10 



OF WOOD IN GENERAL 



often broad and conspicuous ; but in Gymnosperms they are so 

 narrow as not to be visible to the naked eye. From the cambium- 

 layer in one bundle to those in the bundles on either side of it 

 the formation of cambium extends, across the primary pith-rays, 

 so that instead of mere strips of cambium running longitudinally 

 down the stem between the xylem and phloem of each bundle, 

 there is now a cyHndrical sheath of cambium extending from the 

 embryonic tissue of its terminal bud downwards over the whole 

 stem. In transverse section this sheath appears as a ring, and is 

 accordingly sometimes called the cambium-ring. Those parts of 

 it that extend between the bundles are termed interfascicular 

 cambium, in contradistinction to the precisely similar tissue within 



Fig. e.—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. 



the bundles. This cambium-sheath is famihar 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 earHest 

 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 



