Histological aspects of root formation in petioles of detached leaves of Pereskia grandifolia (Cactaceae): natural conditions and effects of GA3 and dark

Adventitious roots arise naturally on petioles of Pereskia grandifolia Haw. held in light. At about the 12th day after the beginning of the experiment, the root primordia arise in a callus tissue developed from the basal portion of the petiole. Associated with the development of the callus, noticeable structural changes occur in the originating organ. Petioles maintained in dark also form callus; however, they die in a few days. On the other hand, petioles treated with GA,, maintained in light, develop callus and survive; but they do not give rise to roots. Some aspects are discussed, such as: the kind of origin observed for the roots, and the possible physiological basis for their formation, as wdl as for the inhibition of their appearance