OF THE DESMIDIEiE. 715 



OF THE GROUP BACILLARIA. 



Sub-group DESMIDIE^ or DESMIDIACE^. 

 (Page 1, Plates I., II., III., XVI.) 



[Class Alg^, Order Chlorosjpermece, Family Desmidiacece of Botanists.] 



Cells of two symmetrical valves, devoid of silex, mostly figured, their junc- 

 tion marked by a pale interruption of the endochrome, frequently also by a 

 constriction ; increasing by the formation of two new half-cells, which become 

 interposed between the older, so that the two newly-produced cells consist 

 each of a new and an old half-cell ; the transverse division complete or incom- 

 plete, the cells thus either free or forming a filament ; endochrome green, 

 occasionally converted into ciliated zoospores (in the single known instance, 

 escaping by an aperture at the apex of one or more specially formed lateral 

 tubes) ; reproduction by conjugation of the contents of two distinct cells, and 

 the formation of sporangia, the contents of which, in after- development, be- 

 come segmented into a definite nimiber of individualized portions, the last 

 generation of which are set free by the bursting or solution of the containing 

 membrane, and become the first fronds of a new vegetative cycle. 



We believe the foregoing diagnosis will apply to and include all the species 

 which we look upon as undoubtedly belonging to this family, and which are 

 introduced into this work. The claims of the genera Cylindrocystis and 

 Mesotaenium, as true members of the Desmidiaceae, not appearing, so far as 

 we can judge, to be satisfactorily established, they are omitted. 



The wonderful variety of form and beautiful symmetrical diversity of out- 

 hne of the members of this family have been dilated on at length in the 

 General History. It seems to us, with regard to the mode of cell-division 

 in the true species of this family, that, normally, the preliminary step in the 

 process is the sej)aration of the cell- contents and the formation of a septum 

 at the central sutiu^e, the two halves of the contents becoming thus indi- 

 vidualized, whereupon ensues the growth and extension of the primordial 

 utricle and contents, concm^rently with the production of the intermediate 

 ceU-waU ultimately to form the two new segments, and either complete sepa- 

 ration taking place, or the cells remaining united in more or less brittle 

 filaments. 



Many of the species, probably all, seem to be liable to an abnormal mode 

 of growth, resulting from the incomplete carrying out of this process, when 

 the new growth forms an intermediate, frequently misshapen structure, pro- 

 ducing with the original segments but one uninterrupted cavity, — this irregu- 

 larity seeming to be primarily due to the omission of the formation of the 

 septum on the recommencement of the vegetative growth (III. 61, Q2) : vide 

 Mrs. H. Thomas, J. M. Sci. vol. iii. pi. 5. figs. 17 & 18 ; also M. de Brebisson, 

 Liste, ifcc. pi. 1. fig. 15 ; and Mr. W. Archer, Proc. Nat. Hist. Soc. Dub. 1859, 

 vol. ii. pi. 1. figs. 9-15.) An inspection of several of the latter figures will, 

 however, show that the intervening structure, in the first instance (from the 

 foregoing cause, as we imagine) rendered abnormal, is not always absolutely 

 shapeless or irregular in its form, but sometimes, its axis of growth striking 

 off at right angles to that of the older segments, assumes the form and often 

 the size of an entire frond. Sometimes, indeed, not only is the axis of growth 



