12 OF WOOD IN GENERAL. 



the bundles. This cambium-sheath is familiar to us all as the 

 layer of delicate thin-walled cells, full of sticky protoplasm, through 

 which we easily tear when we peel a stick. Having what has been 

 termed the quality of perpetual youth, it remains recognizable in 

 a stem many years of age, and with the pith furnishes us with a 

 convenient rough classification of all the structures of such a stem. 

 As we have seen, the pith, not having grown since its earliest 

 condition, remains as a mere central line in such a stem. From 

 this pith to the cambium-sheath is wood or xylem : outside the 

 cambium is the rind, or, as it is commonly but somewhat mis- 

 leadingly termed, bark, made up of the outer and often corky 

 cortex and the inner, largely fibrous, phloem or bast. 



FIG. G. Diagrams of exogenous stem with six bundles, during the first 

 year, at the beginning and at the close of the second year's growth, the last 

 showing the wedge-shaped masses of primary xylem projecting into the 

 central pith, and the formation of the first ring of secondary wood during the 

 second year by the activity of the cambium ring. 



In the first year the xylem and phloem are formed directly by 

 the modification of the inner and outer cells respectively of the 

 procambium-strancl ; but subsequently all wood, bast, and pith- 

 rays originate in the cambium. Accordingly the xylem and 

 phloem of the first year are termed primary, and that formed 

 from the cambium secondary (Fig. 6). 



