26 PHARMACEUTICAL BOTANY 



or, (b) the outermost layer of cortex cells underneath the epidermis be- 

 comes active after being passive for one year, and lays down walls, the 

 inner layer becoming cork cambium, the outer becoming a layer of cork. 

 The cork cuts off water and food supplies from epidermis outside and so 

 epidermis separates and falls off as 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 joins the 

 cambium of the first-formed bundles (intrafascicular cambium) to form 

 a complete cambium ring. By the rapid multiplication of these cam- 

 bial cells new (secondary) xylem is cut off internally and new (secondary) 

 phloem externally, pushing inward the first-formed, or protoxylem, 

 and outward the first-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, and Asclepias, instead 

 of cork cambium arising from outer cortex cells it may arise at any 

 point in cortex. It is the origin of cork cambium at varying depths 

 that causes extensive sheets of tissue to separate off. That is what 

 gives the stringy appearance to the stems of climbers. 



At close of first year in Perennial Dicotyl Stem we note : 



S 

 p. 



o 



flH 



o 



Ph 



1. Epidermis — development of dermatogen or periblem — in process of peeling 



off, later on entirely absent. 



2. Cork tissue or periderm. 



3. Cork cambium or phellogen. 



4. Sometimes zone of thin-waUed cells containing chloroplasts cut off by 



cork cambium on inner face known as phelloderm. 



5. Cortex — in perennial stem cells of cortex may undergo modification into 



mucilage cells, into tannin receptacles, crystal cells, spiral cells, etc. 



6. Fibrovascular bundles of open collateral type which are now arranged 



into a compact circle, and between which are found primary and often 

 secondary medullary r^ys. 



From without inward the following tissues make up f. v. bundles. 



Protophloem j Hard Bast — long tenacious bast fibres. 



Secondary Phloem \ Soft Bast — phloem cells and sieve tubes. 



Cambium — active layer giving rise to secondary phloem on outer and sec- 

 ondary xylem on inner face, and adding to depth of med. rays. 



Secondary xylem — wood fibres, pitted vessels, tracheids. 



Protoxylem — spiral trachere. 



Pith. 



