Internal cambium and intraxylary phloem development in Ipomoea turbinata Lag. (Convolvulaceae)

- Internal cambium in I. turbinata that produce inversely oriented xylem and phloem.
- It initiates from the marginal pith cells and produce quantifiable amount of xylem and phloem on either side of it.
- Interestingly, some samples showed unidirectional internal cambium that produced secondary xylem and phloem in the same direction.
- Such pattern of secondary growth has been reported for the first time after Handa (1937).

Strands of phloem that are present at the periphery of pith are known as intraxylary phloem. Its presence remains restricted to a small portion of eudicots and considered as a characteristic feature for certain families. In the present study, Ipomoea turbinata Lag. showed development of intraxylary protophloem on adaxial tips of protoxylem from the procambial derivatives. Subsequently, additional intraxylary sieve elements were added from the adjacent parenchyma cells that were morphologically different from the pith cells. In thick stems, thin walled cells located between protoxylem and intraxylary protophloem acquired meristematic characters and formed several small segments of internal cambium. Initially, these cambial segments produced only phloem derivatives. Soon after, these segments became bidirectional and began to produce secondary xylem centrifugally and secondary phloem centripetally. Interestingly, in some of the samples development of secondary phloem and xylem was observed in the same direction. The secondary xylem formed by the internal cambium was composed of wide and fibriform vessels, fibres and axial parenchyma. Phloem possessed sieve tube elements, companion cells and parenchyma cells while rays in both xylem and phloem were mostly uniseriate but multiseriate rays were also observed.