Development and functional anatomy of pods of Colophospermum mopane (Caesalpinioideae: Dietarieae)

Abstract

Pod development of Colophospermum mopane was studied from its initiation until it was fully developed and completely filled by the seed. After fertilisation, meristematic activity in various regions of the pericarp causes fruit enlargement. The carpel symmetry and vascularisation displayed by the pods is of the follicular type. One dorsal and two unfused ventral bundles supply the pod. The funicle originates from one of the ventral bundles. The vascular supply of both the dorsal and ventral bundles is elaborate as secondary growth of a cambium increases the diameters of the bundles significantly. During early stages of fruit development the 3–5 innermost parenchyma layers of the mesophyll that borders the inner epidermis differentiate into small thin-walled parenchyma cells that differ considerably in size from the larger outer parenchymatous mesophyll cells. The inner zone of small parenchyma cells eventually differentiates into several collenchyma layers. At a later stage, the innermost parenchyma cells next to the collenchyma layers differentiate into sclerenchyma. As the fruit expands laterally, new vascular bundles continue to differentiate towards the centre of the fruit from the ground parenchyma of the dorsal fruit margin. The area of the fruit margin that is occupied by vascular bundles eventually becomes extensive. The xylem and phloem in the dorsal fruit margin are separated by a cambium. When the pod is mature the cell walls of the parenchymatous mesocarp become thickened and lignified, whereas the collenchymatous stratum becomes partly sclerenchymatous. The zone where the follicle eventually opens is characterised by thick-walled unlignified parenchyma cells between the two ventral bundles at the ventral suture. This unlignified zone is closely connected to the sclerenchymatous flanges of the ventral vascular bundles. The outer epidermis of mature brown pods consists of cells with thick lignified and cutinised walls. The mesophyll of fully developed pods consists of an outer stratum of mesophyll cells with lignified walls and an inner stratum of three or four layers of cells with unlignified walls. The unlignified zone is bordered by a sclerenchymatous stratum that originated from the inner mesophyll layers bordered by another sclerenchymatous stratum that originated from the outer layers of the collenchymatous stratum. The walls of the inner layers of the collenchymatous stratum remain unlignified. Between the sclerenchymatous and non-sclerenchymatous zone of the original collenchyma layers is a transition zone where secondary walls are present but they are unlignified or in various stages of lignification. The inner sclerenchymatous layers of the pericarp probably have a protective function. The inner collenchymatous layers may contribute to fruit opening under dry conditions. The usual method of fruit opening is, however, when the seed forces the fruit valves apart during imbibition.