Inflorescence and flower morphology - Inflorescences are interfoliar, but sometimes seem infrafoliar as they emerge between old, persistent leaf sheaths (Fig. 1a). They are surrounded by the profile (Fig. 1a, b), which has two very marked keels on its dorsal side, located laterally and extending from the base to the apex; the middle part is flat. This structure is covered by a creamy-white sometimes brownish, scattered indumentum on both surfaces. All observed inflorescences are spiciform (Fig. 1c, d). The peduncular bract (Fig. 1b-d) is persistent, covered by a creamy white indumentum, sometimes with some brown appendages; frequent on the abaxial surface and scarce to none on the adaxial. The peduncle is straight in immature inflorescences (Fig. 1a, b) and recurved after anthesis (Fig. 1c, 1d), cylindrical or nearly so (subterete); with scattered brown indumentum. The flowers are arranged in triads (Fig. 1e-h), which are distributed along the entire rachis (Fig. 1e, f). The rachis is covered by brown indumentum (Fig. 1g, h). The dimensions of some of these structures are presented in Table 2.
Table 2 Average values and standard deviation of some morphological variables of the inflorescences and flowers of Bactris simplicifrons.
Variable
|
Mean ± Standard deviation
|
Inflorescence length (cm)
|
4.94±0.72
|
Peduncle length and diameter (cm)
|
2.,64±0.27 for 0.32±0.56
|
Profile length (cm)
|
4.52±0.96
|
Peduncular bract length (cm)
|
8.42±0.74
|
Raquis length and diameter (cm)
|
3.82±0.41 for 0.27±0.032
|
Number of pistillate flowers
|
33.0±1.58
|
Number of staminate flowers
|
72.,25±3.49
|
Size of pistillate flowers (mm)
|
2.96±0.18 for 1.48±0.13
|
Size of staminate flowers (mm)
|
5,21 ±0.82 for 3.03±0.16
|
The size of the flowers differs considering the morphs of the flowers, the staminate flowers being larger. Both floral morphs are trimerous, with heterochlamydeous perianth, imbricated pre-flowering.
In the staminate flowers, the calyx is formed by three sepals briefly united at the base (Fig. 1i, j), of a light cream color, its length is approximately 1mm. The corolla is tubular, cream-colored, and 4-6 mm long (Figs. 1i-k). The six stamens are anisodynamous, epipetalous with dithecal, tetrasporangiate anthers and longitudinally dehisced (Fig. 1k).
In the pistillate flowers, the calyx and corolla are tubular and cream-colored with brown dots forming longitudinal parallel rows in the first, which measures 2˗3 mm in length and the second, 1.6 to 2.5 mm in length (Fig. 1l, m). Three small, leafy staminodes are evident (Fig. 1n). The gynoecium presents an ovoid to tubular ovary, superior, with a relatively long style and a stigma with three slightly pronounced lobes (Fig. 1o-p).
Ontogeny of the reproductive structures - The collection of inflorescences in early stages of development is difficult, due to their interfoliar development. In addition, the presence of various envelopes on the flower buds, as well as their size in the early stages of development, complicate their individualization, making their ontogenetic study under a scanning electron microscope extremely difficult, therefore some aspects of development could not be determined.
The development process was studied from spiked inflorescences, which in their early stages, stage I, are perceptible as a rachis with triad bracts attached to its surface (Fig. 2a, b). They are observed laterally connate (Fig. 2c), generating the false idea that a bract protects two or more triads. A meristematic mass is evident in the axil of these bracts (Fig. 2d). Sometimes the meristematic mass does not develop, very closely packed bracts are visible (Fig. 2e). Such mass is divided into three also meristematic portions (Fig. 2f, g) and, from each of them, a floral bract is differentiated, which is not very long and is less thick than the bract of the triad (Fig. 2h). The development of the flowers occurs almost simultaneously on the rachis and asynchronously between the flowers of the triad; however, it appears to be very fast along the axis. The differentiation of the floral parts of each whorl is centripetal.
The parts of the perianth differentiate in stages II to IV. In both staminate and pistillate flowers the first whorl that is formed is the calyx (which initially covers the dome of both morphs (Fig. 2i-k). Soon, the corolla is differentiated (Fig. 2l, m). In staminate flowers, the sepals are slightly elongated, so it is the corolla that covers the dome (Fig. 2m), while in pistillate flowers both whorls are elongated, and cover the meristematic floral apex (Fig. 2n). Idioblasts with raphides are differentiated at an early stage in the calyx of the female flowers.
The fertile whorls develop from stage V. The staminate flowers complete their differentiation before the female ones. When the fertile whorl will be the androecium, the meristematic dome grows into a short axis and rises a little inside the flower, then it widens and flattens slightly, subsequently initiating the development of the unequal-sized mammiliform staminal primordium, and with a procambial bundle (Fig. 2o-r). The raised dome remains, forming a parenchymal protrusion, which fuses with corolla tissue (Fig. 2s). Each primordium elongates a bit and then begins to differentiate, first the anther, which appears as a widening in the distal part of the primordium (Fig. 2s, t). In the anthers, two regions are initially distinguished, the external one with four layers that will constitute the walls of the anther and the internal one, constituted by the sporogenous tissue (Fig. 2u, v). As the anther matures, the first two layers differentiate into the exothecium and endothecium, the latter with characteristic fibrous thickenings. The third layer forms the middle layers, which are compressed tangentially during microspore development. The fourth layer has prominent cells of various shapes, relatively large and with very evident nuclei and forms the secretory tapetum (Fig. 2w, x), which remains intact until the beginning of microgametogenesis. In the central zone of each sac, mother cells can be distinguished, with polygonal contours, with dense content and a large nucleus (Fig. 2u, v), which go through microsporogenesis until the formation of microspores; tetrads display various forms, they are heteromorphic (Fig. 2x), and remain for some time in the callose envelope, before being released as pollen grains (Fig. 2y, z).
In pistillate flowers, the receptacle appears rounded in cross section (Fig. 3a). Three carpels originate from the meristematic dome, which are distinguishable in longitudinal section, but not in cross section (Fig. 3b); the forming gynoecium adopts a triangular shape, in cross section (Fig. 3c). In addition, the dome laterally produces two or three meristematic primordia, visible between corolla and developing carpels, similar to those observed when stamen formation begins (Fig. 3c, d). The carpels are united dorsally and ventrally in the proximal part, only dorsally in the distal region, where the chambers of each carpel communicate, and are free or only loosely united in the apical part (Fig. 3d-j). They are provided with a dorsal vascular bundle (Fig. 3e). No sutures are visible between them (Fig. 3d, e), except in some regions of the apical area, where it is feasible to verify that they are conduplicated (Fig. 3g). The axis of the ovary is the product of the ventral fusion of the carpels and probably, of the intervention of the meristematic apex (Fig. 3d, e); such axis is prolonged vertically, until it stops its developing, in the zone that will constitute the limit between ovary and style; however, when the structure has not elongated, it is visible almost to the distal end of the gynoecium (Fig. 3f). In this portion, the carpels show indentations in the ventral part, in front of the dorsal vascular bundles (Fig. 3f-h); such slits posteriorly constitute the triradiate stylar canal. The differentiation of the gynoecium occurs from the distal to the proximal part, which is evident by the early distinction of stigmatic papillae and the poor differentiation of the ovary (Fig. 3j, k). The stylar channel is evident as an open (Fig. 3k), triradiate structure (Fig. 3l). In the ovary there are three locules and in the axial zone of each one the placenta is distinguished, underdeveloped, which originates an ovule (Fig. 3n-o). In the wall of the ovary, the epidermis and the procambial bundles immersed in the mesophyll are clearly distinguished. The ovule differentiates as a sessile structure, with a single integument and the nucellus limiting the mother cell of the macrospore, in which the processes leading to the formation of the embryonic sac occur (Fig. 3o, p). The mature embryo sac contains the ovocell, synergid, the central cell with secondary nucleus and the antipodes, the latter persistent (Fig. 3q).
Parts of the inflorescence, anatomical structure - Profile (Fig. 4a, b) with papillose epidermis and abundant appendages on the abaxial surface and with cells from straight to slightly convex walls and thick on the adaxial one. The mesophyll is formed by parenchyma, which collapses in some areas; presents idioblasts with raphides. Collateral vascular bundles are evident, the largest of them with a prominent sclerenchymatic cap and stegmata in the peripheral layer; they are located in greater proportion towards the abaxial surface in the median nerve and in the median zone of the blade.
Peduncular bract (Fig. 4c) with convex-walled epidermal cells on both surfaces, but more pronounced on the abaxial one; appendages scattered on abaxial surface. Mesophyll formed by parenchyma; Stegmata and cells with dark granular contents are evident. Fibrous and fibrovascular bundles are developed and complex, in high frequency towards the abaxial surface, whilst those located towards the adaxial surface are less frequent and less complex
The peduncle (Fig. 4d) and rachis (Fig. 4e, f) present a similar histological arrangement. They are covered by a layer of epidermis of quadrangular to rectangular cells, with stomata and abundant appendages. Next, there is parenchyma with scattered raphides idioblasts, and fibrous bundles in the external zone. Fibrovascular bundles closed collaterals are present towards the center, showing a complex organization pattern in the rachis, due to the traces that emerge towards the bracts and triads. Both types of bundles present stegmata in their outermost layer.
The bract of the triad (Fig. 4g-i) is thickened in the central zone and thinner towards the margins. It is covered by a layer of epidermis with rectangular cells, and convex external walls on the abaxial surface and straight on the adaxial. Mesophyl formed of parenchyma; idioblasts with raphides present. Fibrous and fibrovascular bundles are immersed in the central zone, but mostly visible in the basal part.
The floral bracts (Fig. 4g, j) are covered by epidermis with quadrangular to rectangular cells, with a straight to slightly convex external wall. The mesophyll is occupied by two to three layers of parenchyma and up to five in the central portion. They lack vascular tissue.
Anatomy of staminate flowers - The calyx (Fig. 5a-d) is covered by unistratified epidermis on both surfaces, with quadrangular cells with a thick external wall and ornamented on the abaxial and rectangular, thin-walled on the adaxial. The mesophyll is visible in the proximal area, constituted of 3 to 4 layers of parenchyma, while in the distal portion it presents one to two layers close to the middle nerve and on the sides only the two epidermis. Idioblasts with raphides are visible. Vascular system is formed by a single vascular bundle or without vascularization.
The corolla (Figs. 5e-g) presents connate petals, of irregular thickness, but greater at the base, with unistratified epidermis on both surfaces, with quadrangular to rectangular cells, but on the abaxial surface it is papillose, thick-walled with ornamented cuticle (Fig. 5h); while in the adaxial, the walls are thin, the external one being slightly convex, also ornamented. Stomata evident on the abaxial surface (Fig. 5h, i). Mesophyll with 8-20 parenchyma layers, cells located abaxially larger than those located towards the adaxial surface; some of the cells with granular, dark content and wide diameter idioblasts, formed by one to three cells in longitudinal view, with bundles of raphides; these idioblasts are located mostly towards the abaxial surface. Closed collateral fibrovascular bundles located approximately in the middle region of the mesophyll and interspersed with fibrous bundles, the latter located towards the abaxial surface. Stegmata in the layer that separates each fibrous or fibrovascular cord from the rest of the tissues.
Epipetalous stamens (Fig. 5j-l). Staminal filament is broadened at the base and narrower at the apical portion (Fig. 5m), with a trapezoidal outline at the base and spherical to triangular at the top (Fig. 5n-p). It has a unistratified epidermis, with quadrangular, papillose cells and markedly covered by cuticle ornamented (Fig. 5q). Internally, 3-5 layers of parenchyma, and in the center a closed collateral bundle without associated sclerenchyma (Fig. 5n-p). Anther wall (Fig. 5r-t) with exothecium of rectangular, thin-walled cells, the outer one slightly convex; endothecium with rectangular cells, arranged with their long axis perpendicular to the surface and longitudinal thickenings. Remnants of the tapetum remain recognizable, attached to the inner part of the wall of the microsporangium (Fig. 5r). Stomium is formed by smaller cells (Fig. 5r). There are abundant pollen in pollen sacs. The connective has papillose epidermal cells, followed by parenchyma with some of its cells provided with thickenings similar to those of the endothecium, and in the central zone a closed collateral bundle (Fig. 5r, u).
Anatomy of pistillate flowers - The calyx and corolla are similar in structure (Figs. 6a-c), the epidermis of the adaxial and abaxial surfaces have quadrangular and rectangular cells, with a thin and smooth external wall in the former, and a thick and ornamental one in the second (Fig. 6d). The mesophyll, in the proximal portion of both floral parts, is formed by 12 parenchyma layers in the calyx and corolla with raphid idioblasts located mostly towards the abaxial surface; while towards the distal portion they have 7 or 8 layers. Only in the calyx there are randomly distributed idioblasts with granular phenolic compounds. Fibrous bundles visible mostly towards the abaxial surface and fibrovascular bundles, closed collaterals with prominent caps of sclerenchyma, located in the middle region of the mesophyll, these bundles cover almost the entire thickness of the mesophyll, in the distal part of both whorls. Stegmata visible in both types of bundles.
Three short leafy structures are evident between it and the gynoecium (Fig. 6e, f). They are made up of a unistratified epidermis, with quadrangular to rectangular cells with a straight to slightly convex external wall; followed by one or two layers of parenchyma and a vascular bundle, of little complexity that may or may not be present (Fig. 6g, h).
Syncarpic ovary (Fig. 6i-m) with rectangular cell epidermis with papillose outer wall and appendages. There is parenchyma with fibrous bundle very close to the ventral surface and sclerenchymatous idioblasts Idioblasts with raphides are very abundant above the area where the locules are. It is evident the axis of the ovary with a short extension towards the stylar channel (Fig. 6l). Vascular bundles arranged in ring (Fig. 6j); dorsal bundles and their ramifications are evident in the upper part of the ovary (Fig. 6m). Anticlinally elongated cells of different sizes and with dense protoplasmic content limit the loculus (Fig. 6i, j, l). These cells are continue along with those of the stylar channel. The ovules (Fig. 6l) are orthotropic, unitegumented, and hang from the upper part of the ovary axis. There are cells with different protoplasm in the adjacencies of the micropyle.
Style (Fig. 6n-p) with lengthwise variation in diameter, being larger in the proximal region. It is covered by epidermis of quadrangular and rectangular cells, with a slightly convex external wall; followed by parenchyma with tanniferous idioblasts (cells with granular content), scarce and with externally located fibrous bundles and closed collateral fibrovascular bundles, organized in a ring located towards the interior, the fibrous bundles are not visible in the distal part; In the central zone, there are two to three layers of narrow parenchyma cells followed by abundant papillae of different lengths and dense content, which limit the stylar channel.
Stigma (Fig. 6u-w) with unistratified epidermis of rectangular cells, with a smooth to slightly convex outer wall, followed by 3 layers of parenchyma, and in the central region papillose cells of different sizes, some of them with phenolic content, without evident vascularization.
Appendages - Unicellular tector trichomes of different sizes, thin walls and blunt apices, very flexible (Fig. 7a-c). These trichomes form a tomentous indumentum on the prophyll, peduncle and rachis (Fig. 7d-e); they occur in scattered groups on the peduncular bract (Fig. 7f), and tend to remain individualized and are shorter in the ovary and style (Fig. 7g, h).
Secretory, multicellular, uniserate trichomes, relatively thick-walled with a more prominent terminal cell, variegated in shape, dense content, which is easily detached (Fig. 7i-m). These appendages are less frequent and are found mostly on the rachis and peduncle (Fig. 7n).