of the Desmidieae and Diatomese. 11 



be contracted, before the formation of the septum,into two masses, 

 in contact, but separated by a sharp line of demarcation (two 

 contracted daughter-cells imperfectly cut off from one another, 

 still adhering together at the place of constriction ; fig. 23, 

 Docidium truncatum) . 



tioned above that the youngest part of the cell-wall of Docidia beginning 

 to divide does not become blue by the action of iodized chloride of zinc, 

 while the old shells acquire this colour. Far greater weight must be laid 

 on the answer to the question whether the annular deposit of cellulose 

 substance which appears a considerable time before the division at the 

 upper end, in the internal wall, of the cell about to divide, subsequently 

 invests, as a single cellulose membrane, the contents emerging from the 

 crack of the dehiscent cell (De Bary's view, Abhandl. Senckenb. Gesells. 

 i. p. 41, an opinion which Mohl has adopted; Bot. Zeitung, 1855, p. 720) ; 

 or whether it becomes merely an investing envelope for the already existing 

 proper membrane of the emerging daughter-cells, and is "the local de- 

 posit of a substance between the mother-cell and the uppermost daughter- 

 cell" (Pringsheim, Pflanzenzelle, pp. 35,42). From the first view, it would 

 follow necessarily that the segments of contents of (Edogonium-ceUs under- 

 going division, contracting upon the addition of fluids which act exosmoti- 

 cally, are naked — are entirely destitute of a cell-membrane, even in a semi- 

 fluid condition. Observations of the completed conditions scarcely allow 

 of a certain decision whether the extruded mass of the ring now converted 

 into a membrane, merely adheres to the dehisced old cell-wall, or whether 

 it is immediately continuous with the innermost layer of the latter. But 

 the appearances during the development all speak in favour of Pringsheim's 

 view. The ring of cellulose in question has nothing in common with the 

 infolding of the youngest layer of membrane in the dividing cells of Clado- 

 phora. Not only does it present itself in quite a difi'erent place from the 

 septum, — its structure has no similarity whatever to that of an annular 

 fold of a membrane. It may be seen distinctly, that as the old halves of 

 the wall of the mother-cell gradually separate, the ring of cellulose is 

 stretched out, like a piece of dough. It is not in any way unfolded, but 

 stretched out lengthways in its whole mass. It is thickest in the middle 

 between the margin of the separated pieces of the old cell-wall ; by degrees, 

 as the longitudinal extension increases, its thickness is brought down here 

 also to that which it has at the points of junction with the old cell-mem- 

 brane. The adhesion of the ring to the internal surface of the old cell-wall 

 at the place of dehiscence, sufiiciently explains the cohesion of the joints 

 of the filament during the division. The comparison drawn by Pringsheim 

 between this and the gelatinous substance which envelopes other Algae, 

 thus appears warranted. The resemblance between its development and 

 extension with the completion of the thick gelatinous layer enveloping the 

 whole filaments of Hyalotheca and Didymoprium, in cell- division, is at 

 once evident. 



[Our observations are in favour of the view of Von Mohl and De 

 Bary. We believe that the thick ring of cellulose stretches out to form the 

 first coat of the new cell. With regard to the blue colour taken by the 

 young layers of cellulose in (Edogonium, we find there the youngest most 

 sensitive to this reagent. The last-formed layer in perfect cells be- 

 comes blue when older layers do not; and even the extremely delicate 

 globular vesicle which encloses the zoospore when it first emerges from a 

 dehiscing cell, is coloured blue by sulphuric acid and iodine. This mem- 

 brane is so extremely delicate, that it vanishes (by solution?) very soon 

 after it is exposed externally, thus setting the zoospore free. — A. H.] 



