THE STEM 201 
rates and falls off as a stringy layer. ‘The cork cambium produces 
cork on its outer face and secondary cortex on its inner. 
Between the bundles, certain cells of the primary medullary- 
rays become very active and form interfascicular cambium which 
\periderim 
epidermis 
Fic. 131.—Diagrams, based on the red oak, Quercus rubra, showing the position 
and extent of successively formed periderm or corky layers in a typically woody 
stem. A, a one-year-old twig, the first periderm layer, a complete cylinder, formed 
beneath the epidermis. B, a two-year-old twig, the epidermis and first periderm 
ruptured; new, shell-shaped layers formed deeper in the cortex. C, a three-year- 
old stem, the outer tissues weathered away and more periderm layers formed still 
more deeply in the stem, invading the secondary phloem. D, a four-year-old stem, 
the cortex and outer secondary phloem with their periderm layers weathered away, 
the new cork layers invading the younger phloem. £, the outer tissues of an old 
tree-trunk, showing the narrow band of young, living secondary phloem, and the 
thick, deeply fissured layer of older, dead phloem with its many shell-shaped peri- 
derm layers; a considerable amount of similar tissue has exfoliated. (Krom Eames 
and MacDaniels “Introduction to Plant Anatomy,” McGraw-Hill Book Co., Inc., Publishers.) 
joins the cambium of the first-formed bundles (intrafascicular 
cambium) to form a complete cambium ring. By the rapid multi- 
plication of these cambial cells, new (secondary) xylem is cut off 
internally and new (secondary) phloem externally, pushing 
inward the first-formed, or protoxylem, and outward the first- 
