Secondary Growth of Vascular Cambium
Secondary growth of vascular cambium can be observed in dicot root and secondary growth of stem.
Dicot Root - Secondary growth is observed in vascular cambium and phellogen. It is formed secondarily from conjunctive parenchyma and part of pericycle lying opposite the protocol email points. As initially vascular cambiums are wavy and later it becomes circular. It is derived from pericycle from primary vascular rays. In some other places it forms secondary xylem on the inner side and secondary phloem on the outside. Secondary vascular rays occur at places. Primary vascular rays are muliseriate in the beginning and is uniseriate on later.
Secondary phloem does not persist as the previous phloem get crushed then the new secondary phloem becomes operational. Secondary xylem (primary xylem) are persist. Annual rings are absent because there is little seasonal variation in soil temperature.
Secondary Growth of Stem - Secondary growth of stem is occurred due to vascular cambium and phellogen. Vascular cambium is formed by strips of fascicular cambium (primary meristems) and interfascicular cambium formed by differentiation of medulary ray cells. Vascular cambium is of two types division- anticlinal and periclinal. Anticlinal vascular cambium increases circumference of vascular cambiums (at right angles to surface) and periclinal divisions produce xylem, phloem and vascular rays. Vascular cambium contains two types of cells, ray initials and fusiform initial produces outside secondary phloem and inner secondary xylem. Phloem is not persistent as soon as the older one crushed new one becomes functional. Secondary xylem is persistence. It grows with age and increase the girth of stem. In temperate area spring wood is wider with large, lightly coloured xylem and in autumn wood has smaller, darker and narrower xylem. It produces annual growth ring which help in determining the age of plants.
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