The species Ireartea exorrhiza (Mart) Drude is a palm tree commonly known as paxiúba or paxiubão, which inhabits the floodable lowlands near rivers and streams in the states of AM, MT, and PA. Its pines are used by the Amazonian cablocos as fodder, in the making of woven objects and as rock salt. This work aims to contribute to the knowledge of the internal structures of the pinases (limbus and petiole) of the above mentioned species and the chemical nature of the fiber walls and idioblasts, by means of histochemical tests. The material studied was collected at the Mocambo Reserve, Belém-PA, and, because it was very fibrous, was previously softened in 25% aqueous nitric acid for 30 days for the petiole and 7 days for the petals. The epidermal cells, in frontal view, that cover the central and secondary veins, are heterodimensional, square and rectangular, with the presence of trichome bases, differing from the cells that cover the limbus of the pinnae because they are polyhedral and tangential to the proximities of the veins. The pinna is amphistomatic, with a greater number of stomas in the abaxial face, arranged parallel to the secondary veins, these are mostly of the tetracyclic type. In the mesophyll soon attached to the adaxial and abaxial epidermis, there is the presence of a uniseriate hypodermis formed by rectangular cells, the lacunose parenchyma is formed by irregular cells with small intercellular spaces, and the palisade parenchyma is biseriate formed by high cells, the vascular elements immersed in the lacunose p.are involved by two types of sheaths, one parenchymatic and the other fibrous. Generally at the level of the central vein the sclerenchymatic sheath is more developed, presenting in its vicinity grains of silica. The petiole in cross section shows the vascular elements that are dispersed in the parenchyma, forming nests, surrounded by a ring of small cells with thickened walls. Specific histochemical tests were performed, resulting in positive reactions for lignin, silica grains, starch and cellulose. Some authors cite that the stomata of the genus ireatea are restricted only in the intercostal region of the abaxial face, however, in this work it was observed that ireartea exorrhiza is amphistomatic.
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