284 MICROSCOPIC FORMS OF VEGETABLE LIFE. 



the exact counterpart of the one to which it is opposed, and 

 forms with it a complete cell, so that the original frustule is 

 replaced by two frustules. Sometimes, however, the new valves 

 seem to be a little larger than their predecessors ; so that, in the 

 filamentous species, there may be an increase sufficient to occa- 

 sion a gradual widening^of the filament, although not percepti- 

 ble when two contiguous frustules are compared ; whilst, in the 

 free forms, frustules of difierent sizes may be met with, of which 

 the larger are more numerous than the smaller, the increase in 

 number having taken place in geometrical progression, whilst 

 that of size was uniform. It is not always clear what becomes 

 of the "hoop." In BiddulpMa and Isthmia (I'ig. 96) the two 

 young cells slip out of it, and the hoop at last becomes com- 

 pletely detached ; and the same thing happens with many other 

 Diatoms ; so that the "hoops" are to be found in large numbers, 

 in the settlings of water in which they have been growing for 

 some time. In Meloseira (Figs. 97, 98), and perhaps in the fila- 

 mentous species generally, on the other hand, the "hoops" 

 appear to keep the new frustules united together for some time. 

 But in some other cases, all trace of it is lost ; and it may be 

 questioned whether it has ever been properly silicified, and 

 whether it does not become fused (as it were) into the gelatinous 

 envelope. During the healthy life of the Diatom, the process of 

 self-division is continually being repeated ; and a very rapid 

 multiplication of frustules thus takes place, all of which (as in 

 the cases already cited, §§ 150, 165) must be considered to be 

 repetitions of one and the same individual. Hence it may hap- 

 pen (as among the Desmidiacese, § 168) that myriads of frustules 

 may be found in one locality, uniformly distinguished by some 

 peculiarity of form, size, or marking ; which may yet have had 

 the same remote origin as another collection of frustules, found 

 in some difiierent locality, and alike distinguished by some pecu- 

 liarity of its own.. For there is strong reason to believe, that 

 such difierences spring up among the progeny of any ti-ue gene- 

 rative act (§ 178) ; and, when that progeny is dispersed by cur- 

 rents into different localities, each will continue to multiply its 

 own special type, so long as the process of self-division goes on. 

 177. It is uncertain whether the Diatomacese also multiply by 

 the breaking up of their endochrome into "gonidia," and by 

 the liberation of these, either in the active condition of " zoo- 

 spores," or in the state of "still" or "resting" spores. Certain 

 recent observations by Focke,' however, taken in connection 

 with the analogy of other Protophytes, and with the fact that 

 the sporangial frustules undoubtedly thus multiply by gonidia 

 (§ 178), seem to justify the conclusion that such a method of 

 multiplication does obtain in this group. And it is not at all 

 improbable, that very considerable differences in the size, form, 



' " Physiologisch. Studien,'' Heft ii, 1853; quoted in " Micrographical Dictionary,'' 

 p.201. 



