DIATOMACE^E. 



[ 260 ] 



DIATOMACE^E. 



increase by cell-division, and form a new 

 filament of far greater diameter than that 

 to which it owed its birth. The sporangial 

 frustules of the free forms doubtless increase 

 by cell-division in the usual way (see CON- 

 JUGATION). 



In some cases the first product of the 

 conjugated mass is a siliceous sheath, inside 

 which a new frustule is developed, and 

 finally set free by dehiscence of the sheath 

 (see NAVICULA). 



A great difficulty meets ua here. The 

 necessary consequence of the conjugation 

 just described is, that every species in 

 which it occurs must be represented by two 

 forms, one small and the other large, be- 

 tween which a gap exists, over which we 

 have at present no means of " 



cept by supposing that the two new 

 formed in cell-division need not always be 

 equal, and that, by a dwindling away through 

 a succession of steps of this kind, the pro- 

 geny of the sporangial frustules may be re- 

 duced to the original size. The size of the 

 frustules is said also to vary with the depth 

 of the sea, in marine species. The effect of 

 all this seems to have been disregarded in 

 systematic treatises on the Diatomaceae. 

 Some of the book-species appear to produce 

 other book-species by conjugation ; accord- 

 ing to Focke, Surirella spkndida produces 

 S, bifrons, a very distinct form ; and it is not 

 improbable that S. spkndida is produced by 

 the conjugation of S. Microcora (Focke). 

 There is great probability, however, that 

 the observations made by Focke upon the 

 contents in certain species will lead to the 

 discovery of another mode of increase 

 a reproduction by gonidia, either active 

 or quiescent, such as occurs in the Desmi- 

 diaceae and the other Confervoids. Indeed 

 the contents of the cells of Melosira have 

 been observed to display a motion like 

 1 swarming.' Such spores or gonidia dis- 

 charged from the large ' sporangial ' frus- 

 tules might reproduce the small form, just 

 as the young filaments developed from the 

 zoospores of Chcstophora &c. are very slen- 

 der compared with those of full-grown fila- 

 ments. Focke describes and figures appear- 

 ances in the contents of the frustules of 

 Pinnularia viridis, Surirella bifrons, and 

 others, very like what occur occasionally in 

 the cell-contents of Closterium, namelv en- 

 cysted globules (PI. 10. fig. 10) resembling 

 the resting-spores of Volvox and the fila- 

 mentous Confervas ((EDOGONIUM) j and he 

 considers that such bodies produced in S. 



bifrons may probably reproduce S. Micro- 

 cora. In some of Thwaites's figures of 

 conjugating Diatomaceae (PI. 10. fig. 6), 

 there are appearances which would lead to 

 the idea that spores were occasionally pro- 

 duced in this process. 



The principal attraction of the Diatoma- 

 cese to microscopists, however, lies at 

 present in the structure of the siliceous 

 coats. 



Some remarks upon the method of ren- 

 dering the markings visible have been made 

 in the INTRODUCTION, p. xxviii (IttumitM- 

 tiori) ; and upon the cause of their becoming 

 visible under proper illumination, in the 

 article ANGULAR APERTURE. The grounds 

 for the belief that most the finer markings 

 are depressions have also been mentioned 

 (INTRODUCTION, p. xxxix, I.). But the 

 general view now entertained is, that the 

 finer markings of the valves are the optical 

 expression of hemispheres of silica (PL 15. 

 fig. 46) ; although it is curious, that neither 

 Schultze nor Abbe agree with this view. 

 Yet the formation of the various costae, 

 vittae, and other siliceous structures in the 

 valves, cannot be referred to any simply 

 crystalline or crystalloid hemispherical de- 

 position. 



Preparation of the valves, to render them 

 as distinct as possible, is essential. This 

 may be effected in two ways : 1. By inci- 

 nerating them upon a very thin plate of 

 mica or platinum foil over the flame of a 

 spirit-lamp. This is the quickest "method ; 

 but it has the disadvantages of the valves 

 often becoming semifused or agglutinated 

 to each other by the effects of the heat in 

 the presence of the alkaline salts contained 

 in all organic matters, especially those which 

 are of marine origin. 2. Boiling with strong 

 nitric acid; this is the best method. The 

 water containing the Diatomaceae is allowed 

 to settle for twenty-four hours, the superna- 

 tant liquid poured off, and the deposit dried 

 in a porcelain dish. Strong nitric acid is 

 then added, the whole mixed with a feather 

 or glass brush, and poured into a flask or 

 test-tube and boiled tor some time, a portion 

 being removed occasionally with a narrow 

 dip-tube to determine when the valves are 

 perfectly clean. When this is the case di- 

 stilled water is added to the mixture, and 

 the whole allowed to settle. The super- 

 natant liquid is then carefully decanted, 

 more water added, and the mixture again 

 allowed to settle, poured off, and these ope- 

 rations repeated until a drop of the liquid 



