THE VEGETABLE CELL. 25 



with its substance. The higher the degree in which the latter 

 occurs, the harder the membrane becomes, as is shewn by the 

 relation of the heart- wood to the sap-wood, and in a still greater 

 measure in many seed-coats of a bony consistence, e, g , the peri- 

 carp of Litfiospermum, which contains much lime, the epidermis 

 ot Equiseturti and Calamus^ in which a great quantity of silica is 

 deposited. However, we are without any accurate knowledge of 

 these conditions, in spite of the countless analyses of ashes which 

 we possess, for these give the product of ash of the cell-contents 

 and cell-membrane together. 



The deposition of organic substances is not less general than 

 that of inorganic compounds, at least in particular layers of cell- 

 membrane. Among these the nitrogenous compou.nds are cer- 

 tainly the most widely distributed. They do not occur in the 

 membranes of cells which are just at the commencement of their 

 development, for these are not coloured yellow by tincture of 

 iodine, yet scarcely a full-grown cell is met with in which this is 

 not the case. That these nitrogenous compounds belong, in many 

 instances, and especially in the cells of the wood, to the series of 

 proteine compounds, we have evidence (as Mulder pointed out) in 

 the violet colour which hydrochloric acid produces after long oper- 

 ation, and in the yellow colour which ammonia produces after a 

 previous action of nitric acid. Tlie presence of these compounds 

 explains how, accoiding to Chevandier's analysis, wood contains 

 0*67 to 1*52 per cent, of nitrogen. The 

 darker yellow a cell -membrane is co- F%g,ZL 



loured ij nitrogen, the more Wy it 

 withstands the action of sulphuric acid, 

 and the more difficult it is to obtain the 

 blue colour by the combination of this 

 and iodine. In most parenchymatous 

 cells, especially in the tL walfed, this 

 blue colour usually appeal's so intensely 

 that the original yellow tint totally dis- 

 appears ; ink tick walled cells, on the 

 contrary, especially those of wood, the 

 stronff yellow colour is not altogether ^ ^ „ ^ ^ 



^ *^ - , T T T - / Liber cell of Cocoa ootryophora, 



overcome, and the colour assumes a dirty a, PnimtiTe membrane, h, secon- 



green tint; lastly, in others no blue co- ^iTA'Z^X^"' 

 lour is produced at all, and the membrane 



offers such resistance, even to concentrated sulphuric acid, that 

 it either only swells up sHghtly or remains (juite unaltered, only 

 becoming coloured deep brown ; as is the case particularly in ex- 

 ternal layers of epidermis-cells and the outermost layers of almost 

 aU full-grown, cells, especially those of wood. This outermost layer 

 may readily be taken for the primary membrane of the cell ; but 

 as a rule it is composed of several super-imposed layers, and fre- 

 quently contains the outer ends of the pit canals (fig. 34), whence 



