Development, and Structure of the Vegetable Cell. 433 



The constricted line of union between the two connected 

 daughter cells is not unfrequently colourless; nevertheless it 

 cannot be positively ascertained whether, as appears upon alter- 

 ing the focal distance, and as would correspond with my notion 

 of the origin of the septum, the circularly torn thickened septum 

 is continued as a delicate membrane throughout this uniting 

 band, or whether a fluid constitutes the stratum of separation 

 between the coloured secretion-matter contained in the two 

 daughter cells. 



In most instances, even this connective band is concealed by 

 the coloured contents, which are pressed into the median line 

 by the contracted membranes of the secondary cells ; and in these 

 circumstances the scar of the lacerated septum exhibits the 

 appearance of a notch in this lamina in the mass of chlorophyll. 



The appearances in cells where new septa are in course of 

 lignification are altogether different when rather stronger dios- 

 motic agents are brought into contact with them. In such cases 

 one of the young joint-cells is frequently more strongly con- 

 tracted than its neighbour; the delicate septum perceptible be- 

 tween them, which usually, in its youngest and unthickened 

 condition, protrudes in a convex form into the upper cell (PI. VI. 

 fig. 42 a), does so, under the condition in question, to a greater 

 degree ; the contiguous secretion-vesicles likewise gradually re- 

 cede into the less extended cell, until at length the septum can 

 no longer withstand the constantly increasing pressure, but 

 gives way before it, and allows a sudden rush of the chlorophyll- 

 and starch-corpuscles from their cavity into the neighbouring 

 one. These phenomena may be readily witnessed in the vicinity 

 of the cut portions of the joint-cells (p. 421). 



This translation of the chlorophyll- and starch-corpuscles from 

 one side of the septum to tlie other, within the expanding cell, 

 and which may especially be seen for a considerable time after 

 the rupture of the septum, might lead those who do not take 

 into consideration the great extensibility of the unthickened 

 cell-wall, and who may not have observed the ultimate rupture 

 of the young septum, to the erroneous belief that no septum 

 existed betwixt the two closely approximated and contracted 

 daughter cells. 



However, if, after extension has proceeded for a time, the 

 abrupt and forcible movement of the chlorophyll- vesicles, &c. 

 be first observed, followed by a more gentle one, the opera- 

 tion of diosmosis remaining unchanged, no other inference can 

 be drawn than that an existing obstacle has been suddenly re- 

 moved, and evidence thereby afforded of the previous existence 

 of a septum, overlaid by those secretion-vesicles, at the spot 

 where the movement occurred. 



