nuclei having disappeared, and in D 

 the large nuclei have fused. F, Achnanthes longipes Ag. showing formation of two partheno- 

 genetic auxospores from one protoplast. E, Surirella saxonica showing auxospore formed 

 by conjugation. (.4 D, and E, after Karsten ; F, after W. Smith.) 



individual diatoms. A large non-motile gamete (aplanogamete) emerges 

 from each cell by the rupture of the girdle, and the two gametes fuse to form 

 a zygote, which, as a rule, increases considerably in size, as in the case of 

 those auxospores formed without conjugation. The cytology of this con- 

 jugation has been worked out by Karsten ('00) in the two species Surirella 

 saxonica and Cocconeis Placentula. In the latter, division of the nucleus 



