202 PHARMACEUTICAL BOTANY 
formed, or protophloem, thus increasing the diameter of the stem. 
The primary medullary-rays are deepened. Cambium may also 
give rise to secondary medullary-rays. 
Sometimes, as in Grape Vines, Honeysuckles, Hickories, 
Cherries, Viburnums, and Oaks, etc., instead of cork cambium 
arising only from the epidermis or the outer layer of cortex 
cells, it may arise at any point in the cortex or even as far inward 
as the pericycle or outer layers of phloem, and frequently several 
cork cambia appear successively. As each new cork cambium 
arises and cuts off cork cells on its outer face, all the living tissues 
outside of the cork are deprived of nutrient sap and die. It is the 
origin of cork cambia at varying depths that causes extensive 
sheets of tissue to separate off. This dead tissue has been termed 
borke by the German botanists (see Fig. 131). That is what gives 
the stringy appearance to the stems of climbers. 
At close of first year in a Perennial Dicotyl Stem we note: 
1. Epidermis—development of dermatogen—in process of peeling off, later on 
entirely absent. 
2. Cork tissue. 
3. Cork cambium or phellogen. 
4. Sometimes a zone of thin-walled cells containing chloroplasts cut off by 
cork cambium on inner face and known as phelloderm or secondary cortex. 
5. Primary cortex—in perennials, stem cells of cortex may undergo modifica- 
tion into mucilage cells, into tannin receptacles, crystal cells, spiral cells, 
ete, 
6. Endodermis, or innermost layer of cells of cortex, usually similar to other 
cortex layers in character of cells. 
7. Pericycle, a parenchymatous region usually appearing similar to the cortex, 
but sometimes containing sclerenchyma fibers or stone cells or both. 
8. Vascular bundles of open collateral type which are now arranged in a 
compact circle, and between which are found primary- and often secondary 
medullary rays. The longest bundles represent Primary Bundles which 
have been deepened by the cutting off of secondary xylem and phloem by 
the cambium. 
From without inward the following tissues make up the bundles: 
Protophloem Hard Bast—long tenacious bast fibers (sometimes 
absent). ; 
Secondary Phloem _) Soft Bast—phloem cells, companion cells and sieve 
tubes. 
Cambium—active layer giving rise to secondary phloem on outer and 
secondary xylem or inner face, and adding to depth of med. rays. 
Secondary xylem—wood fibers, pitted vessels, tracheids. 
Protoxylem—spiral trachez. 
9. Pith. 
