PLANT ORGANS AND ORGANISMS 143 



Growth of Perennial DicotylStem and its Histology. A perennial 

 dicotyl stem in the first year does not differ in structure from an 

 annual. By the close of the year a cork cambium (phellogen) has 

 originated beside the epidermis. In origin of cork cambium one 

 of two methods: (a) either the epidermis may divide into an outer 

 layer of cells that remains epidermis and an inner layer of cells that 

 becomes cork cambium, or, (b) the outermost layer of cortex cells 

 underneath the epidermis becomes active after being passive for 

 one year, and lays down walls, the inner layer becoming cork cam- 

 bium, 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 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 joins 

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

 to form a complete cambium ring. By the rapid multiplication 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-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, in- 

 stead 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: 



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- walled cells containing chloroplasts cut off by 



cork cambium on inner face and 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. 



