Root Anatomy of Leptadenia Reticulata and Leptadenia Pyrotechnica (Asclepiadaceae)

Development of cambial variant has been largely studied in stems of climbing species and other members while similar studies on roots are mostly neglected. Therefore, development of interxylary phloem is studied in the roots of Leptadenia reticulata (Retz.) Wight & Arn. and L.pyrotechnica (Forssk.) Decne. In both species, root diameter increases by the activity of single ring of vascular cambium. The cambium divides bi-directionally and produce secondary xylem centripetally and phloem centrifugally. However, small segments of the cambium temporarily ceased to form lignified xylem elements and produced secondary phloem bidirectionally for a short period. Soon after, the cambium regained its normal activity and started producing secondary xylem internally. Thus, the small islands of secondary phloem become embedded within the secondary xylem. In thick roots, earlier formed phloem islands showed collapse of non-conducting sieve elements, followed by their obliteration. Beside interxylary phloem, radially or vertically arranged isolated or group of sieve elements is also observed in the rays of both species. Secondary xylem was diffuse porous with indistinct growth rings and composed of vessels, fibres, tracheids, axial and ray parenchyma cells. Xylem rays mostly uniseriate but multiseriate rays observed occasionally. Uniseriate rays predominantly storied while multicellular rays were tall and several cells wide.