Secondary growth in diocts is due to the formation of two types of cambia, the vascular and cork cambia. The vascular cambium forms out of the fascicular cambium which lies between the primary xylem and phloem in the vascular bundles and the de-differentiation of the interfascicular parenchyma into a cambial group of cells. These two cambia then unite and produce a continuous annulus of cambial cells which then begin their division where one daugther cell is an initial to replace the mother cell and the other daughter cell differentiates into either secondary xylem or phloem. Usually the secondary xylem is produced in much greater amounts than the secondary phloem.
The other secondary growth is the intiation of the cork cambium from the epidermal or cortical layers. Thus the epidermis is replaced with a tissue called the periderm which possesses several types of cells. The cork cambium or phellogen which divides regularly to produce the two other cell types: the phellem cells, which are heavily suberized and exterior to the cork cambium and the phelloderm cells which are more of a parenchyma nature and located centripetal to the cork cambium. There are no clear images of the periderm as it is very difficult to section with the woody exterior. The vascular cambium was not visually reproduced but the cells which it produced are very evident and the location is pointed out below.
This is a good overall view of secondary growth in the stems of cotton. The polarized light gives an excellent contrast of secondary growth in the vascular tissues all the various tissues discussed above in the primary growth.
The formation of the vascular cambium results from the joining of the fascicular cambium located between the primary xylem and phloem. The interfascicular parenchyma de-differentiates into interfascicular cambium. There is not a suitable set of developmental images showing this however a previous image is re-utilized from which this development can be inferred. As stated above these two cell types are united in a continuous annulus and then secondary growth commences.
Upon the formation of the continuous layer of the vascular cambium, this cell layer becomes meristematic to produce secondary xylem and secondary phloem. The secondary xylem is produced centripetally and the secondary phloem is centrifugally. Both secondary vascular tissues are present as continuous annuli in the stem.
Below are several succesive images with different views of the secondary growth in the cotton stem.
Above are nice views of the secondary growth emerging from the vascular cambium. The phloroglucinol stain clearly shows the secondary xylem staining red with the large vessel members and the phloem centrifugal to that which is not stained. The 10x view shows a closer view of the secondary vascular tissue but the vascular cambium cannot be conclusively identified.
Below is a medial longitudinal section of the cotton stem. The pith is evident in the center of the stem as non-stained cells which are blocky and thin-walled. The bottom of the image shows (order is interior to exterior) the layers of primary and early secondary xylem, early secondary phloem, primary phloem, phloic fibers, cortex and epidermis.
Introduction | Flowers&Fruit | Roots | Stems | Leaves
Section of Plant Biology Division of Biological Sciences
UNIVERSITY OF CALIFORNIA, DAVIS