Citation: Ortúñez, E.; Gamarra, R. Seed Morphology, Life Form and Distribution in Three Bromheadia Species (Epidendroideae, Orchidaceae). Diversity 2023, 15, 195. https://doi.org/10.3390/d15020195 Academic Editors: Pavel Kindlmann, Tiiu Kull, Melissa McCormick and Michael Wink Received: 23 December 2022 Revised: 23 January 2023 Accepted: 24 January 2023 Published: 1 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). diversity Article Seed Morphology, Life Form and Distribution in Three Bromheadia Species (Epidendroideae, Orchidaceae) Emma Ortúñez 1,2, * ,† and Roberto Gamarra 1,2,† 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 * Correspondence: emma.ortunez@uam.es † These authors contributed equally to this work. Abstract: The seed morphology of three species belonging to the genus Bromheadia was analyzed under light and scanning electron microscopy. The seeds of B. cecieliae and B. truncata were studied for the first time. Differences in the qualitative and quantitative characteristics between the terrestrial B. finlaysoniana and the epiphytes B. cecieliae and B. truncata were observed, which were in concor- dance with the life form. Due to the variability of the seed shapes, a new methodology is proposed to analyze the distance between the embryo and the testa cells, with the aim of demonstrating the presence of air space within the seed. The method is compared to previous formulae used to measure free air space. Furthermore, a new measurement, the angle in twisted testa cells of epiphytic orchids, is proposed, to evaluate the degree of torsion in medial cells. Although the wide distribution of B. finlaysoniana could be related to the great buoyancy of their seeds in contrast to the limited distri- bution of B. cecieliae, we consider that environmental factors are more influential than the buoyancy of seeds when understanding the distribution of these taxa. Future studies on seeds morphology in orchid genera with terrestrial and epiphytic taxa will provide new insights into this research. Keywords: testa cells; anticlinal walls; embryo; terrestrial; epiphyte 1. Introduction Orchid seeds are tiny, comprising a pluricellular embryo enclosed in a thin layer of dead cells, which make up the testa [1]. Testa cells show a set of characteristics that have been observed under light and scanning electron microscopes, providing qualitative and quantitative data used to describe the seeds’ morphology [2–6]. Previous studies on seed micromorphology have demonstrated the taxonomic value of several traits [7–9], and emphasize its strong correlation with molecular phylogenies [4,10–12]. In Orchidaceae, seed traits are more conservative than other characteristics [4,5]. Two life forms are dominant in the family Orchidaceae: terrestrial and epiphytic. According to Vij et al. [1], terrestrial taxa are primitive. Seed traits have evolved from terrestrial to epiphytic orchids concerning the cell shape, the seed size, the arrangement of the testa cells, or the internal air space between the embryo and testa. Testa cells have evolved from polygonal (quadrangular to rectangular) in terrestrial orchids to elongated in the epiphytic ones. Seed size has been indicated as being larger in terrestrial species [2,13]; however, Arditti and Ghani [3] rejected this assertion. Parallel testa cells along the longitu- dinal axis are common in terrestrial orchids and twisted ones have only been observed in epiphytic orchids [1,14]. Free air space is variable amongst orchid seeds [3], even though several studies have suggested that air space is larger in terrestrial species [1,15,16]. Arditti et al. [2] proposed the use of two mathematical formulae to estimate the volume of the seed and embryo, by simulating two cones fused at the base in the case of the seed, and using a prolate spheroid in the embryo. With these data, a formula to estimate the percentage of free air space was also proposed. Seed and embryo volumes Diversity 2023, 15, 195. https://doi.org/10.3390/d15020195 https://www.mdpi.com/journal/diversity