Development f and Structure of the Vegetable Cell. 191 



composed of cell-structures, these usually exercise a perceptible 

 influence on the form of the superadded layers. Both the 

 organized contents and the form of the thickening layers caused 

 by them afford grounds for the discrimination of the different 

 layers that concur in the construction of the cell-system of a 

 tissue-cell. 



For as such a cell-system is not only composed of a number 

 of cells, but each of these cells again consists of many super- 

 posed layers, it is often difficult to make out the essential nature 

 of any single layer, especially when these laminae, as not un- 

 frequently happens, are only loosely connected together, or are 

 of dissimilar chemical constitution, or, again, when the mem- 

 branes of various endogenous cells are of homogeneous consist- 

 ence, or for other reasons are undistinguishable or inseparable 

 from each other. Under such circumstances it is the rule that 

 the layers of a cell-wall never contain organized bodies ; where 

 such are present, the nearest external membrane is the mem- 

 brane or the innermost lamina of an organized cell. 



The layers of deposit may indeed at the time of absorption 

 be separated from each other by fluid materials ; but they do 

 not enclose organized forms. On the other hand, many endo- 

 genous cells of the system of a tissue-cell contain only fluid, 

 which makes their recognition as cells difficult. Most commonly, 

 however, at least the secondary and the next cells in the interior 

 enclose organized forms. 



When, among these organized contents of the secondary cell 

 (consisting of vesicles containing secretion- material and fre- 

 quently, when the cell is not engaged in the multiphcation, of a 

 nucleus), one of these secretion-vesicles becomes so much ex- 

 tended at the expense of the others as to attain the size of the 

 mother cell ; the tertiary cell (the cell-nucleus) and the rest of 

 the contents are enclosed between the two membranes, which 

 then become approximated, and from this results the form de- 

 signated by Unger the "parietal {wandstdndiger) cell-nucleus," 

 which led Schleiden into bis above-mentioned erroneous notion 

 of cell-genesis. These forms are developed in fruits which are 

 becoming succulent, as also in cells filled with blue, red, and 

 many kinds of yellow colouring-matter. 



But commonly there is a different state of things, the small 

 secretion-vesicles (chlorophyll, starch, mucus, &c.) becoming 

 adherent, during their development, to the internal surface of 

 the membrane of the secondary cell. At a later period, when 

 this membrane begins to undergo a chemical change, and to 

 thicken, these secretory matters become absorbed ; the vesicles 

 vanish out of sight, but the spots at which they have adhered, 

 or still may adhere, do not undergo thickening. 



