Igor | THE MIDDLE LAMELLA 33 
indicating merely the position of the layer in question, is prefer- 
able to either ‘‘cement”’ or “intercellular substance.’ The latter 
term, arising from a totally false conception as to the origin of 
cells, should certainly be dropped. There is also no evidence 
that the cell wall layers which, taken together, form the middle 
lamella have any significance as a cement for binding the cells 
together, and soit is inappropriate to apply to them a name sug- 
gesting any such function. 
The fact of the origin of the middle lamella as a double 
layer deposited on the surfaces of two plasma membranes, and 
the fact that in the formation of intercellular spaces it always 
splits in a median plane, must always be borne in mind. This 
constant plane of cleavage may be called the primary cleavage 
plane, or, perhaps, the primary cleft when it is spoken of with 
reference to the space between the daughter plasma membranes 
in which the cell wall material is deposited. 
I would distinguish cambial walls as the walls of celis yet 
capable of division, and primary cell walls as those layers added 
during the growth of the cambium cell into a full-sized wood or 
bast element. Secondary thickening would include the strata 
deposited in the subsequent history of such cells. This second- 
ary thickening may be subdivided further where evidence exists 
that its deposition has been interrupted and subsequently 
resumed; we should then have tertiary thickening, and so on. 
For the adult condition of thickened cell walls, the following 
terminology might be proposed as most exact from the stand- 
point of our present knowledge : 
The boundary plane between adjacent cells, commonly invisi- 
ble except where indicated by intercellular spaces, would be, 
following the suggestion already made, the primary cleavage 
plane. Including this on both sides, we should have the appa- 
rently homogeneous middle lamella, including the cambial walls 
and more or less of the primary cell walls, according to the pro- 
portional pectic acid content of the latter. Then would come 
those layers of the primary wall, if there be such, which are pre- 
dominantly non-pectic; and lastly the layers of secondary and 
