110 



Bacillariece 



evidence to show that the first and fourth types are probably of the most 

 frequent occurrence. The most essential feature in the formation of an 

 auxospore is the increase in size of the cell. The first method of auxospore- 

 formation is common in certain species of Melosira, and as the auxospores 

 grow into filaments of cells while still attached to the old threads, the dis- 

 crepancy in size between the new filaments and the old ones is at once 

 obvious (fig. 80 1}. 



The little evidence we possess concerning the cytological details of 

 auxospore-formation tends to show that those auxospores which develop 



Fig. 80. Auxospore-formation in the genus Melosira. 1, M. mtmmuloides Borr. ; 2 and 3, M. 

 Borrcri Grev. ; 4, M. varians Ag. (From Oltmanus, after W. Sm., Karsten, and Pfitzer.) 



from zygotes are sexually produced, and therefore undoubted sexuality exists 

 among diatoms. Such spores can be compared with the zygotes of the 

 Desmidiacese, but whereas the latter remain without rejuvenescence as 

 zygospores, the zygotes formed in the Bacillariege undergo an immediate 

 rejuvenescence to form auxospores. Moreover, if those auxospores produced 

 without conjugation are to be regarded as parthenospores, then the pro- 

 duction of parthenogenetic spores is much more frequent than the production 

 of zygotes in the Bacillariea?, whereas parthenospores are of very rare occur- 

 rence in the Desmidiacese. 



