Leaf development in Begonia hispida var. cucullifera with special reference to vascular organization

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Abstract
The leaf is initiated by periclinal divisions in the second cell layer of the apex. Early in development, it assumes an asymmetrical shape as a result of the non-median position of the apical meristematic region that forms the leaf tip. The other main lobes of the leaf are formed by two additional regions of growth to the side of the tip. In the second plastochron, primordia are initiated on the upper surface of the main ribs of the leaf and these often develop into large, leaf-like appendages. During the third plastochron, the leaf becomes covered by a dense layer of trichomes. Vascular development is well correlated with regions of growth. The median procambial strand extends diagonally into the skewed apical growth region and another strand branches from this and develops on the other side. Lateral procambial strands usually form in an abaxial to adaxial sequence in the petiole: one strand of the first pair to develop forms part of the median rib and the other forms a third main rib on the opposite side. After the initiation of primordia on the leaf surface, a 'ventral system' of procambia is developed supplying mainly the ventral rib surface and consequently the epiphyllous appendages. This consists of branches from the adaxial-most peripheral bundles in the petiole as well as a bundle in the centre. The vascular anatomy of the stem and nodal region is also described in relation to that of the leaf.

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