LEAF DEVELOPMENT IN DOXANTHA UNGUIS-CATI (BIGNONIACEAE)

Leaf structure in Doxantha unguis-cati is polymorphic. The usual mature compound leaf is composed of two lanceolate leaflets and a terminal tripartite spine-tendril. Leaf primordia are initiated simultaneously in pairs on opposite flanks of the shoot apical meristem by periclinal cell divisions in the third subsurface layer of the peripheral flank meristem. Two leaflet primordia are the first lateral appendages of the compound leaf. Initiation of these leaflet primordia occurs on the adaxial side of a compound leaf primordium 63–70 μm long. Lamina formation is initiated at the base of a leaflet primordium 70–90 μm long and continues acropetally. Mesophyll differentiation occurs in later stages of development of leaflets. The second pair of lateral appendages of the leaf primordium differentiate as prongs of the tendril. Initiation of the second pair of lateral appendages occurs on the adaxial side of a primordium approximately 168 μm long. Acropetal procambialization and vacuolation of cells extend to the apex of tendrils about 112 μm long, restricting the tendril meristem to the adaxial side of the primordium and resulting in curvature of the tendril. The tendril meristem is gradually limited to a more basipetal position as elongation of apical cells continues. Initiatory divisions and early ontogenetic stages of leaflets and tendrils are similar. Their ontogeny differs when the lateral primordia are approximately 70 μm long. Marginal and submarginal initials differentiate within leaflets but not in tendrils. Apical growth of tendrils ceases very early in ontogeny as compared with leaflets.