Comparative Histocytology of the Petiole and the Main Pulvinus in Mimosa pudica L.

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Abstract
In Mimosa pudica, the main pulvinus, which brings about leaf movements, presents unusual structural characteristics in comparison with the petiole. Peculiar cellular features which exist in the cortex, epidermis, parenchyma and endodermal regions include the shape of the cells, their disposition and the location of the organelles. The central cylinder of the petiole is surrounded only by a few parenchyma layers whereas the central cylinder of the pulvinus forms a narrow central core enclosed in numerous cortical parenchyma layers. The phloem of the pulvinus contains collenchymatous cells towards the outside and possesses companion cells with wall ingrowths; these phloem members do not exist in the petiole. Xylem and protoxylem parenchyma cells of the petiole possess wall ingrowths which do not occur in homologous cells of the pulvinus. Moreover the pith of the pulvinus is composed of small fibriform elements similar to the xylem fibriform elements of the organ. The structures observed may facilitate exchanges between cells in the petiole and in the pulvinus. The predominant functions of the organs relative to lateral and longitudinal transfer of nutrients and conduction of stimuli are discussed.

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