PLANT ORGANS AND ORGANISMS 
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growth is lessened and small ducts and much mechanical woody 
fiber are formed. Thus the open, loosely arranged product of the 
spring growth abuts on the densely arranged product of the last 
summer and autumn growth and the sharp contrast marks the 
periods of growth. To the spring, summer and autumn regions of 
growth of each year is given the term of “annual ring .” By count¬ 
ing the number of these rings it is possible to estimate the age of 
the tree or branch. 
Fig. 74.—Part of a transverse section of a twig of the linden, four years old. 
m, Pith; ms, medullary sheath; x, secondary wood; Ph, phloem; 2, 3, 4, annual 
rings; c, cambium; pa, dilated outer ends of medullary rays; b, bast; pr, primary 
cortex; k, cork. (From Sayre after Vines.) 
Bark. —Bark or bork is a term applied to all that portion of a 
woody exogenous plant axis outside of the cambium line. 
In pharmacognic work, bark is divided into three zones, these 
from without inward being: 
1. Outer Bark or Cork. 
2. Middle Bark or Cortical Parenchyma. 
3. Inner Bark or Phloem. 
Periderm. —Periderm is a name applied to all the tissue produced 
externally by the cork cambium (P hello gen). This term appears 
01 ten in pharmacognic and materia medica texts. 
Phelloderm. —Phelloderm or secondary cortex is all that tissue 
produced by the cork cambium on its inner face. Its cells frequently 
contain chloroplasts. 
