DIATOMACE^ 



423 



this way constantly diminish in size, until the original size is restored by 

 the formation of an auxospore, resulting from the concents leaving the 

 siliceous valves, which fall away from one another, and increasing in 

 size, either by simple growth or by the coalescence of two auxospores 

 produced in the same mother-cell. In other cases two distinct auxo- 

 spores appear to be produced from the contents of a single mother-cell. 

 The auxospore finally becomes invested in a new siliceous cell-wall. In 

 those cases in which the process has been most carefully followed out, 

 the auxospore does not appear to owe its origin to any process of true 

 sexual union. 



In some genera what is regarded by some as a true process of con- 

 jugation has been observed, a zygosperm being produced as the result of 

 the coalescence of the protoplasmic contents of two different individuals. 

 The conjugating diatoms are here placed 

 side by side enclosed in a common gela- 

 tinous sheath ; the contents of each escape 

 by the falling apart of the two valves, and 

 unite into a single zygosperm. In other 

 cases two zygosperms result from the con- 

 jugation of a pair of cells. The protoplasm 

 of each cell, as it escapes from its siliceous 

 wall, puts out two protuberances ; these 

 meet in pairs, and the whole contents of 

 the pair of mother-cells finally pass into 

 the two zygosperms, which complete their 

 development in precisely the same way as 

 the auxospores. Buffham states (Journ. 

 Quek. Micr. Club, 1885, p. 131) that in the conjugation of Rhabdomena 

 (Ktz.) the 'male' frustule is always smaller than the 'female' frustule, 

 and that the union is effected by the 'male' frustules attaching them- 

 selves in numbers to any part of the. girdle of the 'female' frustule. 

 De Bary and Pfitzer do not regard the fusion of the cell-contents 

 of diatoms as in any sense a true process of sexual conjugation. De 

 Bary (Bot. Zeit., 1858, Supplement, p. 61) thus summarises the four 

 modes in which diatoms are reproduced by means of auxospores or 

 zygosperms : — (i) Two products of conjugation are formed by the union 

 of the contents of two distinct individuals; (2) a similar process results 

 in the formation of a single product of the same nature ; (3) a single 

 act of conjugation (production of auxospore) takes place between two 

 portions of the contents of the same individual ; (4) two such acts of 

 conjugation take place simultaneously between different portions of the 

 contents of the same individual. In all cases the formation of a new in- 

 dividual is completed by the simple division of the product of union 



Fig. 



'.onema constric- 

 ^wwz Ehrb. attached by gelatinous 

 stalks to a fresh-water alga 

 (greatly magnified). 



