ISSN 2226-3063 e-ISSN 2227-9555 Modern Phytomorphology 11: 117–130, 2017 doi: 10.5281/zenodo.1078523 RESEARCH ARTICLE Vegetative anatomical adaptations of Epidendrum radicans (Epidendroideae, Orchidaceae) to epiphytic conditions of growth Muthukumar Thangavelu *, Shenbagam Muthu Root and Soil Biology Laboratory, Department of Botany, Bharathiar University, 641046 Coimbatore, Tamilnadu, India; * [email protected] Received: 13.08.2017 | Accepted: 18.11.2017 | Published: 04.12.2017 Abstract The anatomical properties of leaf, stem, and root of Epidendrum radicans Pav. ex Lindl., belonging to the subfamily Epidendroideae (Orchidaceae) were investigated for adaptations to stressed habitats. The anatomical investigation revealed that leaves of E. radicans have a thick cuticle (3–4 µm) and paracytic type of stomata. Foliar epidermal cells are conical on the adaxial surface and rectangular in the abaxial surface, distinct hypodermis absent, and uniseriate fiber bundles are arranged in both sides of the leaves. The foliar mesophyll is homogenous and starch grains and raphides present. The leaf sheath covering the stem have cuticle restricted to the outer surface and air spaces are present. The stem has a cuticulerized uniseriate epidermis and a uniseriate hypodermis. The cortex and a parenchymatous ground tissue of the stem are separated by a layer of sclerenchymatous band. Vascular bundles are collateral and their size generally increases from the periphery towards the center. A sclerenchymatous patch covers the phloem pole, whereas the xylem is covered by thin-walled parenchymatous cells. The roots possess Epidendrum-type velamen. Cover cells present. Uniseriate dimorphic exodermis consists of U-thickened long cells and thin-walled passage cells. The endodermal cells O-thickened, pericycle sclerenchymatous, xylem 10–14 arched. The pith is sclerenchymatous, but parenchymatous at the center. The anatomical examination of E. radicans revealed adaptations to moisture stress conditions like thick cuticle covering the leaves and stem, water storage cells, multilayered velamen and dimorphic exodermis. Keywords: Epidendrum radicans, cover cells, cuticle, idioblasts, raphides, starch grains, stegmata, velamen, water stress © The Author(s) 2017. Published by Novikoff A.V., State Natural History Museum NAS of Ukraine on behalf of Modern Phytomorphology. This is an open access article under the Creative Commons BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/) freely available on https://phytomorphology.org/ . 118 Muthukumar T., Shenbagam M. Introduction anatomy of some Epidendroideae reported the presence of fiber and compound midrib Epidendrum L. is the largest genus in the vascular bundles and the absence of trichomes, neotropical subtribe Laeliinae Benth. with hypodermis and motor cells in leaves of around 1500 species and is native to the E. radicans Pav. ex Lindl. In their comparative American continent (van den Berg et al. 2005; anatomy of the vegetative parts of Laeliinae, Pinheiro & Cozzolino 2013). The high diversity Stern & Carlsward (2009) examined the leaf in Epidendrum is due to the different habitats anatomy of E. anceps Jacq. and E. nocturnum they occupy ranging from dry tropical jungles Jacq., among other members of the subtribe. The to humid forests. In nature, most of these main foliar anatomical features noted were the orchids grow as epiphytes, while some exist as absence of hairs, adaxial and abaxial (E. anceps) lithophytes or terrestrial forms (Hágsater & Soto- or abaxial (E. nocturnum) distribution of the Arenas 2005). Further, species of Epidendrum stomata, reniform guard cells, lack of fibre are also aggressive colonizers of the disturbed bundles and hypodermis, undifferentiated and denuded areas. Epidendrum taxa are mesophyll, and stegmata restricted to fibre proposed as a potential model for investigation bundles and vascular bundles. More recently, on many evolutionary hypothesis including Moreira et al. (2013) compared the anatomical habitat selection and reproductive biology due adaptations of the epiphytic E. secundum Jacq. to their wide variation in morphology, genetic and Dichaea cogniauxiana Schltr., growing in diversity, ecological functionality and easy a nebular forest located at an amporupestre propagation (Pinheiro & Cozzolino 2013). The area at Serra da Piedade of Brazil to stresses of circumscription of Epidendrum is of great debate their microhabitats. In E. secundum the leaves and several species that were once in this genus growing in open light had more stomata and a are now transferred to other genera (Chase et al. thicker cuticle than D. cogniauxiana growing in 2015). Economic importance of Epidendrum the shade (Moreira et al. 2013). is limited when compared to other orchid Stem anatomy of Epidendrum is not well genera. Stems of E. mosenii Rchb. f. are used studied when compared to leaves and roots. as analgesic and E. rigidum Jacq. is reported to Weltz (1897) examined the anatomy of possess herbicidal activity. Infusion prepared stems in eight genera of Laeliinae and noted from leaves of E. chlorocorymbos Schltr. is that the hypodermis in Epidendrum spp. was known to reduce blood cholesterol levels, cure homogeneous, consisting of similar appearing spots on the skin, treat ear ache and stimulate cells. Investigations on the stem anatomy of dreaming (Asseleih et al. 2015). Nevertheless, E. anceps and E. nocturnum revealed the absence species of Epidendrum are highly popular and of hairs and stomata (Stern & Carlsward 2009). most sought after by gardeners and hobbyists The cauline epidermis consisted of cells that for their beautiful flowers and are popularly was either thick- (E. nocturnum) or thin-walled known as Poor man’s orchid or Crucifix orchid (E. anceps), and the hypodermis contained (Stern & Carlsward 2009). cells with thickened walls. Vascular bundles Anatomy helps in understanding the trends were distributed in the ground tissue and the in plant evolution and adaptations although phloem and xylem were covered by crescent most of the recent concepts in these areas are shaped sclerenchymatous caps. Stegmata often based on molecular studies (Seago & Fernando associated with the phloem sclerenchyma 2013). Despite their huge species diversity (Stern & Carlsward 2009). and widespread popularity in horticulture, Dycus & Knudson (1957) while examining anatomical studies on species of Epidendrum the role of velamen in aerial roots of orchids are very limited. Baker (1972) noted that the observed that the number of velamen layers lengths of guard cells in E. parkinsonianum in E. xanthium Lindl., E. boothianum Lindl., Hook., ranged between 60 and 63 µm. Khasim E. ibaguense Lindl. and E. radicans ranged & Mohana-Rao (1990) in their study on the from 3 to 5. Pridgeon et al. (1982) examined Modern Phytomorphology 11, 2017 Vegetative adaptations of Epidendrum radicans to epiphytic growth 119 A B Fig. 1. Habit of Epidendrum radicans. A – vegetative shoot with roots (arrow heads) and the stem covered by leaf sheath (ls); B – flowering shoot. Scale bars = 5 cm (A) and 2 cm (B). 13 species of Epidendrum for the presence in E. campestre was heterogeneous with of tilosomes in roots. Of these, elaborate periclinally flattened exovelamen and an tilosomes were present only in E. coriifolium endovelamen with finely reticulate thickened Lindl., and tilosomes of limited nature were walls. Cover cells were present in both the observed in E. ensatum A. Rich. & Gal., species. The velamen consisted of 5–6 cell E. pseudepidendrum (Rchb. f.) Rchb. f. and layers in both species, exodermal cells were E. secundum Jacq. Moller & Rasmussen O-thickened (E. campestre) or U-thickened (1984) indicated the occurrence of conical (E. secundum), cortex was 9–12-layered, silica structures in Epidendrum. Porembski endodermis was O-thickened and the xylem & Barthlott (1988) in their classification of arches ranged from 11 to 17. The root cortex velamen radicum grouped orchids with velamen of E. campestre had fungal hyphae and water resembling E. bifidurm Abul. as Epidendrum- storing idioblasts. Nevertheless, velamen cell type. Epivelamen cells in this velamen layers, as well as the water storing idioblasts, type are smaller than the radially elongated in roots were observed in the both species endovelamen cells. Further, endovelamen cells (Oliveira & Sajo 1999). These anatomical have thickenings that are fused into composed modifications indicate adaptation of the ledges and large pores. Tilosomes are infrequent two orchids to their microhabitats (Moreira and walls of the endodermis lightly thickened. et al. 2013). The anatomy ofE. anceps and Cortex may possess tracheoidal idioblasts E. nocturnum roots was characterized by the (Porembski & Barthlott 1988). Zankowski et al. lack of tilosomes and cortical cell wall banding, (1987) investigated the developmental anatomy and exodermal cells with U-thickened walls. of velamen and exodermis in aerial roots of Endodermal and pericycle cells in these taxa E. ibaguense and concluded that casparian strips were isodiametric with O-thickened walls in the exodermis are obscure and are not a (Stern & Carlsward 2009). functional equivalent of endodermis. Oliveira Among the different species ofEpidendrum , & Sajo (1999) studied the root anatomy of the ground dwelling E. radicans (Fig. 1), E. campestre Lindl. and E. secundum growing at commonly known as fire star orchid or ground Instituto de Botânica de São Paulo. Root hairs root orchid, is indigenous to Central America were