Stem and root anatomy of Zanonia indica L. (Cucurbitaceae) and significant adaptations of the aerial roots

Summary The physiological and ecological adaptions of higher plants are driven by anatomical changes that contribute to the evolution of the climbing habit. This not only makes them able to survive in a dry environment but also supports them to undergo secondary growth and complete their reproduction. Some plants even form aerial roots as a survival strategy. In the present study, the anatomy of both aerial and underground roots and stems in Zanonia indica L. (Cucurbitaceae) is investigated. In young stems, vascular bundles (VB) had sieve elements on the inner margin of the protoxylem elements (i.e., bicollateral VB). In stems as well as aerial and underground roots, the axial elements of the secondary xylem were arranged in radial plates separated by wide medullary rays. Aerial and underground roots shared similar features in having exarch protoxylem and distinct narrow piths. Structurally, the secondary xylem was composed of vessels, tracheids, fibres, axial and ray parenchyma cells while extra-fascicular sieve elements were observed only in stems and aerial roots. The formation of aerial roots is a survival mechanism whose significance is discussed with reference to the available literature.