Radial secondary growth and formation of successive cambia and their products in Ipomoea hederifolia L. (Convolvulaceae)
Rajput KS; Raole VM; Gandhi D
Botanical Journal of the Linnean Society
Ipomoea hederifolia stems increase in thickness using a combination of different types of cambial variant such as\nthe discontinuous concentric rings of cambia the development of included phloem the reverse orientation of discontinuous cambial segments the internal phloem the formation of secondary xylem and phloem from the internal cambium and differentiation of cork in the pith. After primary growth the first ring of cambium arises\nbetween the external primary phloem and primary xylem producing secondary phloem centrifugally and secondary xylem centripetally. The stem becomes lobed flat undulating or irregular in shape as a result of the formation of both discontinuous and continuous concentric rings of cambia. As the formation of secondary xylem is greater in one region than in another this results in the formation of a grooved stem. Successive cambia formed after the first ring are of two distinct functional types: (1) functionally normal successive cambia that divide to form secondary xylem centripetally and secondary phloem centrifugally like other dicotyledons that show successive rings and (2) abnormal cambia with reverse orientation. The former type of successive rings originates from the parenchyma cells located outside the phloem produced by previous cambium. The latter type of cambium develops from the conjunctive tissue located at the base of the secondary xylem formed by functionally normal cambia. This cambium is functionally inverted producing secondary xylem centrifugally and secondary phloem centripetally. In\nlater secondary growth xylem parenchyma situated deep inside the secondary xylem undergoes de-differentiation and re-differentiates into included phloem islands in secondary xylem.