Development, and Structure of the Vegetable Cell. 417 



and iodine is employed^ when the shrivelled membrane is scarcely 

 coloured pale yellow. 



Moreover these phenomena are modified not only by the 

 stage of development of the plant, but probably also by variations 

 in the conditions of nutrition, as the researches on Spirogyra, 

 hereafter to be described, have further shown. 



From the foregoing remarks it follows that the joint-cells of 

 Cladophora glomerata do not contain at all periods of their de- 

 velopment a mucilaginous, granular, viscid fluid — the "goni- 

 mical contents'' of Kiitzing or the "protoplasm" of Mohl; but 

 that, at certain periods, they are entirely filled, as if occupied 

 by a perfect cellular tissue, with thin-walled strongly diosmotic 

 cells, which, on the one hand, enlai'gc in water with extraordi- 

 nary rapidity, and, on the other, shrivel up in solution of gum. 



The nature of these endogenous cells is twofold. Thus some 

 of them contain, besides a colourless watery fluid, often also 

 without this, a larger or smaller quantity of a green mucilaginous 

 matter (both materials being very probably enveloped in special 

 cell-membranes), and, floating freely in the latter substance, 

 cither small vesicles of starch and chlorophyll (the latter frequently 

 containing a starchy nuclear vesicle) or smaller and larger cells, 

 in which these bodies are then contained. Their membrane is 

 distended, and soon becomes liquefied in water. 



The cells of the second description are filled only with a clear 

 transparent liquid, and very rapidly absorb water until they are 

 burst and destroyed by it. They are therefore difficult of ob- 

 servation, unless the water is duly mixed with gum, so as to 

 retard the distention of the diosmotic membrane, when it be- 

 comes possible at times to demonstrate that the membrane of 

 many of these cells is corroded, but not dissolved, by water and 

 solution of chloride of zinc and iodine. 



Between these two kinds of cells intermediate forms' occur, 

 showing that the two forms are not of an entirely different 

 nature, but only difi'erent stages of development of one and the 

 same kind of cell. 



The phenomena observed during the action of fluids on the 

 cut joint- cells and on the expanding endogenous cells which 

 issue from them, as also the position of the latter in the unin- 

 jured and normally grown joint-cells, indicate that the cells 

 consisting of watery contents and more resistent membranes 

 occupy the median line of the cylindrical joint-cell of the Con- 

 ferva, whilst those filled with chlorophyll, and whose membranes 

 are soluble in water, occur nearer to the surface than this central 

 tissue. 



Moreover, at the extremities of the joint-cells of perfectly 

 normally developed plants, the limpid cells — pressed together 



