DIATOMACE^. 



[ 249 ] 



DIATOMACE^. 



sulphuric acid, and also often bj drying ; 

 and H. L. Smith has shown that it exhibits 

 the spectroscopic reactions of chlorophyll. 

 Oil-globules, soluble in ether, are also found, 

 sometimes of large size, in particular stages 

 of growth, probably representing here the 

 starch-grains found in other Confervoids, 

 or indeed the oil which occurs in them and 

 other plants in seasons of rest. No starch 

 has been detected in this family. A trans- 

 parent rounded nuclear body is often ob- 

 served in the centre of the contents. 

 Schmidt found in Frusculia salina, after 

 removing the oil by ether and the proto- 

 plasiui by potash, a substance identical iu 

 composition with the cellulose of Lichens. 

 This was probably derived from the organic 

 matter of the silicified membranes of the 

 frustules. 



The ordinary mode of increase of the cells 

 of the Diatomacese is, like that of all other 

 vegetable cells, a process of division. In 

 Melosira, Isthnia, &c. this bears a close re- 

 semblance to the process which occurs in 

 Spiror/i/ra ; and it is only a modified form of 

 the same process that is found in the free 

 Diatomacete. It may be briefly described 

 thus : — the primordial utricle, enclosing the 

 contents, divides into two portions, which 

 separate from one another in a plane parallel 

 with the sides of the individual frustules ; 

 the two valves of the parent-cell gradually 

 separate from one another, remaining con- 

 nected by the simidtaneous gradual widen- 

 ing of the hoop. In the space thus afforded, 

 the two segments of contents secrete each 

 a new layer of very delicate, easily crum- 

 pled membrane without markings, which 

 ultimately becomes silicified over the sur- 

 faces, constituting two new half-frustules, 

 back to back, corresponding to and con- 

 joined with the two half-frustules of the 

 parent to form two new individuals. The 

 markings on the new valves are sometimes 

 different from those upon the parent. The 

 history and ultimate fate of the hoop seems 

 to be variable. Sometimes it becomes 

 solidly silicified, but not much expanded in 

 breadth, and falls oft" when the two frus- 

 tules are complete, allowing them to sepa- 

 rate ; this is the case in Pleurosigma, and 

 probabl)' in all the allied forms; these 

 hoops are often to be found in large num- 

 bers in the settlings of water in which 

 Diatomacese have been kept a long time. 

 Perhaps the most remarkable development 

 of the silicified hoop occiu's in Biddulphia 

 (PI. 19, fig. 9), Isthmia, and similar forms : 



the new half-frustules formed inside the 

 hoop of these genera slip out from it like 

 the inner tubes from the outer case of a 

 telescope. In Melosira (PI. 10. fig. 8) the 

 hoops appear to keep the new frustules 

 united together for some time ; and here 

 there is no overlapping. 



The development of the stipes to which 

 the frustules of many genera are attached, 

 is obscure ; Cox suggests that in Isth- 

 mia, a separate internal ceU exists, near 

 the point at which the frustule is attached, 

 having the office of secreting the gelatinous 

 substance of the stipes. 



The only mode of reproduction, besides 

 the division, known certainly to exist in the 

 Diatomacefe, is that in which conjugation 

 takes place. This has been observed in a 

 number of genera, and presents considerable 

 variation in its details. In Fraqilaria 

 (PI. 10. fig. 4) and Surirella (PI. 10. fig. 6) 

 the conjugation takes place between two free 

 frustiUes lying near together, each of which 

 opens at the suture and extrudes its con- 

 tents iu a mass (probably enclosed in the 

 primordial utricle) ; the masses of contents 

 coalesce, the whole meanwhUe becoming 

 involved in a mass of gelatinous substance. 

 After a while, the body resulting from the 

 conjugation is seen to assume the form of a 

 frustide, of larger size than the parents, 

 which Thwaites, the discoverer, called a 

 sporangial frustrde. In the majority of 

 cases, however, as iu Eunotia (PI. 10. fig. 6), 

 Gomphonema Cocconema, &c., the conjuga- 

 tion is double, as is the case in Closterium 

 lineatmn (Con-tugation) ; the contents of 

 the parent-frustules apparently divide into 

 two portions (as if for cell-division) before 

 conjugating ; and then there is a collateral 

 conjugation of the two pairs, two sporan- 

 gial frustules being the result. In Melosira 

 (PL 10. fig. 8) and Orthosira (PI. 10. fig. 9) 

 the appearances presented seem to indicate 

 that the conjugation takes place between 

 two segments of a frustide which have 

 separated as if for ordinary cell-division ; 

 but, instead of forming new half-frustules, 

 have coalesced again and secreted a coat 

 over the entire surface, thus constituting 

 one new independent sporangial frustule of 

 larger size. In Achnanthes and lihabdo- 

 netna, two sporangial frustules are formed 

 after the conjugation of the two halves of 

 one (just-divided) frustule. Probably this 

 is a case of very early division of the conju- 

 gation-body. In Melosira (PI. 10. fig. 8) 

 the conjugation-body has been observed to 



