248 Transactions of tJie [Sess. 



(3) sporular multiplication. In the first of these processes, the 

 nucleus divides, then the soft protoplasmic substance, and finally 

 a wall is formed along the divisional plane, in connection with 

 which the siliceous matter for the valve of each daughter frustule 

 is deposited. As a result of conjugation, according to Smith, 

 two parent frustules may give rise to one or two sporangia ; or the 

 valves of a single frustule may separate, and the contents increase 

 so as to form two sporangia, or in other case finally condense into 

 a single sporangium. In the case of sporular multiplication, which 

 has been regarded as sufficient to account for the enormous multi- 

 tudes of some species, — the other modes of multiplication explaining 

 the rarity of others, — the protoplasmic contents break up into a 

 number of sporules, which form the starting-points of new frustules. 

 Examples of this have been observed by O'Meara, Castracane, and 

 others. The exact nature of the so-called auxospores has been 

 disputed. By Pfitzer they are looked upon as the starting-point of 

 a new descending series of forms, being produced only after any 

 given species has been reduced to its minimum size by successive 

 temnogenetic processes. This interpretation of their function thus 

 implies that increase of the silicified cell-wall does not take place 

 during or subsequent to fission. But Smith has given figures of 

 frustules that go to show that such increase may go on ; and the 

 belief has been held by some that the auxospores are more pro- 

 perly abnormal structures, to be regarded rather as the expiring 

 phases of the Diatom than as the means of inaugurating a new 

 and vigorous series of forms. 



From the geological point of view, Diatoms play an important 

 role. Their habits as marine or fresh-water may be made use of in 

 determining the exact mode of origin of geological deposits ; while 

 the enormous banks of frustules in course of formation, found in 

 recent times by Hooker in the Antarctic Ocean, and more lately 

 by the naturalists of the Challenger/ go to show how these great 

 deposits may have originated. The persistence of some genera 

 and species from Carboniferous or even Silurian times to the 

 present day is noteworthy. 



It is of great importance to observe that Diatoms sometimes 

 occur on the surface of the sea in enormous masses, and give a 

 characteristic tinge to the ocean water. This is not unfrequently 

 the case with Rhizosoleni^ and Thalassiosira, isolated specimens 

 of which not unfrequently occur in the Firth of Forth ; and such 

 aggregations are of importance from the fact that they may influence 

 the formation of deposits at the mouths of rivers, of sediment in 

 harbours, and so on. 



It may be interesting to append a short list of some of the 

 species of Diatoms which have recently been observed floating on 



^ ' Proc. Roy. Soc. of London, ' vol. xxiv. p. 533. 



