66 GENERAL HISTORY OF THE INFIJSOEIA. 



Structures similar to those represented by Smith, of Synedra radians, oecui-red 

 in extreme abundance in the end of the autumn of 1854, in company with 

 Synedra Ulna, Here the cells, which, like those observed by Smith in the 

 allied species, had a diseased aspect and an abnormal arrangement of the 

 coloured contents, were imbedded in a granular jelly, of a reddish colour by 

 transmitted light. I very much doubt whether these last were in a condition 

 capable of further development ; while in reference to the cysts of CydoteUa 

 ojyerctdata, I share Smith's opinion. 



" The estabhshment of the assertion that the commencement of conjugation 

 in the Desmidieae and Diatomeae is but little distinguished from the com- 

 mencement of vegetative cell-division, renders some discussion of the latter 

 requisite. Pringsheim has already dii^ected attention to the resemblance of 

 this process in the Desmidieae to the vegetative cell-multiplication of the 

 joints of (Edogonium. In fact, it is an absolutely general phenomenon in the 

 true Desmidieae, so far as observation reaches, that the older parts of the 

 membrane of a cell about to divide, do not, as in other cases (for example, 

 in Zygnemeae), regularly increase in size with the parent-cell by growth in 

 all directions ; but the older, outer layers of the integument spht open with 

 an annular crack at the equator of the cell, shortly after (or during ?) the 

 division. They still remain sticking on, covering the ends of the cell with a 

 thick envelope, but become removed gradually fiu'ther apart by the interpo- 

 sition of new cellulose between their fractured edges. The interposed new 

 coat is the dii'ect continuation of that which hues the internal surface of the 

 cracked halves of the old shell. It is the margins of the half-sheUs which 

 constitute the rings, parallel to the end-surfaces, upon the cyhndrical lateral 

 surfaces of the cells of Hyahtheca dissdiens and H. mucosa, the wrinkled pro- 

 jections of the membrane in the middle of the deep constriction of the cell of 

 Micrasterias and the large Euastra, of the flat constriction of the cell of 

 Docid'mm, as also the ring at the equator of the external surface of Closte- 

 rium : in CJosterium and in Docidmm, frequently as many as six may be 

 counted, — a phenomenon which, in Docidium truncatum and the large Clos- 

 teria, may be recognized at fii^st sight as dependent upon a number of halves 

 of cracked cells regularly encasing their successors. 



" The dehiscence of the coat of the dividing cell is, in all obsei^ved cases, 

 preceded by the formation of the septum dividing the cell into two halves 

 \Cosmar'mm margaritiferum). The gradual development of this from the 

 margin of the cell- wall inwards, as a gradually- widening annular fold of the 

 innermost layer of the integument, has not yet been observed, and, from 

 analogy with the processes in (Edogonium, is scarcely probable. But, as in 

 (Edogonium, the contents of the cell may be contracted, before the fonnation 

 of the septum, into two masses, in contact, but separated by a sharp hne 

 of demarcation (two contracted daughter- cells imperfectly cut off from one 

 another, still adhering together at the place of constriction). 



" From the half-shells of cells of the same Docidium which dehisced under 

 the eye of the observer, emerged, within half-an-hom% to the extent of 4th or 

 ^th of the length of the half- shells, the daughter- cells, still intimately con- 

 nected at the point of contact. They could henceforth be perceived to be 

 enclosed by a cellulose coat, fii^m although dehcate. Treated with reagents 

 strongly extracting water, such as glycerine, one or both of the extruded 

 pieces frequently di-ew back into the halves of the shells of the mother- cell, 

 the projecting pieces of membrane becoming doubled inwards. The just- 

 emerged coats of the daughter- cells of Docidium did not take a blue colour 

 when treated with iodized chloride of zinc, while the old halves of the mem- 

 brane of the divided cell assumed the blue colour immediately. 



