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Article

Characterizing the Seed Coat in the Subtribe Angraecinae (Orchidaceae, Vandeae) and Its Taxonomic Value

by
Roberto Gamarra
1,2,
Emma Ortúñez
1,2,*,
Pablo De La Fuente
3,
Guillermo Valdelvira
1 and
Álvaro Hernando
4
1
Departamento de Biología, Universidad Autónoma de Madrid, 28049 Madrid, Spain
2
Centro de Investigación en Biodiversidad y Cambio Global (CIBC-UAM), Universidad Autónoma de Madrid, 28049 Madrid, Spain
3
Departamento de Biodiversidad, Ecología y Evolución, Universidad Complutense de Madrid, 28040 Madrid, Spain
4
Departamento de Farmacología, Farmacognosia y Botánica, Universidad Complutense de Madrid, 28040 Madrid, Spain
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(4), 280; https://doi.org/10.3390/d17040280
Submission received: 25 March 2025 / Revised: 10 April 2025 / Accepted: 14 April 2025 / Published: 16 April 2025
(This article belongs to the Section Plant Diversity)
Browse Figures
Versions Notes

Abstract

:
The seeds of 121 species belonging to 38 genera of the subtribe Angraecinae (Orchidaceae) were studied in terms of their morphological characteristics under a scanning electron microscope (SEM). This study provided new data about the seed micromorphology of 17 genera and 100 species. Ten qualitative traits of the seed coat were analyzed, of which four were common for all the examined samples: elongated testa cells, anticlinal zone, curved transverse anticlinal walls, and sunken and narrow-to-not visible periclinal walls. These features are consistent with the epiphytic life-form of the genera studied. However, variation among genera was observed with reference to the seed shape, the morphology of the apical and the basal poles, the arrangement of the medial cells, the morphology of the longitudinal anticlinal walls, and the presence of testa extensions. A cluster analysis was performed, and two large groups were segregated according to the seed shape. Within a genus, all the examined species showed the same pattern of seed coat, except in Diaphananthe, Mystacidium, and Rhipidoglossum. On the other hand, the variation in the seed coat observed in Angraecum sensu lato supported its segregation into different genera. Our results perfectly fitted with recent taxonomic proposals.

1. Introduction

Orchid seeds are formed by a thin testa with dead cells along a longitudinal axis ending in two poles (the apical or chalazal and the basal or micropylar) enclosing a pluricellular embryo. Their tiny size and the presence of free air space surrounding the embryo suggest adaptation to an anemochorous dispersal [1,2,3].
Preliminary studies on seed coats stated that qualitative traits have significant importance in classifying orchids on a suprageneric level [4]. Later, studies on the tribe Orchideae demonstrated that some features of the seed coat (seed shape, the morphology of the cells and the anticlinal walls, the ornamentation of the periclinal walls) are helpful in resolving generic delimitations and supporting phylogenetic results based on molecular analyses [5,6,7].
It is accepted that Orchidaceae initially evolved as a terrestrial group and later expanded to the epiphytic habitat and its strong diversification [8,9,10]. According to this evolution, the ancient seed coat traits are present in terrestrial orchids of the subfamilies Apostasioiedae and Cypripedioideae, whose seeds are characterized by ovoid to fusiform shape, isodiametric to slightly elongated medial cells, straight longitudinal and transverse anticlinal walls, and visible periclinal walls [3]. Vij et al. [11] suggested that several seed morphological traits (fusiform in shape, quadrangular cells of the testa, the simple arrangement of the cells along the longitudinal axis) appeared to be basic in orchids, and that the development of heavily thickened testa cells in epiphytic orchids seemed to have an adaptive significance that contributed to protecting the embryo under higher humidity. Barthlott et al. [3] regarded the rectangular shape in the testa cells as ancestral, and the straight longitudinal anticlinal walls as a common condition in orchids. Some characteristics, such as the elevated and curved transverse anticlinal walls or the presence of testa extensions, are only found in epiphytic orchids of the subfamily Epidendroideae [3,12], the most diverse group within Orchidaceae [13,14].
Within Epidendroideae, the tribe Vandeae Lindl. included the subtribe Angraecinae Summerh., which according to Pridgeon et al. [15], comprises 49 genera and about 760 species, mostly found in Africa (from West Tropical Africa to Southern Africa and the Western Indian Ocean), with 2 species in Sri Lanka (Aerangis hologlottis (Schltr.) Schltr. and Angraecum zeylanicum Lindl.), and 2 genera (Campylocentrum Benth. and Dendrophylax Rchb.f.) that grow exclusively in America (from the southeastern U.S.A. to Argentina) [16]. The vast number of species are epiphytes, a few are also lithophytes [15].
The molecular analyses demonstrated the monophyly of this subtribe against Aeridinae [13,17]. Within Angraecinae, molecular analyses confirmed that many genera, such as Aerangis, Ancistrorhynchus, Bolusiella, Campylocentrum, Dendrophylax, Microcoelia, or Solenangis, were monophyletic, but others, such as Diaphananthe, Rhipidoglossum, or Tridactyle, were polyphyletic or paraphyletic [18,19,20,21,22,23].
On the other hand, the genus Angraecum Bory sensu lato has been confirmed as polyphyletic in all the molecular analyses [13,17,18,19,20], conducting its segregation and merging other genera such as Afropectinariella M.Simo & Stévart, Conchograecum Szlach., Grochocka, Oledrz. & Mytnik, Dolabrifolia (Pfitzer) Szlach. & Romowicz, or Lesliegraecum Szlach., Mytnik & Grochocka [20,24,25,26].
Studies on seed coat in the subtribe Angraecinae are extremely scarce considering its high diversity in genera and species [3,12,27]. In a comprehensive work on seed morphology in Orchidaceae, Barthlott et al. [3] analyzed the seed coats of 18 genera of this subtribe, concluding that their seeds belong to the Vanda type, characterized principally by short to elongated scobiform in shape, elongated testa cells along the longitudinal axis, sunken and hardly visible periclinal walls, and strong marginal ridges [1,3,12]. This pattern has also been observed in representatives of the subtribes Adrorhizinae, Polystachyinae, and Aeridinae [3,28,29], which belong to the tribe Vandeae [15]. Gamarra et al. [12] concluded that twisted medial cells, a thickened anticlinal zone with prominent ridges, raised and thickened cell corners, narrow-to-not visible periclinal walls, and the presence of waxes were common traits in epiphytic orchids in African Vandeae.
Studies on seed morphology are essential to recognize adaptation mechanisms, dispersal, and germination in tropical orchids, because they are vulnerable to habitat destruction and climate change, and these studies can help conservation programs of orchids [30,31].
Our study is part of an ongoing investigation into fruits and seed coats in the tribe Vandeae (Epidendroideae) [28,32]. One of the main goals of this research is to characterize the seed micromorphology in the subtribe Angraecinae, contributing to the better knowledge of the genera studied and providing new tools that are essential to understand the dispersal, germination, and distribution of these orchids in tropical habitats. This study will also contribute to conservation programs considering the alteration of habitats and future scenarios regarding climate change. Another goal is to check if the qualitative traits of seeds remain constant within a genus, as we have verified in precedent investigations on orchid seeds, using this information to assess their potential taxonomic value.

2. Materials and Methods

2.1. Plant Material

Mature seeds from 121 species representing 38 genera of the subtribe Angraecinae were analyzed, of which 100 species and 17 genera were studied for the first time. Samples were obtained from mature capsules of the sheets deposited in the K (Royal Botanic Gardens, Kew, UK) and MA (Real Jardín Botánico de Madrid, Spain) herbaria, according to the conservation protocols of these institutions and curators’ permission. Dry seeds were preserved in paper envelopes at room temperature. A list of voucher specimens is presented in Table S1. Scientific names and authorities are presented according to POWO [16].

2.2. Microscopic Study

Samples of each specimen were mounted on scanning electron microscope (SEM) stubs and coated with gold in a sputter-coater (SEM Coating System, Bio-Rad SC 502, Bio-Rad Laboratories, Madrid, Spain). Ten seeds of each sample were observed in a Philips XL30 (Philips, Amsterdam, The Netherlands) at an accelerating voltage of 20 kV in the Interdepartmental Service of Investigation at the Universidad Autónoma de Madrid (SIDI-UAM), and SEM micrographs were taken.
For each species, the morphological observations were focused particularly on the variability of ten qualitative traits: the seed shape, morphology of both poles, typology of the medial cells and their arrangement, presence of anticlinal zone, morphology of the longitudinal and the transverse anticlinal walls, periclinal walls, and the presence of testa extensions. Seed structure terminology follows Vij et al. [11], Barthlott et al. [3], Verma et al. [33], and Gamarra et al. [12].

2.3. Cluster and Principal Component Analysis

A standardized basic matrix was utilized for the qualitative traits studied. Following the criteria in Vij et al. [11] and Barthlott et al. [3], a binary code was provided for each trait (0: ancestral state; 1: derived state). Using this codification, a cluster analysis was performed with the UPGMA agglomeration hierarchical method and the similarity among taxa was calculated using the Jaccard similarity index with PAST 4.02 statistics software [34]. The Jaccard index was chosen due to its effectiveness in showing the seed morphology coincidence, and its reliance on presence/absence data. The relationships among the qualitative traits were determined using principal component analysis (PCA) performed using PAST 4.02 statistics software [32]. A bi-plot graphic was generated based on the main principal components.

3. Results

A set of ten qualitative traits of the seed coat were analyzed via SEM for all the specimens included in Table S1. From these, four were common in all the seeds studied: elongated medial cells, thickened anticlinal zone, curved transverse anticlinal walls, sunken and narrow-to-not visible periclinal walls (Figure 1 and Figures S1–S4).
The rest of the qualitative features show differences. Seed shape varies from fusiform to filiform (Figure 2A,B and Figures S1–S4), the apical pole from conical to pointed (Figure 2A,B and Figures S1–S4), the basal pole from truncated to pointed (Figure 2C,D and Figures S1–S4), the arrangement of the medial cells along the longitudinal axis from straight to twisted (Figure 3A,B and Figures S1–S4), and the morphology of the longitudinal anticlinal walls from straight to undulating (Figure 3C,D and Figures S1–S4).
Testa extensions have been observed in 11 of the genera studied, in both poles in the genera Dendrophylax, Dinklageella, Microcoelia, and Solenangis, or only in the basal pole in Campylocentrum, Conchograecum, Cryptopus, Erasanthe, Lesliegraecum, Neobathiea, and Oeonia. These extensions arise from the anticlinal walls at the ends of the testa cells and vary in morphology from long trichomes to grappling hooks (Figure 4A–C and Figures S1–S4). The rest of the genera lack testa extensions and the ends of the testa cells are similar in morphology, with curved transversal anticlinal walls except in Afropectinariella, which shows cucullated protrusions (Figure 4D).
Seed qualitative traits are common for all the examined species within a genus (Table 1, Figure 5 and Figures S1–S4), except for the genera Diaphananthe, Mystacidium, and Rhipidoglossum.
In Diaphananthe, the main difference is the arrangement of the medial cells (Table 1). They are straight in D. ichneumonea, D. letouzeyi, D. odoratissima, D. pellucida (Figure 6A and Figure S2), and D. plehniana, and twisted in D. bidens, D. fragrantissima, D. sarcorhynchoides, and D. vesicata (Figure 6B and Figure S2). In Rhipidoglossum, all the seeds have a pointed apical pole (Figure 6C and Figure S4), which is conical in R. brachyceras (Figure 6D). In Mystacidium, the apical pole is conical in M. pulchellum (Figure 6E), but pointed in M. tanganyikense (Figure 6F).
All the examined species in Angraecum show the same pattern of seed coat, with fusiform seeds, conical apical pole, truncated basal pole, straight medial cells along the longitudinal axis, straight longitudinal anticlinal walls, and absence of testa extensions (Figure 7A and Figure S1). This pattern shows differences in the seed coat of related genera such as Afropectinariella, Conchograecum, Dolabrifolia, and Lesliegraecum, based on the seed shape, the morphology of the apical pole, the arrangement of the medial cells along the longitudinal axis, or the presence of testa extensions (Table 1). The main difference in Afropectinariella is the presence of cucullated protrusions in the curved transverse anticlinal walls (Figure 4D). Dolabrifolia differs by the twisted arrangement of the medial cells (Figure 7B). On the other hand, the seeds of Conchograecum and Lesliegraecum are filiform in shape, with testa extensions in the basal pole, but the longitudinal anticlinal walls are straight in Conchograecum (Figure 7C and Figure S1) and undulating in Lesliegraecum (Figure 7D and Figure S3).
The uncommon qualitative traits reported in Table 1 were scored as binary (Table 2) for the 121 species studied, and a matrix was performed.
The UPGMA phenogram resulting from the cluster analysis is shown in Figure 8, which illustrates relations based on seed coat among the genera of Angraecinae. The cophenetic coefficient value using the Jaccard similarity index is 0.9658. Two major clusters are segregated according to the seed shape, with a similarity index higher than 0.54: filiform (upper part of the phenogram) or fusiform (lower part of the phenogram).
Within the cluster with filiform seeds in a shape with a similarity index higher than 0.80, the first partition segregates the genus Sphyrarhynchus, which lacks testa extensions. The genera with testa extensions are fragmented in a third cluster level: with pointed apical and basal poles and testa extensions in both poles (Dendrophylax, Dinklageella, Microcoelia, Solenangis), and with a pointed apical pole and truncated basal pole with testa extensions (Campylocentrum, Conchograecum, Cryptopus, Erasanthe, Neobathieae, Oeonia). On this last cluster level, the genus Lesliegraecum is detached via the presence of undulating longitudinal anticlinal walls.
The genera with seeds fusiform in shape show a similarity index higher than 0.75. Two cluster levels according to the morphology of the apical pole are segregated. The first group includes the genus Aerangis, which is segregated by the presence of twisted medial cells with regard to Mystacidium tanganyikense and the genus Rhipidoglossum (except R. brachyceras), which have straight medial cells along the longitudinal axis. The second group distributes the genera into two subgroups according to the arrangement of the medial cells: straight in Afropectinariella, Ancistrorhynchus, Angraecopsis, Angraecum, Bolusiella, Cyrtorchis, Eurychone, Jumellea, Lemurella, Listrostachys, Mystacidium pulchellum, Oeoniella, Plectrelminthus, Podangis, Rhipidoglossum brachyceras, Sobennikoffia, Tridactyle, Ypsilopus, and some species of Diaphananthe, and twisted in Aeranthes, Calyptrochilum, Dolabrifolia, Nephrangis, Rangaeris, Summerhayesia, and some species of Diaphananthe.
PCA was carried out based on the six uncommon qualitative traits. The first two principal components explained 84.18% of the variance (PC1 accounted for 68.78% and PC2 for 15.40%) of the seeds among the species (Table 3). Factor loadings for the first two components are shown in Table 4.
Figure 9 displays the bi-plot graphic of the PC1 and PC2. According to the PCA, the arrangement of the medial cells is the least similar among the qualitative traits, and the most similar is the morphology of the longitudinal anticlinal walls.

4. Discussion

In this study, we provide new data about seed morphology in 17 genera and 100 species, which could aid in the identification of taxa. Seed morphology is essential to understand processes such as dispersal or germination and contributes to the design of future conservation programs in orchids, considering the alteration of habitats and climate change in recent decades [30,31].
We have observed four common qualitative traits in all the examined genera: elongated medial cells, thickened anticlinal zone, curved transverse anticlinal walls, and sunken and narrow-to-not visible periclinal walls. However, variability is manifested in the other six qualitative traits: seed shape, morphology of the apical and the basal pole, arrangement of the medial testa cells, morphology of the longitudinal anticlinal walls, and the presence of testa extensions (Table 1). The common traits, together with the arrangement of the twisted medial cells and the presence of testa extensions, are qualitative features related to the epiphytic life-form in tropical orchids [12]. Our results are in concordance with the assignment of the seeds to the Vanda type [3,28,29], providing new information about the morphology of both poles, the arrangement of the medial testa cells, the presence of the anticlinal zone, and the morphology of the longitudinal anticlinal walls. The qualitative traits are the same as those observed in representatives of the subtribes Adrorhizinae, Polystachyinae, and Aeridinae, within the tribe Vandeae [3,28,29].
Testa extensions (in the form of hooked projections or resembling long trichomes) have been observed in 11 genera. Previously, they have been observed in other orchids of the tribe Vandeae (subtribe Aeridinae) and the tribe Cymbidieae (subtribe Oncidiinae), mainly in the so-called twig orchids [3,12,35]. In epiphytic orchids of Angraecinae, these structures could be related to the attachment to the bark of twigs, branches, and trunks of woody plants, but to verify this assertion, accurate information about the species ecology is essential and future investigations on tropical orchids should provide data about the type of ramifications and the height at which the epiphytes grow. Due to the tiny size of orchid seeds, research in the field is far too complex, but experiments in laboratories using different models of bark surface could contribute to a better understanding of the attachment mechanisms in orchid seeds.
To date, the analysis of quantitative traits has demonstrated differences mainly in the percentage of free air space inside the seeds, higher in terrestrial and lower in epiphytic habitats [2,11,29]. This trait is in concordance with a higher seed flotation time in the air and a higher dispersal ability in terrestrial orchids [29,36]. Further research should explore the differences in qualitative traits, mostly in the characteristics of the anticlinal and periclinal walls among terrestrial and epiphytic orchids. In Vandeae, the presence of the anticlinal zone and the sunken and narrow-to-not visible periclinal walls are common traits, and these characteristics are probably related to the dispersal ability, so future experiments are essential to check if these traits are in concordance with the habitat in which the epiphytes grow.
The taxonomic and phylogenetic value of qualitative traits in seed morphology in Orchidaceae has been highlighted in previous studies [4,5,7]. In this study, our results are consistent with the results of molecular analyses. Mostly of the genera show a similar pattern of seed coats for all the examined species, confirming their monophyletic range such as in Aerangis, Ancistrorhynchus, Campeylocentrum, Dendrophylax, or Microcoelia [20,37,38]. On the other hand, we have observed variability in the seed coats of Diaphananthe, Mystacidium, and Rhipidoglossum.
In Diaphananthe, we have observed two types of seeds according to the arrangement of medial cells: twisted in D. bidens, D. fragrantissima, D. sarcorhynchoides, and D. vesicate, and straight in D. ichneumonea, D. letouzeyi, D. odoratissima, D. pellucida, and D. plehniana. Simó-Droissart et al. [18] concluded that this genus is polyphyletic, recognizing three subclades. Farminhão et al. [20] considered that the phylogeny of the genus was unresolved. According to our study, two groups are well differentiated within the genus, but more samples are needed to check the phylogenetic relationships with the variability of the seeds.
The seeds of the analyzed species of Mystacidium show differences in the morphology of the apical pole: conical in M. pulchellum and pointed in M. tanganyikense (Table 1, Figure 6E,F). Barthlott et al. [3] described the seeds of M. gracile Harv. as filiform with testa extensions at both poles, not seen in the taxa studied in our research. Martos et al. [21] concluded that Mystacidium is polyphyletic, with M. tanganyikense closer to Rhipidoglossum, and the rest of the species included in a clade are closer to Angraecopsis. Our results show concordance with this conclusion, because the seeds of M. tanganyikense share the same pattern with most of the species of Rhipidoglossum, and those of M. pulchellum with Angraecopsis. However, further studies with more species are necessary to verify these results.
The seed coat in Rhipidoglossum shows common characteristics, with the exception of the morphology of the apical pole, which is conical in R. brachyceras (Table 1), formerly embedded in the genus Cribbia Senghas [15]. Simó-Droissart et al. [18] considered the genus Rhipidoglossum as paraphyletic with respect to Cribbia. In Martos et al. [21] and Farminhão et al. [20], the species usually included in Cribbia, such as C. brachyceras, were nested within Rhipidoglossum. More species of the former genus Cribbia should be sampled to corroborate if they share a common pattern of seed coat based on the morphology of the apical pole.
All the molecular analyses concluded that the genus Angraecum sensu lato is polyphyletic, and many genera were segregated from it [18,20,37,38,39]. In our study, the seeds of Angraecum cadetii, A. calceolus, A. conchiferum, A. dives, A. eburneum, A. sacciferum, and A. stolzii are fusiform in shape, with a conical apical pole, truncated basal pole, straightly arranged medial cells, longitudinal anticlinal walls, and absence of testa extensions. This set of taxa appears in a wide clade within Angraecum s.str., including the genus Oeoniella [20], whose seeds have the same morphological pattern. In our study, we have observed variations in some qualitative traits of the seed coat in the examined species of Afropectinariella, Dolabrifolia, Conchograeum and Lesliegraecum, previously segregated from Angraecum [18,20,37,38,39]. Afropectinariella differs from Angraecum by the presence of cucullated protrusions in the curved transverse anticlinal walls (Figure 4D); Dolabrifolia by the twisted arrangement of the testa cells (Figure 7B); Conchograecum and Lesliegraecum by the seed shape, the presence of testa extensions, and the morphology of the longitudinal anticlinal walls (Figure 7C,D). According to Farminhão et al. [20], Lesliegraecum (Angraecum) chamaeanthus appeared in a clade with the genera Cryptopus, Erasanthe, Neobathiea, and Oeonia, whose seeds are filiform in shape, with testa extensions in the basal pole. Although the number of analyzed species in our study is low, our results support the polyphyly of Angraecum sensu lato and the segregation of these genera, even though more research with an increased number of samples is needed.

5. Conclusions

The results of this study highlight the importance of the qualitative traits in the seed coat in the subtribe Angraecinae, especially the elongated and twisted medial cells, the presence of a thickened anticlinal zone, the curved transversal anticlinal walls, the sunken and narrow-to-not visible periclinal walls, and the presence of testa extensions, corroborating its relationship with the epiphytic life-form of this group.
This study also provides new data about seed morphology in 17 genera and 100 species, which could aid in the identification of taxa and to understand processes such as dispersal ability, germination, and the distribution of these orchids.
In 17 of the genera studied, we have observed the same pattern of seed morphology for all the species within a genus. Our results reveal a strong relationship between the qualitative traits of the seed coat and the taxonomy of these genera, supporting its monophyly. Only in the genera Diaphananthe, Mystacidium, and Rhipidoglossum did we observe differences among qualitative traits in the studied species within a genus, but our results are in concordance with the phylogenetic results in these genera.
Considering its high diversity, more species of the subtribe Angraecinae should be sampled and new accurate information about the species ecology is needed to better understand the relationships between the qualitative traits and processes, such as the seed dispersal or germination, which should provide the information required for conservation programs, considering the alteration of the habitats in which the orchids grow and the climate changes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/d17040280/s1. Table S1: Studied material with locality, collectors, and voucher (* new genera studied; ** new species studied). Figure S1. Seed coat: (A) Aerangis appendiculata; (B) Aeranthes polyanthemus; (C) Ancistrorhynchus capitatus; (D) Angraecopsis parviflora; (E) Angraecum conchiferum; (F) A. stolzii; (G) Campylocentrum poeppiggii; (H) Conchograecum erectum. Figure S2. Seed coat: (A) Cryptopus elatus; (B) Dendrophylax porrectus; (C) Diaphananthe fragrantissima; (D) D. ichneumonea; (E) D. vesicata; (F) Erasanthe henrici; (G) Eurychone rothschildiana; (H) Jumellea comorensis. Figure S3. Seed coat: (A) Lemurella culicifera; (B) Lesliegraecum chamaeanthus; (C) Listrostachys pertusa; (D) Microcoelia globulosa; (E) Neobathiea grandidieriana; (F) Oeonia rosea; (G) Podangis rhipsalisocia; (H) Rangaeris muscicola. Figure S4. Seed coat: (A) Rhipidoglossum curvatum; (B) R. schimperianum; (C) R. subsimplex; (D) Sobennifoffia robusta; (E) Solenangis scandens; (F) Sphyrarhynchus schliebenii; (G) Tridactyle tridactylites; (H) Ypsilopus erectus.

Author Contributions

R.G. and E.O. designed the research and obtained samples. All authors (R.G., E.O., Á.H., G.V. and P.D.L.F.) analyzed the samples using SEM and performed the cluster analyses. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

All data generated or analyzed in this study are included in this published article.

Acknowledgments

We are much indebted to the curators of the herbaria K and MA for their permission for the examination of specimens of the genera studied and the loan of materials. Technical assistance from Esperanza Salvador and Isidoro Poveda at the SEM laboratory (SIDI-UAM) is gratefully acknowledged. To Patricia Barberá, who reviewed the manuscript, and Maria Teresa Boquete, for the statistical advice.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Common qualitative traits: (A) elongated medial cells in Dolabrifolia bancoense; (B) anticlinal zone in Erasanthe henrici; (C) curved transverse anticlinal walls in Angraecopsis tenerrima; and (D) sunken and narrow-to-not visible periclinal walls (arrow) in Campylocentrum poepiggii (medial cells, MCs; anticlinal zone, AZ; and transverse anticlinal walls, TAWs).
Figure 1. Common qualitative traits: (A) elongated medial cells in Dolabrifolia bancoense; (B) anticlinal zone in Erasanthe henrici; (C) curved transverse anticlinal walls in Angraecopsis tenerrima; and (D) sunken and narrow-to-not visible periclinal walls (arrow) in Campylocentrum poepiggii (medial cells, MCs; anticlinal zone, AZ; and transverse anticlinal walls, TAWs).
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Figure 2. Seed shape and morphology of the apical and basal poles: (A) fusiform with conical apical pole in Nephrangis filiformis; (B) filiform with pointed apical pole in Microcoelia caespitosa; (C) fusiform with truncated basal pole in Summerhayesia zambesiaca; and (D) filiform with pointed basal pole in Dinklageella liberica (apical pole, AP; basal pole, BP).
Figure 2. Seed shape and morphology of the apical and basal poles: (A) fusiform with conical apical pole in Nephrangis filiformis; (B) filiform with pointed apical pole in Microcoelia caespitosa; (C) fusiform with truncated basal pole in Summerhayesia zambesiaca; and (D) filiform with pointed basal pole in Dinklageella liberica (apical pole, AP; basal pole, BP).
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Figure 3. Arrangement of the medial cells: (A) straight in Oeoniella aphrodite; (B) twisted in Calyptrochilum christyanum. Morphology of the longitudinal anticlinal walls: (C) straight in Ancistrorhynchus clandestinus; (D) undulating in Lesliegraecum tenellum.
Figure 3. Arrangement of the medial cells: (A) straight in Oeoniella aphrodite; (B) twisted in Calyptrochilum christyanum. Morphology of the longitudinal anticlinal walls: (C) straight in Ancistrorhynchus clandestinus; (D) undulating in Lesliegraecum tenellum.
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Figure 4. Testa extensions: (A) trichomes in the basal pole in Dendrophylax porrectus; (B) hooked extensions in the apical pole in Solenangis scandens; (C) hooked extensions in the basal pole in Microcoelia caespitosa; and (D) cucullated protrusions on the curved transverse anticlinal walls in Afropectinariella subulata.
Figure 4. Testa extensions: (A) trichomes in the basal pole in Dendrophylax porrectus; (B) hooked extensions in the apical pole in Solenangis scandens; (C) hooked extensions in the basal pole in Microcoelia caespitosa; and (D) cucullated protrusions on the curved transverse anticlinal walls in Afropectinariella subulata.
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Figure 5. Seed coat: (A) Angraecopsis parviflora; (B) A. ischnopus; (C) Bolusiella talbotii; (D) B. iridifolia; (E) Campylocentrum crassirhizum; (F) C. jamaicense; (G) Microcoelia aphylla; and (H) M. exilis.
Figure 5. Seed coat: (A) Angraecopsis parviflora; (B) A. ischnopus; (C) Bolusiella talbotii; (D) B. iridifolia; (E) Campylocentrum crassirhizum; (F) C. jamaicense; (G) Microcoelia aphylla; and (H) M. exilis.
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Figure 6. Variability of qualitative traits in the seed coat of the genera Diaphananthe, Rhipidoglossum, and Mystacidium: (A) straight medial cells along the longitudinal axis in D. pellucida; (B) twisted medial cells along the longitudinal axis in D. sarcorhynchoides; (C) fusiform seed with pointed apical pole in R. kamerunense; (D) fusiform seed with conical apical pole in R. brachyceras; (E) fusiform seed with conical apical pole in M. pulchellum; and (F) fusiform seed with pointed apical pole in M. tanganyikense.
Figure 6. Variability of qualitative traits in the seed coat of the genera Diaphananthe, Rhipidoglossum, and Mystacidium: (A) straight medial cells along the longitudinal axis in D. pellucida; (B) twisted medial cells along the longitudinal axis in D. sarcorhynchoides; (C) fusiform seed with pointed apical pole in R. kamerunense; (D) fusiform seed with conical apical pole in R. brachyceras; (E) fusiform seed with conical apical pole in M. pulchellum; and (F) fusiform seed with pointed apical pole in M. tanganyikense.
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Figure 7. Variation in seed coat in Angraecum and closely related genera: (A) fusiform seed with straight medial cells in Angraecum calceolus; (B) fusiform seed with twisted medial cells in Dolabrifolia bancoense; (C) filiform seed with testa extensions in Conchograecum multinominatum; and (D) filiform seed with testa extensions and undulating longitudinal anticlinal walls in Lesliegraecum tenellum.
Figure 7. Variation in seed coat in Angraecum and closely related genera: (A) fusiform seed with straight medial cells in Angraecum calceolus; (B) fusiform seed with twisted medial cells in Dolabrifolia bancoense; (C) filiform seed with testa extensions in Conchograecum multinominatum; and (D) filiform seed with testa extensions and undulating longitudinal anticlinal walls in Lesliegraecum tenellum.
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Figure 8. UPGMA phenogram showing the clustering among genera of Angraecinae. Apical pole, AP; longitudinal anticlinal walls, LAWs; medial cells, MCs; and testa extensions, TEs. Colors: green, Diaphananthe; yellow, Mystacidium; and blue, Rhipidoglossum.
Figure 8. UPGMA phenogram showing the clustering among genera of Angraecinae. Apical pole, AP; longitudinal anticlinal walls, LAWs; medial cells, MCs; and testa extensions, TEs. Colors: green, Diaphananthe; yellow, Mystacidium; and blue, Rhipidoglossum.
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Figure 9. Principal component analysis based on six qualitative traits of 38 genera of the subtribe Angraecinae. Lines: blue, basal pole; green, seed shape; pink, apical pole; purple, testa extensions; red, arrangement of the medial cells; and yellow, longitudinal anticlinal walls. A: Dendrophylax (2 sp.), Dinklageella liberica, Microcoelia (7 sp.), and Solenangis scandens. B: Campylocentrum (9 sp.), Conchograecum (3 sp.), Cryptopus elatus, Erasanthe henrici, Neobathiea grandidieriana, and Oeonia rosea. C: Lesliegraecum (2 sp.). D: Sphyrarhynchus schliebenii. E: Mystacidium tanganyikense and Rhipidoglossum (11 sp.). F: Eurychone rothschildiana, Cyrtorchis (4 sp.), Bolusiella (3 sp.), Afropectinariella subulata, Ancistrorhynchus (6 sp.), Angraecopsis (5 sp.), Angraecum (7 sp.), Diaphananthe ichneumonea, D. letouzeyi, D. odoratissima, D. pellucida, D. plehniana, Jumellea (5 sp.), Lemurella culicifera, Listrostachys pertusa, Mystacidium pulchellum, Oeoniella aphrodite, Plectrelminthus caudatus, Podangis rhipsalisocia, Rhipidoglossum brachyceras, Sobennikoffia robusta, Tridactyle (8 sp.), and Ypsilopus (3 sp.). G: Dolabrifolia (3 sp.), Aeranthes (2 sp.), Calyptrochilum (2 sp.), Diaphananthe bidens, D. fragrantissima, D. sarcorhynchoides, D. vesicata, Nephrangis filiformis, Rangaeris muscicola, and Summerhayesia zambesiaca. H: Aerangis (10 sp.).
Figure 9. Principal component analysis based on six qualitative traits of 38 genera of the subtribe Angraecinae. Lines: blue, basal pole; green, seed shape; pink, apical pole; purple, testa extensions; red, arrangement of the medial cells; and yellow, longitudinal anticlinal walls. A: Dendrophylax (2 sp.), Dinklageella liberica, Microcoelia (7 sp.), and Solenangis scandens. B: Campylocentrum (9 sp.), Conchograecum (3 sp.), Cryptopus elatus, Erasanthe henrici, Neobathiea grandidieriana, and Oeonia rosea. C: Lesliegraecum (2 sp.). D: Sphyrarhynchus schliebenii. E: Mystacidium tanganyikense and Rhipidoglossum (11 sp.). F: Eurychone rothschildiana, Cyrtorchis (4 sp.), Bolusiella (3 sp.), Afropectinariella subulata, Ancistrorhynchus (6 sp.), Angraecopsis (5 sp.), Angraecum (7 sp.), Diaphananthe ichneumonea, D. letouzeyi, D. odoratissima, D. pellucida, D. plehniana, Jumellea (5 sp.), Lemurella culicifera, Listrostachys pertusa, Mystacidium pulchellum, Oeoniella aphrodite, Plectrelminthus caudatus, Podangis rhipsalisocia, Rhipidoglossum brachyceras, Sobennikoffia robusta, Tridactyle (8 sp.), and Ypsilopus (3 sp.). G: Dolabrifolia (3 sp.), Aeranthes (2 sp.), Calyptrochilum (2 sp.), Diaphananthe bidens, D. fragrantissima, D. sarcorhynchoides, D. vesicata, Nephrangis filiformis, Rangaeris muscicola, and Summerhayesia zambesiaca. H: Aerangis (10 sp.).
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Table 1. Comparative data of uncommon qualitative traits of the seed coat (spp., species studied in each genus).
Table 1. Comparative data of uncommon qualitative traits of the seed coat (spp., species studied in each genus).
TaxaSeed ShapeApical PoleBasal PoleArrangement of the Medial CellsLongitudinal Anticlinal WallsTesta Extensions
Aerangis (10 sp.)FusiformPointedTruncatedTwistedStraightAbsent
Aeranthes (2 sp.)FusiformConicalTruncatedTwistedStraightAbsent
Afropectinariella subulataFusiformConicalTruncatedStraightStraightAbsent
Ancistrorhynchus (6 sp.)FusiformConicalTruncatedStraightStraightAbsent
Angraecopsis (5 sp.)FusiformConicalTruncatedStraightStraightAbsent
Angraecum (7 sp.)FusiformConicalTruncatedStraightStraightAbsent
Bolusiella (3 sp.)FusiformConicalTruncatedStraightStraightAbsent
Calyptrochilum (2 sp.)FusiformConicalTruncatedTwistedStraightAbsent
Campylocentrum (9 sp.)FiliformPointedTruncatedStraightStraightPresent
Conchograecum (3 sp.)FiliformPointedTruncatedStraightStraightPresent
Cryptopus elatusFiliformPointedTruncatedStraightStraightPresent
Cyrtorchis (4 sp.)FusiformConicalTruncatedStraightStraightAbsent
Dendrophylax (2 sp.)FiliformPointedPointedStraightStraightPresent
Diaphananthe (5 sp.) exceptFusiformConicalTruncatedStraightStraightAbsent
D. bidens, D. fragrantissima,
D. sarcorhynchoides, D. vesicata		FusiformConicalTruncatedTwistedStraightAbsent
Dinklagella libericaFiliformPointedPointedStraightStraightPresent
Dolabrifolia (3 sp.)FusiformConicalTruncatedTwistedStraightAbsent
Erasanthe henriciFiliformPointedTruncatedStraightStraightPresent
Eurychone rothschildianaFusiformConicalTruncatedStraightStraightAbsent
Jumellea (5 sp.)FusiformConicalTruncatedStraightStraightAbsent
Lemurella culiciferaFusiformConicalTruncatedStraightStraightAbsent
Lesliegraecum (2 sp.)FiliformPointedTruncatedStraightUndulatingPresent
Listrostachys pertusaFusiformConicalTruncatedStraightStraightAbsent
Microcoelia (7 sp.)FiliformPointedPointedStraightStraightPresent
Mystacidium pulchellumFusiformConicalTruncatedStraightStraightAbsent
 M. tanganyikenseFusiformPointedTruncatedStraightStraightAbsent
Neobathiea grandidierianaFiliformPointedTruncatedStraightStraightPresent
Nephrangis filiformisFusiformConicalTruncatedTwistedStraightAbsent
Oeonia roseaFiliformPointedTruncatedStraightStraightPresent
Oeoniella aphroditaeFusiformConicalTruncatedStraightStraightAbsent
Plectrelminthus caudatusFusiformConicalTruncatedStraightStraightAbsent
Podangis rhipsalisociaFusiformConicalTruncatedStraightStraightAbsent
Rangaeris muscicolaFusiformConicalTruncatedTwistedStraightAbsent
Rhipidoglossum (11 sp.)FusiformPointedTruncatedStraightStraightAbsent
 R. brachycerasFusiformConicalTruncatedStraightStraightAbsent
Sobennikoffia robustaFusiformConicalTruncatedStraightStraightAbsent
Solenangis scandensFiliformPointedPointedStraightStraightPresent
Sphyrarhynchus schliebeniiFiliformPointedTruncatedStraightStraightAbsent
Summerhayesia zambesiacaFusiformConicalTruncatedTwistedStraightAbsent
Tridactyle (8 sp.)FusiformConicalTruncatedStraightStraightAbsent
Ypsilopus (3 sp.)FusiformConicalTruncatedStraightStraightAbsent
Table 2. Codes for the qualitative traits in the cluster analysis.
Table 2. Codes for the qualitative traits in the cluster analysis.
TraitVariability
Seed shape0 (fusiform); 1 (filiform)
Apical pole0 (conical); 1 (pointed)
Basal pole0 (truncated); 1 (pointed)
Arrangement of medial cells0 (straight); 1 (twisted)
Longitudinal anticlinal walls0 (straight); 1 (undulating)
Testa extensions0 (absent); 1 (present)
Table 3. Principal component analysis based on six qualitative traits of 38 genera of the subtribe Angraecinae.
Table 3. Principal component analysis based on six qualitative traits of 38 genera of the subtribe Angraecinae.
PCEigenvalue%Variance
10.637568.783
20.14278415.406
30.06902717.4477
40.04611354.9754
50.02061282.224
60.01079191.1644
Table 4. Morphological variables of seed morphology used in the PCA with the factor loading for the first two components.
Table 4. Morphological variables of seed morphology used in the PCA with the factor loading for the first two components.
Component 1Component 2
Seed shape0.55900.0697
Apical pole0.55960.1740
Basal pole0.21550.0518
Arrangement of medial cells−0.19490.9776
Longitudinal anticlinal walls0.04720.0047
Testa extensions0.58540.0798
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Gamarra, R.; Ortúñez, E.; De La Fuente, P.; Valdelvira, G.; Hernando, Á. Characterizing the Seed Coat in the Subtribe Angraecinae (Orchidaceae, Vandeae) and Its Taxonomic Value. Diversity 2025, 17, 280. https://doi.org/10.3390/d17040280

AMA Style

Gamarra R, Ortúñez E, De La Fuente P, Valdelvira G, Hernando Á. Characterizing the Seed Coat in the Subtribe Angraecinae (Orchidaceae, Vandeae) and Its Taxonomic Value. Diversity. 2025; 17(4):280. https://doi.org/10.3390/d17040280

Chicago/Turabian Style

Gamarra, Roberto, Emma Ortúñez, Pablo De La Fuente, Guillermo Valdelvira, and Álvaro Hernando. 2025. "Characterizing the Seed Coat in the Subtribe Angraecinae (Orchidaceae, Vandeae) and Its Taxonomic Value" Diversity 17, no. 4: 280. https://doi.org/10.3390/d17040280

APA Style

Gamarra, R., Ortúñez, E., De La Fuente, P., Valdelvira, G., & Hernando, Á. (2025). Characterizing the Seed Coat in the Subtribe Angraecinae (Orchidaceae, Vandeae) and Its Taxonomic Value. Diversity, 17(4), 280. https://doi.org/10.3390/d17040280

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