DICOTYL STEM. 93 



xylem portions of numerous bundles which have grown together 

 and the only evidence left of their having once bee! separated is a 

 number of radial rows of elongated cells running through the 

 ring from the pith to the outer edge. These rows of cells are the 

 medullary rays. The cells were once soft parenchyma cells 

 between the bundles, but have subsequently become lignified 



At the outer edge of the ring of wood cells is a narrow zone 

 of small cells, thin-walled, unstained, rectangular in shape and 

 more or less in radial rows. These form the cambium zone or 

 growing layer. 



Outside the cambium is a zone of unstained cells composed 

 of sieve tissue and parenchyma cells. This zone is bordered 



U. 



m.r. 

 wood 



bast 



Fig. 56. Segment of cross-section of stem of Bittersweet, cort, parenchyma of 

 middle bark; scl.f., bast fibres; m.r., medullary ray (Greenish). 



exteriorly by a broken circle of small thick-walled bast fibres. 

 The area between the cambium and bast fibres, including the 

 latter, is composed of the phloem portions of the bundles. The 

 medullary rays are continued through the phloem areas. This 

 region forms the so-called inner or liber bark, spoken of in the 

 lesson on bast and wood fibres. Next to the inner bark is the 

 middle bark, composed of large ordinary parenchyma cells. 

 Beyond this is the outer bark, composed of a layer of cork cells. 

 Only dicotyls have a true bark, and the structure of such a bark 

 is seen in this section. When the bark is peeled off the rupture 

 takes place at the cambium zone, which is soft and easily torn. 



