Secondary growth in stem
Secondary growth occurs only in dicot stem a little away from the shoot apex and helps the plant to grow in girth (thickness) and makes it very strong to stand upright for many years. The growth in thickness becomes possible due to formation of some new tissues entirely by the activities of the lateral meristems. There are two types of lateral meristem; vascular cambium and cork cambium or Phellogen. These tissues are known as secondary tissues and the growth in girth or thickness thus accomplished is referred to as secondary growth.
Activity of vascular cambium
The secondary tissue produced by the vascular cambium is called the secondary xylem and secondary phloem. A few living parenchyma cells of the medullary rays, which retained the potentialities of cells division and new strips of meristems, are formed in a line with the fascicular cambium. They are known as interfascicular cambium. It is secondary in origin and joins up with the fascicular cambium and thus a distinct continuous cambium ring is formed. Cambium divides and adds cells on internal side (towards pith) which mature into secondary xylem and cells added towards external side (periphery) mature into secondary phloem. Amount of secondary xylem produced is more than secondary phloem.
Activity of cork cambium
The cork cambium (Phellogen) is the lateral meristem that produces cork or Phellem towards the outside and secondary cortex or Phelloderm towards the inner side. The cork cells or Phellem are waxy, impregnated with suberin, a lipid material that makes them waterproof but also causes them to die. Areas of loosely packed cells penetrate cork layers and enable gas exchange to occur. Phelloderm consists of living parenchyma cells, which may be photosynthetic and store nutrients. Phellogen produces much more Phellem on the outerside than Phelloderm on the inner. The cells constituting Phellem or called cork cells.
Phellogen, Phelloderm and Phellem together constitute the periderm. Due to internal increase in thickness, periderm replaces the epidermis, becomes protective in function. All the dead cells lying outside the active Phellogen constitute the bark.
In Betula bhojpatra bark peels off like sheets of paper. Ancient manuscripts are still preserved on them. Cork tissue becomes very thick in Cork tree (Quercus suber) and is used commercially as, bottle-stoppers, insulators, shoe soles etc.