Development, and Structure of the Vegetable Cell. ^83 



after the thickening of their walls and of those of their mother 

 cell has commenced, coincidently with the more distinct protru- 

 sion of the dissepiment, that the application of solution of chlo- 

 ride of zinc and iodine produces a blue coloration of the external 

 membranes of the new joint-cells and of their now likewise 

 thickened and closely embracing mother cell ; and this happens 

 contemporaneously with the detachment of a secondary cell, 

 which up to this period could not be isolated. 



The difficulty, or perhaps, at present, more correctly speaking, 

 the impossibility, of effecting, by the agency of endosmosis, the 

 separation of the closely approximated membranes of the two 

 endogenous cells from their mother cell (the secondary membrane 

 of the joint-cell), or, before the thickening of their walls, from 

 the secondary cells within them, is owing in part to the nature 

 of the intercellular substance, and in part to their very similar 

 chemical and physical (diosmotic) properties. 



The similarity or identity of these membranes is so great, in- 

 deed, that it is extremely difficult to determine whether the young 

 cells consist of a single or double cell-wall, and whether the en- 

 veloping secondary membrane of the mother cell is still present 

 or has been destroyed. It is moreover somewhat difficult to 

 make out with certainty the presence of the large vesicles filled 

 with colourless fluid amid the corpuscles of chlorophyll and 

 starch within the cavity of the cell. 



In many species of the genus Spirogyra, and still more easily 

 in Cladophora glomerata, we may ascertain, by cutting through 

 their joint-cells under water, or by the action of different re^* 

 agents, that these cells are filled with a delicately walled cellular 

 tissue. And we may sometimes be so fortunate as to witness 

 the complete extrusion of the new joint-ceils (themselves also 

 occupied by cellular contents) from the mother cell before theit 

 membranes are thickened. 



. In (Edogonium, the joint-cells of which acquire cellular con- 

 tents only after they have attained their full size, and simulta*. 

 neously with the commencement of the thickening of their walls, 

 these are no longer protruded from their mother cell ; and in their 

 younger phases (in which, as they then only exhibit fluid con- 

 tents, they cannot be distinguished from the large neighbouring 

 secretion-cells) they are furnished with walls as delicate as those 

 of the latter, and are immediately dissolved by contact with 

 water. Indeed it is only when sections of such plants are made 

 in a weak solution of gum-arabic, instead of water, that we can suc- 

 ceed in observing for any length of time the very delicately walled, 

 easily overlooked, endogenous, non-nucleated cells emerging 

 from their parent cell ; and even then it is impossible, for rea- 

 sons already stated, to distinguish whether these are only trans- 



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