Mr. G. H. K. Thwaites on the Cell-Membrane of Plants. 17 



means of a short tube developed from each, and through the 

 canal formed by the union of these the endochrome of one of the 

 cells passes into the other cell, becomes mixed with its endo- 

 chrome, and subsequently around this mixed endochrome a cell- 

 membrane is developed. This membrane would certainly appear 

 to be developed by the endochrome and not by one of the ori- 

 ginal cell-walls, otherwise we could not expect it to be entirely 

 influenced as to its form and size by the contained endochrome, 

 but that there would be indications of its being independent of 

 this. The spore-membrane, however, not only corresponds in 

 extent with the contained endochrome, but if, as is sometimes 

 accidentally the case, the mass of endochrome has become divided 

 into two portions, each of these portions becomes covered with a 

 cell-membrane ; thus showing that the relation is between these, 

 and not between either and the original cell-wall. That a spore 

 of Zygnema represents a cell of the same plant is well-shown by 

 the mode of fructification of an allied genus, Vesiculifera, where 

 it is evident that such is its character (see p. 333). 



Amongst the Algse the number of cells is often very much in- 

 creased by fissiparous division ; that is to say, a single cell be- 

 comes divided into two (sometimes four) : the way in which this 

 takes place is interesting, and I think throws light upon the 

 ordinary production of cells. The process of the fissiparous di- 

 vision of cells may be well-seen in the large species of Zygnema ; 

 in these the endochrome is arranged in one or more spiral coils 

 within the cell. When the latter is about to become divided, a 

 slight disturbance of the regularity of the spirals may be observed 

 just in that part of the cell where the division will take place ; 

 their continuity is subsequently broken at this spot, and soon 

 afterwards the original cell may be seen to have become converted 

 into two, with no apparent disturbance of the endochrome except 

 just at the point where separation took place. [The large nucleus 

 has also become divided into two.] Various explanations have 

 been given of the mode in which the division of the cell takes 

 place, but I believe the correct one is to consider that each half- 

 endochrome developes around it a new cell-membrane — the old 

 one remaining or becoming absorbed. I have certainly seen 

 traces of the original cell-membrane in a fragile species of Zyg- 

 nema found in this neighbourhood. In Isthmia, Meloseira and 

 other genera which possess a siliceous cell-wall, it is distinctly 

 seen that two perfect cells are developed within the original one, 

 and this would lead us to expect the same thing to occur in all 

 species where this mode of division obtains. 



We may now proceed to the consideration of the ordinary mode 

 of development of cells, and there is perhaps scarcely a species in 

 which this can be studied to greater advantage than in the very 



Ann. $ Mag. N. Hist. Vol. xviii. C 



