3.2.1 Floral bud
In the basal region, below the receptacle, the sections of the peduncle of the flower bud showed a flat circular shape (Figure 2A) with undulated uniseriate epidermis. Adjacent to the epidermis, the cortical region is filled by 2-4 layers of fundamental parenchyma, and internally there is a thick continuous layer of sclerenchyma forming a ring that surrounds the entire phloem (Figure 2A). The vascular system has seven collateral bundles: three larger ones facing the adaxial face and four smaller ones positioned abaxially. The medullary parenchyma is well developed, with larger cells in relation to the cortical parenchyma, and with lignified cell walls (Figure 2A).
Above the peduncle, the calyx has two short sepals (Figure 2B) positioned externally to the others that are more elongated. Internally to the sepals, there are petals that are thinner in relation to the sepals (Figure 2C), creating a wrap protecting the reproductive whorls. Sections of the median part of the bud showed how the androecium was arranged on the bud axis: the pollinating anthers located abaxially (Figure 2D) and the feeding anthers adaxially (Figure 2E). In this part of the bud, it was possible to reach only one staminode that is placed to the left of the feeding anthers (Figure 2F). The gynoecium is positioned in the center of the bud (Figure 2G).
3.2.2 Calyx
The calyx of S. aversiflora is heteromorphic, consisting of five sepals, two smaller external ones and three larger internal ones. In paradermal section, the sepals present an epidermis with irregular cell shapes, with anticlinal walls varying from curved to sinuous on both surfaces (Figure 3A-B), a striated cuticle (Figure 3C), non-glandular trichomes restricted to the margins (Figure 3D), and glandular trichomes at the base. Sepals are hypoestomatic with plenty anisocytic (Figure 3A-B) and tetracytic stomata. Stomata occur mostly in the basal and median region of the sepals, being rare or absent at the apex (Figure 3D-E). In cross-section, the epidermis of both the adaxial and abaxial surface is uniseriate with cell shapes ranging from oval to rectangular, and adjacent to the epidermis, there is a homogeneous mesophyll (Figure 3F) with circular cells of different sizes and few intercellular spaces (Figure 3G). Collateral vascular bundles occur sparsely along the mesophyll (Figure 3 F-H).
3.3.3 Corolla
The corolla of S. aversiflora is composed of five petals that differed morphologically into, consisting of three groups: concave petals, smaller non-concave petals, and larger non-concave petals (Figure 4A). Internally, consistent anatomical patterns are observed, with few variations in the analyzed traits. In paradermal section, the concave petals present varied cell shapes and anticlinal wall contours: on the adaxial surface, in the basal region, the cells are polygonal with curved and straight anticlinal walls (Figure 4B) while in the median and apical regions, the cells are irregular and have sinuous anticlinal walls (Figure B). In turn, on the abaxial surface, the anticlinal walls are evenly sinuous throughout the petal (Figure 4C). The other petals present irregular cell shapes with sinuous anticlinal walls on both surfaces (4D-O). Stomata are distributed on both epidermal surfaces (4H-I), with the tetracytic and anisocytic types being most commonly observed (Figure 4I), and the anomocytic type the more rare. The sinuosity of the epidermal cells forms crests and valleys and between these points we observed the formation of bordered pit chambers in all petals, more conspicuously in the smaller non-concave petals (Figure 4J, K-L). Simple trichomes are present in all petals, sparsely distributed in the basal region and more densely on the veins (Figure 4M-N). The petals are glabrous in the apical region. Sessile glandular trichomes are rarely found, being observed only by SEM (Figure 4O).
In cross section, the petals have a uniseriate epidermis on both surfaces, with cells of tabular to oval shapes (Figure 5A), simple trichomes sparsely distributed on the epidermis (Figure 5B), and rare sessile glandular trichomes (Figure 5C). The mesophyll is homogeneous, consisting only of braciform cells (Figure 5D). The cells in the concave petals have visually shorter arms that connect to each other forming a kind of network with visually smaller intercellular spaces (Figure 5A and D) in relation to the other petals. In the latter, the cellular arms are longer and more loosely connected, leaving intercellular spaces that are visually larger in relation to the concave petals (Figure 5E). The petals have planoconvex veins (Figure 5I), except the concave petals, whose lateral veins are biconvex (Figure 5J). The vascular system is collateral, formed by a single central bundle that occurs in the center of the veins (Figure 5I-J).
3.3.4 Androecium
Senna aversiflora has heterantherous flowers. The androecium is formed by three larger pollinating anthers, four smaller feeding anthers, and three staminodes. Although the external morphology differs, the internal structural organization of the anthers shows a pattern (Figure 6A-B). In cross section, the anthers are composed of a pair of thecae (Figure 6A-B). The exothecium is papillose and uniseriate (Figure 6C), glabrous, and covered by a striated cuticle. Adjacent to the epidermis, there is a fibrous endothelium (Figure 6C) and, more internally, 1-2 extracts of parenchymal tissues followed by the collapsed tapetum (Figure 6C). The lobes are separated by connective tissue and within each lobe, the parenchymal tissue separates the lobes into two locules (totaling four locules). In the central region of each locule, the parenchymal tissue protrudes vertically and at its end forms a convoluted swelling (Figure 6D). The vascular tissue is located in the central region of the connective site (Figure 6D) consisting of three collateral bundles, one smaller central one and two lateral ones (Figure 6E). The filament has a convex concave shape (Figure 6F) with a uniseriate epidermis and the cortical region consists of 1-2 layers of collenchyma and 5-7 layers of parenchyma. The vascular system is collateral, formed by a single U-shaped central bundle (Figure 6F). Analyses of light microscopy images showed no observable structural modifications between intact and damaged anthers.
3. 3. 5 Gynoecium
The gynoecium of S. aversiflora is formed by a single carpel with an ovary varying in shape from linear to curved, with a reduced style (Figure 7A). In cross section, the ovary is a rectangle with dilated angles and a slight narrowing of the sides (Figure 7B). The epidermis is composed of uniseriate tabular cells with thickened cell walls and simple trichomes that are densely distributed throughout the epidermis. A hypodermis is found adjacent to the epidermal tissue, with visually larger cells than the epidermis. Internally to the hypodermis, the filling tissue consists of parenchyma with compressed cells, without intercellular spaces. Two conspicuously smaller cell layers occur more internally towards the locule, and adjacent to these layers, we observed the parietal tissue with 1-3 layers of elongated and juxtaposed cells (Figure 7C).
The ovule is placed in the center of the locule. In longitudinal section, the ovule is seen located at the base of the ovary (Figure 7D). The ovary is continuous with the style (Figure 7E). As the style approaches the stigma, the locule narrows (Figure 7F). The vascular system is collateral, formed by four V-shaped bundles arranged at the angles of the ovary and the style (Figure 7G). The stigma is obovate with a uniseriate epidermis and hypodermis, fundamental parenchyma filling the central region, and vascular bundles reduced to two U-shaped bundles located at the ends of the stigma (Figure 7H-I). At one end, the vascular bundle surrounds the slit of the stigma (Figure 7I).