MPfNFUSS: CLASSIFICATION OF THE ALGAE 153 



Wallicli did not realize the significance of his discove^-. In 1869 Macdonald 

 and Pfitzer independently of each other i)ointed out that, since at division a 

 new valve and a new connecting band are formed within each of the two con- 

 necting bands of the parent cell, one of the daughter cells is smaller than the 

 other, which is of the same size as the parent. (That there occurs a decrease 

 in the size of the frustule of a species at each cell division was suspected previ- 

 ously by Griffith and Ilenfrey, 1856, p. 201.) Through continued division, cells 

 are thus formed whose dimensions are api)reciably below the maximum charac- 

 teristic of the species. Ultimately the cells would be too small to undergo further 

 division and the race would perish unless a periodic reestablishment of maximal 

 size occurred. Both Macdonald and Pfitzer considered the process of conjuga- 

 tion as probably providing the required rejuvenescence. This postulate gained 

 substance through the earlier observation of Braun (1851) that the cell de- 

 veloping from a zygote is larger than the parent cells. Pfitzer also noted that in 

 some instances a rejuvenating spore was produced by only one cell. Irrespec- 

 tive of their method of formation, Pfitzer called these spores auxospores (en- 

 larging spores). 



Two years later Pfitzer (1871) furnished abundant evidence favoring not 

 only the concept of a reduction in the size of diatom cells through vegetative 

 cell division but the reestablishment of the maximal size of the species through 

 the process of auxospore formation. In this work he also gave the first detailed 

 account of the living part of the diatom cell, the protoplast, which previously 

 had received only slight attention, and introduced the characters presented by 

 the plastids in the classification of these organisms. 



It will be recalled that largely because of their movement certain (or all) 

 diatoms were for a long time regarded as animals. The exact method of move- 

 ment of these organisms remained a matter of conjecture for more than a 

 hundred years after the first species were described. In a series of papers start- 

 ing in 1889 0. Miiller produced evidence that the movement of the cells is owing 

 to cytoplasmic streaming along the raphe. Although the detailed mechanics of 

 the process are not yet entirely understood, IMiiller's interpretation is still ac- 

 cepted as the most plausible explanation of the phenomenon. In 1895 Miiller 

 also published a valuable paper on the axial relations and planes of symmetry 

 in diatoms, and coined, among others, the terms epitheca and hypotheca to de- 

 note the larger and smaller halves, respectively, of the frustule. 



Klebahn in 1896, working on Rhopalodia gihha, a member of the order Pen- 

 nales, was the first to obtain cytological results suggesting that diatoms are 

 diploid and that meiosis occurs during gametogenesis. Further evidence of this 

 was produced by Karsten in 1899, and in 1912 he gave convincing proof of the 

 diploid nature of another member of the Pennales. Since that time various au- 

 thors (Geitler, 1927a, 1927b, 1928; Cholnoky, 1928, 1933a; Meyer, 1929; Subrah- 

 manyan, 1947) have confirmed the fact that the Pennales are diploid and that 

 meiosis precedes auxospore formation. Since a conjugation of cells was not 

 known to occur during auxospore formation in the Centrales, it was believed 

 for a number of years (cf. Oltmanns, 1922a) that these forms, to the contrary, 

 were haploid and that auxospore formation is an asexual process. The first per- 

 son to show that the Centrales were likewise diploid and that here auxospore for- 

 mation also is a sexual process (autogamy) was Persidsky (1929, 1935). His 



