PLANT ORGANS AND ORGANISMS 
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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. 
