1901 | THE MIDDLE LAMELLA 29 
walls in this case are not composed of pure pectic acid, or 
that it exists in them in a somewhat different form. Such a dif- 
ference in chemical nature might easily occur, since the tangen- 
tial walls are being laid down anew, while the radial walls are 
older, and at most are simply added to as the cells increase in 
number and size. There is no evidence of the existence here of 
a mass corresponding to Dippel’s Zwéschensubstanz. 
The power of the cell to secrete cell wall materials of very 
different chemical composition at different periods in its history 
is much greater even than is indicated by Mangin’s work. That 
the compounds stained by ruthenium red are not derived from 
the decomposition of a previously deposited cellulose wall is 
proved by the complete staining of the young cambium wall. 
Still, that there is a possibility of a closer relation than Mangin 
supposes between cellulose and other cell wall materials on the one 
hand, and pectic substances on the other, perhaps even involving 
transformations of material from one class into the other, is to 
be inferred from the action of ruthenium red upon the walls of 
the cells surrounding the resin pores in pine. But that the 
power of secreting pectic compounds is not limited to a single 
period in the history of the cell is shown by the changes already 
discussed in the mass of the middle lamella. Since the very 
youngest and thinnest cambium walls are stained by ruthenium 
red, and at a later stage the thicker walls are still completely 
Stained, the power of depositing pectic acid must last for a con- 
siderable time. That it continues in some cases even beyond 
the cambium stage is indicated by the variations in the thickness 
of the middle lamella in older tissues. Pectic substances, though 
perhaps not in the form of pectic acid, are also secreted at later 
Stages in cell wall development, usually in combination with 
other materials, as cellulose, callose, and nitrogenous substances. 
In most of the thickened walls studied, evidence of the presence 
of pectic substances was found in the secondary thickening 
layers. The general rule, that pectic compounds are deposited 
in the cell wall early in the life of the cell, and that layers depos- 
ited later are predominantly non-pectic, is seen to have exceptions 
