[Chap. XXVII GENERAL REGIONS AND PROCESSES IN STEMS 265 



adjacent innermost layer of cells in the cortex is sometimes referred to as 

 an endodermis, but in numerous stems it does not differ in structure 

 from cortical cells immediately external to it. One may therefore visualize 

 a woody twig as consisting of the pith, the vascular cylinder ( stele ) , the 

 cortex, and the epidermis if it is still present. The vascular cylinder in 

 the youngest parts of stems is not completely closed but consists of 

 several separate vascular bundles with parenchyma between. 



Between the bark and the wood cylinder is a cylinder of meristematic 

 cells, the vascular cambium. Since this cambium is but one cell, or at 

 most only a few cells, in thickness, it is too thin to be seen without a 

 microscope. Although in cross section it appears as a ring of very small, 

 thin-walled cells, it should be visualized as a cylinder of meristematic 

 cells immediately surrounding the wood cylinder and lining the inner- 

 most layer of the bark. As the cambium cells divide, new xylem cells de- 

 velop at the outer surface of the wood cylinder, and new phloem cells 

 develop at the inner surface of the bark. It is the formation of new cells in 

 the cambium that results in the annual increase in diameter of perennial 

 stems. In spring, when the cambium cells are dividing, the bark is 

 easily separated from the wood cylinder. Those who have made whistles 

 out of twigs or helped peel spruce logs for pulp mills are fullv aware 

 of this fact. 



One should continue to whittle and observe until the relative arrange- 

 ment of all these general regions is distinctly visualized. With the excep- 

 tion of the pith, each general region of woody stems should be visualized 

 as a cylinder surrounding other cylinders of the stem. The wood cvlinder 

 surrounds the pith, and in turn is surrounded by a cylinder of cam- 

 bium; the cylinder of bark surrounds both the cambium and the wood 

 cylinder. Within the bark next to the cambium is a cylinder of phloem, 

 followed in order by cylinders of pericycle, cortical chlorenchvma, cork, 

 and epidermis, provided one is examining a twig or small branch. 

 Within a few years the epidermis, cortex (cortical cork and chloren- 

 chyma), pericycle, and other outer layers of the bark die and slough 

 off. Consequently, the bark surrounding the trunk of a large tree con- 

 sists only of the phloem and layers of cork that develop annually from 

 cork cambiums, which in turn develop each year from living phloem 

 cells. Most of these features of a woody stem are represented in Fig. 95. 



It is well known that the age of any part of a branch may be ascer- 

 tained by noting its location in reference to the terminal bud scale scars. 



