Anatomia caulinar e foliar de Pilea peperomioides Diels (Urticaceae) com ênfase nas suas estruturas secretoras

Abstract

Pilea peperomioides, commonly known as the "Chinese money plant," is native to China but is commercially cultivated in Brazil due to its ornamental appeal. Given the scarcity of anatomical studies on this species, the present research aimed to anatomically characterize the stem and leaf of Pilea peperomioides, with an emphasis on its secretory structures, contributing to taxonomic and ecological studies on the species. Botanical material was obtained from floriculture shops, and two individuals were used. Samples of the leaf blade, petiole, and stem were fixed in FAA 50. After embedding in Paraplast, the leaf blade and petiole samples were sectioned using a rotary microtome. Hand sections were performed on the leaf blade for epidermis analysis and on the stem. Paradermal sections of the leaf blade were stained with safranin, while transverse sections of the blade, petiole, and stem were subjected to double staining with safranin and Astra blue or basic fuchsin and Astra blue in tartaric acid. Semi-permanent slides were mounted in glycerin, and permanent slides were mounted in glass varnish. The leaf is epistomatic, with anisocytic stomata. In the leaf blade, the epidermis is uniseriate; stomata, lithocysts, and trichomes occur on the adaxial surface and are positioned at the same level as the epidermal cells. The chlorophyllous parenchyma is concentrated on the adaxial side, while the abaxial side has several layers of water-storage hypodermis. The vascular system is composed of collateral vascular bundles. The midrib has a plano-convex contour with a single collateral vascular bundle shaped like an open arch; lithocysts occur sporadically on the most convex portion of the contour. The petiole has arounded contour with a uniseriate epidermis. Lithocysts are observed in transverse sections with spherical cystoliths. Internally, there are two to three layers of collenchyma, followed by several layers of filling parenchyma. The stem has a rounded contour and exhibits secondary growth. The epidermis is uniseriate, with spherical cystoliths. Beneath the epidermis, periderm formation occurs, followed by five to six layers of angular collenchyma. The cortex and medullary cells store starch, which is abundant in the medulla. The secretory system consists of hydathodes, trichomes, and secretory sacs. Hydathodes are distributed throughout the abaxial surface of the leaf blade, forming depressions in the epidermis. Glandular trichomes, scattered on the adaxial surface of the leaf, are peltate, consisting of a basal cell, a short stalk, and a multicellular head. Secretory sacs are associated with larger vascular bundles in the interveinal regions, the central vein, the leaf margin, and the petiole, where they occur adjacent to vascular bundles. In the stem, secretory sacs are distributed in the cortex and medulla, the latter located near the primary xylem. Longitudinal sections of the stem revealed that these cavities have an elongated lumen and can therefore be classified as secretory ducts. P. peperomioides exhibits anatomical characteristics already described for Urticaceae and the genus Pilea. This study provides a detailed characterization of the hydathodes in P. peperomioides, an aspect that had not been previously investigated. Additionally, the species has anatomical features associated with adaptation to the mountainous and humid environments where it naturally occurs. Versão