PLANT ORGANS AND ORGANISMS 147 



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 (Phellogen). 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. 



