6 PROTOPLASM AND PLANT CELLS 



conditions the cellulose or pectose may become changed 

 into gums, e.g. gum arable, 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 laj^er 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 

 Fig. 2.— Thick- Comparatively thin-walled cells thinner 

 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- 



