148 PLANT ANATOMY, 
may therefore afford information whether an adulteration with 
other vegetable or even inorganic powders has taken place. 
The estimation of ash must, of course, always be preceded bya 
microscopical analysis of the substance itself. 
II. The Cell-wall. 
The integument of the cell is called the cell-membrane or cell- 
wall. 
The cell-wall of young cells is a thin membrane which con- 
sists of cellulose, and only at a later period becomes variously 
changed, either chemically through the deposition of other sub- 
stances, or morphologically through the insertion of molecules 
of the same kind. Even the membrane of young cells, however, 
is not perfectly pure cellulose, since it owes its first formation to 
the protoplasm, remains for a long time in contact with the 
nitrogenous substances of the latter, and is penetrated by them. 
Only cellulose which has been purified by means of chemical 
solyents corresponds to the formula C,,H,,0.,. 
In living cells, the wall is in most intimate contact with the 
protoplasm-sac. To this contact is to be referred the growth 
of the cell. 
The growth of the cell takes place in a twofold manner, on 
the one hand by a change of form, and on the other by a trans- 
formation of the chemical nature of the cellulose, which, in the 
course of deyelopment of the cells, is capable of assuming a series 
of new chemical and physical properties. 
The change in form of the cell concerns either chiefly its out- 
line, and in this case may be considered surface-growth, or the 
development of the cell is specially expressed by a thickening of 
the wall, so that the growth in thickness determines the appear- 
ance of the cell. Although both directions of growth are not 
sharply to be separated, and are essentially based upon the 
same processes, they nevertheless deviate widely from each other 
in their results. 
2, lupulin about 8, starch less than 1, cacao about 4, mustard seed and 
flaxseed from 4 to 4.5, and pepper about 5 per cent of ash. 
