DIATOMACE.E. 



[ 200 ] 



DIATOMACE.E. 



is very improbable, otherwise they woukl be 

 met with frequently in other minute unicel- 

 lular organisms. No true vibratile cilia have 

 yet been detected upon the Diatomacefe, 

 although Mr. Thwaites imagined, from the 

 appearance of currents in the water, that 

 they exist on Bacillaria. Some are not un- 

 frequently found bearing tufts of or fringed 

 with rigid cilia, like those often seen at the 

 ends of the filaments of Oscillatorice ; these 

 would seem to be formed like the fringes met 

 with in the Desmidiacese, by a modification 

 of the gelatinous envelope ; they never ex- 

 hibit motion. 



In the foregoing paragraphs the Diato- 

 macese have been treated in reference chiefly 

 to their own peculiar characters. We 

 must not, however, pass over the physiolo- 

 gical relations of these organisms to other 

 families, nor omit to remark upon the un- 

 philosophical treatment they have received 

 at the hands of systematic naturahsts. 



In placing the Diatomaceae among plants, 

 we assume an agreement between the frus- 

 tule of a Diatomacean and an individual cell 

 of any undoubted vegetable, such as Proto- 

 coccus, and between the series of frustules 

 such as we find in Frayilaria (PI, 12. fig, 33), 

 or Melosir a (PI. 13. fig. 5 a), with the cellu- 

 lar filament of a Conferva or a Zygnema. 

 This agreement does undoubtedly exist, and 

 the siliceous shell is really only a result of 

 the incrustation or permeation by silica of a 

 true vegetable cell-membrane, just in the 

 same way as takes place in the epidermis of 

 Equisetum. It is not yet ascertained in 

 either case whether the silica is outside or 

 in the substance of the cell-membrane ; cer- 

 tainly it is not inside, as that would be in- 

 compatible with the known jjliaenomena of 

 division. It may be removed by hydrofluoric 

 acid, leaving the basement-membrane in 

 situ ; but this proves nothing. The proba- 

 bilities are that the substance of the mem- 

 brane is imbued with it. The application of 

 the term "epiderm" to the membrane (Smith) 

 is altogether inadmissible, as there is no ho- 

 mology w hatever ; and the supposition that 

 the reticulations on the valves of some ge- 

 nera denote a compound cellular tissue, is at 

 once without foundation in fact, and contrary 

 to what the general character of such or- 

 ganisms would lead us to expect, since we 

 find spores, pollen-grains, the outer walls of 

 epidermal cells, the membranes of the Des- 

 midiacese, &c., generally exhibiting patterns 

 of some kind dependent upon the mode of 

 development of the simple membrane form- 



ing their external coat. The cell-contents 

 of the Diatomaceaj require far more careful 

 study than they have yet received. It is 

 most probable that there exists a layer of 

 protoplasm, forming a primordial utricle, in- 

 side the cell-membrane, and enclosing the 

 rest of the contents ; the coloured substance 

 constituting the mass of the endochrome 

 appears to be a modification of chlorophyll ; 

 it takes a green or greenish-blue tint with 

 sulphuric acid, and also often by drying. 

 Oil-globules, soluble in aether, 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 de- 

 tected in this family.) A transparent rounded 

 bod}' is often observed in the centre of the 

 contents, and has been called a nucleus. 

 Schmidt found in FrustuUa salina, after re- 

 moving the oil by aether and the protoplasm 

 by potash, a substance identical in composi- 

 tion 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 Diatomaceae is, like that of all other 

 vegetable cells, a process of division. In 

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

 semblance to the process which occurs in 

 Spirogyra, and it is only a modified form of 

 the same process that is found in the free 

 Diatomaceans. It may be briefly described 

 thus : — the primordial utricle, enclosing the 

 contents, divided 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 simultaneous gradual widening 

 of the "hoop." In the space thus afforded, 

 the two segments of contents secrete each a 

 new la^-er of membrane (ultimately silicified) 

 over the surfaces where they are in contact, 

 which layers of membrane constitute two 

 new half-frustules, back to back, corre- 

 sponding to and conjoined with the two half- 

 frustules of the parent, to form two new in- 

 dividuals. The history and ultimate fate of 

 the "hoop" seems to be variable. Some- 

 times it becomes solidly silicified, but not 

 much expanded in breadth, and falls off 

 when the two frustules are complete, allow- 

 ing them to separate; this is the case in 

 Gyrosigma, and probably all the allied forms; 

 these "hoops" are often to be found in large 

 1 numbers in the settlings of water in which 



