REPORT ON THE DIATOM ACE. E. 7 



of the frustules increase by reason of the process of fission. Dr Pfitzer ' of Bonn, by 

 denying the possibility of such growth, was induced to imagine the theory of the existence 

 of auxospores, and to believe that these constituted the only real reproduction of Diatoms 

 by a sexual process. According to this observer, after the frustules had, in the succession of 

 divisions, reached the smallest dimensions compatible with the species, the contents of the 

 small frustules escaped and united with each other so as to form one or two sporangia. 

 Within these one or two sporangial frustules called auxospores appeared, and by their 

 larger dimensions brought the Diatom to the beginning of another series of new graduated 

 forms. But such a theory, taken in a general sense, and assumed as the process of repro- 

 duction common to all genera of Diatoms, is fundamentally false, because it rests on the 

 gratuitous supposition that the diatomaceous walls are incapable of any increase in size. 

 That such an increase, however, does take place has been proved by the Rev. Professor 

 W. Smith in his classic Synopsis of the British Diatomacese (plate lii. fig. 335), where 

 some sporangial frustules of Orthosira diclcieii are represented in which it is evident 

 that the siliceous walls increase with the growth of the contents. It is also to be noted 

 in this connection that the distinguished Hugo von Mohl maintains that the cytoderm of 

 a Diatom is not entirely inorganic, but only an organic membrane which is impregnated 

 with silex, it having been already shown that silica is sometimes substituted for carbon in 

 the formation of cellulose. 



Again, if during the process of duplication an expansion of the cell wall did not take 

 place, a hundred frustules of a Fragilarian filament would exhibit some difference in their 

 longitudinal diameter. But no such difference is observable. Moreover, as it is impossible 

 to understand the formation of the two new dividing walls in the centre of the parent cell 

 in all their minute details unless it be admitted that the new frustules are stereotyped 

 upon the old ones, it follows that such a process cannot be verified except in the genera in 

 which the two valves of the frustule are perfectly identical in a symmetrical position. 

 It is to be remarked that fissiparous division has not yet been observed in a single case 

 to form an exception to the above rule. 



Finally, against the theory which regards the sporangial frustule as destined to initiate 

 a new descending series, Dr Wallich' 2 remarks that that frustule, instead of being, as 

 heretofore assumed, the primary or parent frustule of a new and vigorous generation, 

 constitutes in reality the expiring phase in the life of an old one. Professor H. L. Smith 

 seems to be of the same opinion, for, considering the possibility of Stauroneis phcenicen- 

 teron, Ehrenb., being a sporangial form of Stauroneis gracilis, Ehrenb., he points out that 

 it is only an abnormal and transient form from which the Stauroneis gracilis, differing 

 from it so much as it does, could never originate. 



1 Untersuchungen iiber den Bau und Entwickelung der Bacillarien (Diatornaceen), Bonn, 187E 



2 On the Relation between the Development, Reproduction, and Markings of the Diatomaceae, Month. 

 Micr. Journ., vol. svii. p. 61, 1877. 



