126 Prof. H. Karsten on the Formation, 



habitat when these have been long upon the slide, and brought 

 into contact with different kinds of water. 



A less degree of this depression appears, however, to occur even 

 in plants growing in a natural state ; and this is of interest here, 

 because it induces the fold-formation which was formerly re- 

 garded as the cause of cell-multiplication, when the depression 

 occurs at the precise period at which the two more or less sphe- 

 rical daughter-cells, touching the large secretion-cells with their 

 peripheral surface, and hampered by these in their rapid growth, 

 bring the previously free parts of their central surface into 

 complete contact, and thus enclose this impressed membrane 

 between them. The depth to which the folds of the secondary 

 cell is enclosed in the septum in course of formation depends 

 upon the greater or less extent of contact of the central surfaces 

 of the daughter cells at the time of this process. 



By a curvature or depression of one or the other of the chloro- 

 phyll-sacs, the side of the joint-cell is already perceptible, on 

 which the liquefaction of the small secretion-cells situated about 

 the nucleus takes place more rapidly than the enlargement of 

 the neighbouring young joint-cells, which usually occurs simul- 

 taneously with it. 



Newly formed septa not unfrequently occur, which on one 

 side do not enclose the smallest trace of a fold of the mother cell 

 between them, but show the well-preserved chlorophyll-sacs 

 distinctly at their circumference (as represented in fig. 58 a, 

 in S. nitida), whilst on the other side of the periphery of the 

 mother cell a fold of this kind is engaged, more or less deeply, 

 between the two plates of the septum. 



These enclosed folds of the membrane of the secondary cell, 

 which are no doubt subsequently absorbed, are at first thickened, 

 reminding us of the folds of Cladophora, described at pp. 420 

 and 425 (vol. xiii.), as well as its peripheral portion, whilst the 

 chlorophyll- sacs appressed to them are immediately absorbed. 



The thickening of the membranes of the daughter cells, which 

 takes place immediately, and their amalgamation with those of 

 the mother cell commence in the portions forming the septum 

 even before the completion of the absorption of the chlorophyll- 

 sacs which surround them. 



When the absorption of these secretion-materials is much de- 

 layed, the new, half-thickened septum may be seen, in certain 

 positions, already united on each side to the membrane of the 

 mother cell, after the joint-cell has been treated with endosmotic 

 fluids, whilst it is still free beneath the chlorophyll-sacs. Figs. 

 74 and 75 show this in one sac. 



But phenomena do occur which seem to show that in the 

 Spirogyr<s the development into new joint-cells does not always 



