302 DESCRIPTION OP [^foli/gastrica. 



Instead; also, of the pair of conjugated frastules producing between 

 them two sporangia, they may develope but a single one, as happens 

 in Fragilaria pedinalis. In this species, too, the sporangium, at first 

 cylindi'ical, soon assumes a flattened, somewhat quadrangular form, 

 and, in many cases, undergoes fissiparous division before it has put on 

 the exact appearance of the fmstule of a Fragilaria. 



"The Ifeloseiree {Gallionella, Ehi\), and the Bid(lulphi<^, (Mr. 

 Thwaites remarks), would seem, in their development of sporangia, to 

 offer an exception to most BiatomacecB ; for in those genera no 

 evident conjugation has been seen. However, something analogous 

 to it must take place; for, excepting the mixture of endochromes of 

 two cells, the phenomena are of precisely similar character. Thus, 

 instead of the conjugation of two frustules, a change takes place in 

 the endochrome of a single frustule, — that is, a disturbance of its 

 previous arrangement, a mo\T.ng towards the centre of the frustule, 

 and a rapid increase in its quantity : subsequently to this, it becomes 

 a sporangium, and out of this are developed sporangial frustules, as 

 in the other Biatomacece. In a single cell, therefore, a process, 

 physiologically precisely similar to that occurring between two con- 

 jugating cells, takes place ; and it is not difficult to believe, taking 

 into view the secondary character of cell-membrane, that the two 

 kinds of endochrome may be developed at the opposite ends of one 

 frustule, as easily as in two contiguous frustules, and give rise to the 

 same phenomena as ordinary conjugation." In the Zygnemece, 

 adjoining cells in the same filament are found to conjugate. 



The process of conjugation has now been seen in most genera of the 

 well-defined Diatomacem. Observed first in Eunotia, it was sub- 

 sequently seen in Gomphonema, Cocconema, Fragilaria, Schizonema, 

 and its modified character in Melosira and Biddulphia. 



Kiitzing, unacquainted, at the time his treatise was written, of 

 propagation by conjugation, considered that the Biaiomem multiply 

 in three ways: — 1. By development of their gonimic substance 

 (endochrome) which, as he says, happens in the lower Algae, but in 

 the Biatomea is uncertain. 2. By fission, complete, or incomplete, 

 a ge.ieral mode; and 3. By gemma) or spores, the formation of 

 which he has vv'itnessed in several si^ecics of Melosira, in Schizonema, 

 and in Mivrome^a, 



