6 PROTOPLASM AND PLANT CELLS 
conditions the cellulose or pectose may become changed 
into gums, e.g. gum arabic, cherry gum, slime of flax- 
seed, etc. 
10. The cell wall when first formed is very thin. 
Growth occurs either by apposition (deposition of cell 
wall substance on the inner surface of the wall) in which 
case the wall becomes thicker and may or may not 
appear layered, or by intussusception (the deposition of 
new material among the particles of the old), in which 
case the wall becomes larger as well as often thicker. 
The first layer formed is the thin middle lamella. Upon 
this is deposited, on either side, a thicker layer of some- 
what different composition, the secondary lamella. A 
tertiary lamella is sometimes formed also. These 
different layers are usually of somewhat different chemi- 
cal composition. ‘Thus the middle lamella is often com- 
posed of calcium pectate or some other pectose compound 
while the secondary lamellae are cellulose or a mixture 
of cellulose with other substances. When present, the 
tertiary lamella is usually nearly pure cellulose. 
11. The walls between adjacent living cells are quite 
generally perforated by very minute pores through which 
delicate fibrils of cytoplasm extend from one cell to the 
other, apparently thus binding all the 
living cells of the plant together into one 
more or less coordinated unit. 
12. The thickening of the cell wall is 
not always uniform. Indeed, except in 
Fic. 2.—Thick- ° comparatively thin-walled cells thinner 
ened cell walls. 
areas or spots are almost always left be 
tween the more thickened parts. These thickenings may 
be ridges which are in the shape of rings, spirals or reticu- 
lations or may occupy so much of the surface that the 
unthickened parts appear as pits. Usually these thick- 
