12 TIMBER AND SOME OF ITS DISEASES. [CHAP. 



numbers towards the outside of the cambium cylin- 

 der (i.e, to add to the cortex) but we are now only 



FIG. 5. Portion of cambium of a fir, showing the development of the young wood 

 tracheides from the cambium-cells. The arrow points to centre of the stem. The 

 cambium-cells at length cease to divide, and the walls become thicker (a), except at 

 certain areas, where the bordered pits are developed (b and c). To the right is a 

 medullary ray. Highly magnified, and the contents of the cambium-cells omitted 

 for clearness. 



concerned with the wood, and therefore only regard 

 those cells which are developed on the inside (i.e. 



