194 SEAWEEDS 



former ones, it necessarily follows that every suc- 

 ceeding generation becomes smaller by just as much 

 as the thickness of the membrane at the girdle, since, 

 once formed, the Diatoms do not subsequently in- 

 crease in size. The character of size frequently used 

 by certain students of Diatoms must therefore be 

 fallacious, and no doubt the inordinate multipli- 

 cation of species in this group is partly due to 

 its use. 



When by this process, repeatedly carried out, a 

 certain minimum diminution of size is reached, the 

 original stature is regained by the formation of an 

 auxospore. This may be formed in one of five 

 different ways. (1) In the most simple form the 

 valves open and the contents (generally, but not 

 always, protected by a gelatinous envelope) emerge 

 and increase in volume. The auxospore so formed 

 soon acquires a thin membrane which is not silicified 

 (the perizonium). Two siliceous valves are then 

 formed, first one and then the other, within this 

 membrane, enclosing all the contents, and it is note- 

 worthy that these first valves, while possessing all 

 the ordinary characters of the species, exhibit slight 

 differences in the membrane from that of the 

 daughter-cells, subsequently produced by division 

 from this parent form in the manner described. (2) 

 Instead of one auxospore, two may be formed by the 

 division of the contents of the cell, and these two 

 naked cells emerge from the separated valves and 

 develop each into an auxospore. (3) An auxospore 

 may be formed after conjugation. Two Diatoms 

 secrete a common gelatinous envelope ; the contents 



