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).