106 



Bacillariede 



In the filamentous colonies of the genus Melosira, O. Muller ('03) has observed cells 

 with thick walls and coarse markings, and other thin-walled cells with much finer and 

 differently disposed markings. In some cases, indeed, one valve of a cell would be of the 

 first type and the other of the second type. M tiller's first interpretation of these 

 differences was that he was dealing with mutations, although later he changed his view 

 to agree with that expressed by Gran that it was a case of polymorphism. 



Gran ('04) has recorded what he considers to be seasonal dimorphism in Rhizosolenia 

 hebetata, with a winter form, forma hiemalis, and a summer one, forma semispina ; but it 

 seems probable, as pointed out by Kofoid, that there are really two distinct species which 

 under the influence of certain environmental factors mutate the one to the other. 



REPRODUCTION BY AUXOSPORES. Diatoms reproduce themselves by a 

 type of spore known as an auxospore. Although formed frequently in some 

 centric forms, in the vast majority of diatoms auxospores do not appear to 

 be of common occurrence. The normal auxospore can be regarded as pro- 

 duced by the conjugation of two gametes, but differing from a zygospore 

 by its almost immediate increase in size. There is a sort of rejuvenescence 

 of the spore as soon as it is formed. Those auxospores produced without 

 conjugation are probably parthenogenetic. It has been commonly accepted 

 that the formation of auxospores counteracts the results of repeated cell- 

 division, whereby the individuals have been greatly reduced in size. This 

 view regards auxospore-formation as a method of regaining the maximum 

 size of the species. It is probable that the rejuvenescence is much more 

 important than the mere re-establishment of size, and it must be noted that 

 auxospores are not by any means always formed from the most diminutive 

 cells. In nearly all cases there is a considerable mucous secretion enveloping 

 those cells taking part in the formation of auxospores. 



There are five methods of auxospore-formation, the distinctions between 

 which were first clearly drawn up by Klebahn ('96). Karsten ('00) only 

 recognizes four types. 



(1) The protoplast of one of the smaller, reduced cells swells' up, forces 

 apart the halves of the cell-wall, and escapes through the rupture enveloped 

 in a thin cellulose membrane. This is the auxospore, which rapidly increases 

 in size and assumes an outward shape more or less like the original cell. 

 The wall soon becomes silicified and sometimes the markings characteristic 

 of the species are at once acquired (fig. 78 C and D). In other cases the 

 individual organized from the auxospore remains somewhat irregular, but 

 very soon undergoes cell-division. The new valves are much more perfect 

 in shape and sculpture, and the individuals of each succeeding generation 

 rapidly regain their characteristic form and elegance. Miquel ('92 '98), 

 from numerous experimental cultures of various species of diatoms, states 

 that the re-establishment of the maximum size is habitually brought about 

 by the formation of this simple type of auxospore. It is merely the re- 

 juvenescence of a single cell accompanied by an increase in size. 



