Pulvinus and petiole comparative anatomy in a species of Mimosa with slow nyctinastic leaf movement

The pulvinus is the thickening at the base of leguminous plant leaves responsible for rapid or slow leaf movements. The structural characteristics of pulvini have usually been studied in plants with rapid leaf movements, especially Mimosa species and information on the pulvini of species with slow leaf movement is scarce. We investigated whether the anatomical characteristics of pulvini from a Mimosa species presenting slow nyctinastic leaf movements were different from those of the species in this genus with rapid leaf movements and if these structural aspects differed from those in the petioles. Primary and secondary pulvini and petioles of Mimosa caesalpiniifolia Benth. (Leguminosae, Caesalpinioideae) were cross and longitudinal sectioned and analyzed using a light microscopy. Calcium oxalate crystals were analyzed under polarized light. Histochemical tests were performed for detection of starch, phenolic compounds, polysaccharides, lignin and calcium oxalate. Primary and secondary pulvini had characteristics common to the pulvini of other legume species with different types of leaf movements, such as large parenchyma cortex, reduced vascular system and lack of lignification, which explains the pulvinus’ greater flexibility compared with the petiole. The petiole exhibited reduced cortex, peripheral vascular system and wide pith. Starch grains and calcium oxalate prismatic crystals were observed in the endodermis of pulvini and petioles. The detection of calcium oxalate crystals in the pulvinus endodermis of a Mimosa species is a novelty. However, this feature is common in pulvini with slow movements, corroborating the idea that the pulvinus endodermis content is related to the type of leaf movement, regardless of taxonomic considerations. Septate pericyclic fibers with pectocellulosic walls containing primary pit fields occurred in the pulvini and indicates symplastic communication between cortex and vascular cylinder; in the petiole, the pericyclic fibers were lignified. Internal phloem occurred in both the pulvini and petioles; this is uncommon in leguminous species and a novelty for this genus.