Vascular Epiphytic Medicinal Plants as Sources of Therapeutic Agents: Their Ethnopharmacological Uses, Chemical Composition, and Biological Activities

This is an extensive review on epiphytic plants that have been used traditionally as medicines. It provides information on 185 epiphytes and their traditional medicinal uses, regions where Indigenous people use the plants, parts of the plants used as medicines and their preparation, and their reported phytochemical properties and pharmacological properties aligned with their traditional uses. These epiphytic medicinal plants are able to produce a range of secondary metabolites, including alkaloids, and a total of 842 phytochemicals have been identified to date. As many as 71 epiphytic medicinal plants were studied for their biological activities, showing promising pharmacological activities, including as anti-inflammatory, antimicrobial, and anticancer agents. There are several species that were not investigated for their activities and are worthy of exploration. These epipythes have the potential to furnish drug lead compounds, especially for treating cancers, and thus warrant indepth investigations.


Introduction
Epiphytes are plants that grow on other plants and are often known as air plants. They are mostly found in moist tropical areas on canopy tree-tops, where they exploit the nutrients available from leaf and other organic debris. These plants exist within the plantae and fungi kingdom. The term epiphyte itself was first introduced in 1815 by Charles-François Brisseau de Mirbel in "Eléments de physiologie végétale et de botanique" [1]. Epiphytes can be categorized into vascular and non-vascular epiphytic plants; the latter includes the marchantiophyta (liverworts), anthocerotophyta (hornworts), and bryophyta (mosses). The common epiphytes are mosses, ferns, liverworts, lichens, and the orchids. Epiphytes fall under two major categories: As holo-and hemi-epiphytes. While orchids are a good example of holo-epiphytes, the strangler fig is a hemi-epiphyte. Although geological studies have proposed the existence of epiphytes since the pleistone epoch, an epiphyte was first depicted in "the Badianus Manuscript" by Martinus de la Cruz in 1552, which showed the Vanilla fragrans, a hemi-epiphytic orchid, being used by the tribal communities in latin America for fragrance and aroma, usually hung around their neck [1].
Epiphytes have been a source of food and medicine for thousands of years. Since they grow in a unique ecological environment, they produce interesting secondary metabolites that often show exciting biological activities. There are notable reviews on non-vascular epiphytes, bryophyta, regarding their phytochemical and pharmacological activities [2][3][4][5]. There are also extensive reviews on epiphytic lichens covering secondary metabolites and their pharmacological activities [6][7][8][9]. The only available review on vascular epiphytes related to medicinal uses was focused on Orchidaceae [10]. Therefore, to the best of our knowledge, there is no extensive database of vascular epiphytes regarding their medicinal contribution.
There are 27,614 recorded species of vascular epiphytes belonging to 73 families and 913 genera [11]. Vascular epiphyte species are commonly found in pteridophyta, gymnosperms, and angiosperms plant groups, which are mostly found in the moist tropical areas on canopy tree tops, where they exploits the nutrients available from leaf and other organic debris [12,13]. In this study, information on vascular epiphytic medicinal plant species was collected using search engines (Web of Science, Scifinder Scholar, prosea, prota, Google scholar), medicinal plant books (Plant Resources of South-East Asia: Medicinal and Poisonous Plants [14][15][16], Plant Resources of South-East Asia: Cryptogams: Ferns and Fern Allies [17], Mangrove Guide for South-East Asia [18], Medicinal Plants of the Asia-Pacific [19], Medicinal Plants of the Guiana [20], Indian Medicinal Plants [21,22], Medicinal Plants of Bhutan [23], Medicinal and aromatic plants of Indian Ocean islands: Madagascar, Comoros, Seychelles and Mascarenes [24]), and the Indonesian Medicinal Plants Database [25]. Scientific names of the epiphytic medicinal plant species were compared against the Plantlist database for accepted names to avoid redundancy [26]. The time-frame threshold for data coverage was from the earliest available data until early 2020. Nevertheless, empirical knowledge regarding traditional medicinal plants was passed through generations using verbal or written communication, with verbal communication highly practiced by remote tribes [27,28]. It is possible that some oral traditional medical knowledge may not be reported and therefore not captured in this review. In this current study, we collected and reviewed 185 epiphytic medicinal plants reported in the literature, covering ethnomedicinal uses of epiphytes, their phytochemical studies and the pharmacological activities. The data collection approach used is presented in Figure 1.

Vascular Epiphytic Medicinal Plant Species Distribution within Plant Families
In this component of the study, we collated and analysed 185 of the medicinally used epiphytic plants species using ethnopharmacological information. This data (Table 1) includes the name of species, plant family, areas where the epiphytes are used in traditional medicines, part(s) of the plant being used in medication, how the medicine was prepared, and indications. Of the 185 medicinally used epiphytes, 53 species were ferns (mostly polipodiaceae), with 132 species belonging to the nonfern category. The Orchidaceae family contains the Dendrobium genus that contains the highest number of medicinal epiphytes, including 64 orchid species and 20 Dendrobium species. The

Vascular Epiphytic Medicinal Plant Species Distribution within Plant Families
In this component of the study, we collated and analysed 185 of the medicinally used epiphytic plants species using ethnopharmacological information. This data (Table 1) includes the name of species, plant family, areas where the epiphytes are used in traditional medicines, part(s) of the plant being used in medication, how the medicine was prepared, and indications. Of the 185 medicinally used epiphytes, 53 species were ferns (mostly polipodiaceae), with 132 species belonging to the non-fern category. The Orchidaceae family contains the Dendrobium genus that contains the highest number of medicinal epiphytes, including 64 orchid species and 20 Dendrobium species. The Orchidaceae epiphytes were the majority of non-fern epiphytes. Cassytha filiformis L, Bulbophyllum odoratissimum (Sm.) Lindl. ex Wall., Cymbidium goeringii Rchb.f.) Rchb.f., Acrostichum aureum Limme, and Ficus natalensis Hochst. were the five most popular vascular epiphytic medicinal pants used ( Figure 2).

Distribution of Vascular Epiphytic Medicinal Plant Species by Country
Based on the available records, the data curation and analysis revealed that the Indigenous Indonesians have used 58 diverse epiphytic medicinal plant species throughout the archipelago and have the highest record compared to other tropical countries ( Figure 3). China is second and is well known for its traditional medicine, including the use of epiphytes in medicament preparation. This is followed by the Indigenous Indians, with the well-established Ayurveda as a formal record of Indian medicinal plants. The traditional medicinal plant knowledge of Indonesa has been heavily influenced by Indian culture and enriched by Chinese and Arabian traders since the kingdom era [27].

Parts of Vascular Epiphytic Medicinal Plant Species Used in Traditional Medicines
This review determined that leaves were the main plant components used in the traditional medicines ( Figure 4). This was expected given they are more easily harvested (without excessive

Distribution of Vascular Epiphytic Medicinal Plant Species by Country
Based on the available records, the data curation and analysis revealed that the Indigenous Indonesians have used 58 diverse epiphytic medicinal plant species throughout the archipelago and have the highest record compared to other tropical countries ( Figure 3). China is second and is well known for its traditional medicine, including the use of epiphytes in medicament preparation. This is followed by the Indigenous Indians, with the well-established Ayurveda as a formal record of Indian medicinal plants. The traditional medicinal plant knowledge of Indonesa has been heavily influenced by Indian culture and enriched by Chinese and Arabian traders since the kingdom era [27].

Distribution of Vascular Epiphytic Medicinal Plant Species by Country
Based on the available records, the data curation and analysis revealed that the Indigenous Indonesians have used 58 diverse epiphytic medicinal plant species throughout the archipelago and have the highest record compared to other tropical countries ( Figure 3). China is second and is well known for its traditional medicine, including the use of epiphytes in medicament preparation. This is followed by the Indigenous Indians, with the well-established Ayurveda as a formal record of Indian medicinal plants. The traditional medicinal plant knowledge of Indonesa has been heavily influenced by Indian culture and enriched by Chinese and Arabian traders since the kingdom era [27].

Parts of Vascular Epiphytic Medicinal Plant Species Used in Traditional Medicines
This review determined that leaves were the main plant components used in the traditional medicines ( Figure 4). This was expected given they are more easily harvested (without excessive

Parts of Vascular Epiphytic Medicinal Plant Species Used in Traditional Medicines
This review determined that leaves were the main plant components used in the traditional medicines ( Figure 4). This was expected given they are more easily harvested (without excessive tools) and processed compared to other plant parts, e.g., the root and stem. As some epiphytes have a small biomass compared to higher trees, the whole plant is commonly harvested in medicament preparation. Interestingly, almost half of epiphytic medicinal plants were ferns, in which the stem-like stipe is prepared for medicine. Without haustoria (a specialised absorbing structure of a parasitic plant), the root and rhizome of epiphytic medicinal plants are easily harvested and prepared.
Biomolecules 2020, 10, x 4 of 57 tools) and processed compared to other plant parts, e.g., the root and stem. As some epiphytes have a small biomass compared to higher trees, the whole plant is commonly harvested in medicament preparation. Interestingly, almost half of epiphytic medicinal plants were ferns, in which the stemlike stipe is prepared for medicine. Without haustoria (a specialised absorbing structure of a parasitic plant), the root and rhizome of epiphytic medicinal plants are easily harvested and prepared.

Modes of Preparation and Dosage of Administration of Vascular Epiphytic Medicinal Plant Species in Traditional Medicines
Generally, medicinally active secondary metabolites have a water solubility problem likely related to the lipophilic moieties in their structures [29]. Using boiling water, decoctions are able to increase the yield of secondary metabolites extracted from medicinal plants. Therefore, it is not surprising that decoctions are commonly used in traditional medicine preparations from plants ( Figure 5). External applications are also commonly practiced in traditional medicinal therapies, including poultice (moist mass of material), raw, or less processed medicine. Poultices were commonly prepared for skin diseases while a decoction was ingested for internal infectious diseases (i.e., fever).

Modes of Preparation and Dosage of Administration of Vascular Epiphytic Medicinal Plant Species in Traditional Medicines
Generally, medicinally active secondary metabolites have a water solubility problem likely related to the lipophilic moieties in their structures [29]. Using boiling water, decoctions are able to increase the yield of secondary metabolites extracted from medicinal plants. Therefore, it is not surprising that decoctions are commonly used in traditional medicine preparations from plants ( Figure 5). External applications are also commonly practiced in traditional medicinal therapies, including poultice (moist mass of material), raw, or less processed medicine. Poultices were commonly prepared for skin diseases while a decoction was ingested for internal infectious diseases (i.e., fever).

Category of Diseases Treated by Vascular Epiphytic Medicinal Plant Species
Interestingly, epiphytes have been used for treating various ailments, including both infectious and non-infectious diseases. Traditional communities described infectious diseases related to skin diseases (wounds, boils, ulcers, abscesses, smallpox) and non-skin diseases (fever, diarrhoea, ulcers, colds, worm infections, and malaria). A total of 54 epiphytic medicinal plant species were prescribed to treat skin diseases while 81 species to treat non-skin infectious diseases ( Figure 6). Hygiene has been a serious issue in traditional communities as it gives rise to infectious diseases. Fever is a common symptom of pathogenic infection and has been treated using medicinal plants, including epiphytes. Hygiene issues are also a common cause of skin disease, wounds, dysentery, and diarrhoea in traditional communities.

Category of Diseases Treated by Vascular Epiphytic Medicinal Plant Species
Interestingly, epiphytes have been used for treating various ailments, including both infectious and non-infectious diseases. Traditional communities described infectious diseases related to skin diseases (wounds, boils, ulcers, abscesses, smallpox) and non-skin diseases (fever, diarrhoea, ulcers, colds, worm infections, and malaria). A total of 54 epiphytic medicinal plant species were prescribed to treat skin diseases while 81 species to treat non-skin infectious diseases ( Figure 6).

Category of Diseases Treated by Vascular Epiphytic Medicinal Plant Species
Interestingly, epiphytes have been used for treating various ailments, including both infectious and non-infectious diseases. Traditional communities described infectious diseases related to skin diseases (wounds, boils, ulcers, abscesses, smallpox) and non-skin diseases (fever, diarrhoea, ulcers, colds, worm infections, and malaria). A total of 54 epiphytic medicinal plant species were prescribed to treat skin diseases while 81 species to treat non-skin infectious diseases ( Figure 6). Hygiene has been a serious issue in traditional communities as it gives rise to infectious diseases. Fever is a common symptom of pathogenic infection and has been treated using medicinal plants, including epiphytes. Hygiene issues are also a common cause of skin disease, wounds, dysentery, and diarrhoea in traditional communities. Hygiene has been a serious issue in traditional communities as it gives rise to infectious diseases. Fever is a common symptom of pathogenic infection and has been treated using medicinal plants, including epiphytes. Hygiene issues are also a common cause of skin disease, wounds, dysentery, and diarrhoea in traditional communities.

Phytochemical Composition of Vascular Epiphytic Medicinal Plants
Epiphytes belong to a distinctive plant class as they do not survive in soil and this influences the secondary metabolites present. Epiphytes are physically removed from the terrestrial soil nutrient pool and grow upon other plants in canopy habitats, shaping epiphyte morphologies by the method in which they acquire nutrients [30]. Nutrients, such as nitrogen and phosphorus, are obtained from different sources, including canopy debris (through fall) and host tree foliar leaching [30], the latter influencing canopy soil nutrient cycling [31,32]. In the conversion of sunlight into chemical energy, the epiphyte often uses a specific carbon fixation pathway (CAM: Crassulacean acid metabolism) as a result of harsh environmental conditions [33], making them unique and thus worthwhile for scientific studies.
In the early 20th century, laboratory-based research on epiphytes studied the plant's production of alkaloids, cyanogenetic, and organic sulfur compounds, with the plants producing limited quantities of these compounds [34]. Common plant steroids, e.g., β-sitosterol, have been shown to be present in 22 different epiphytic medicinal plants (Figure 7). This is possibly due to the function of the steroids as structural cell wall components, giving rise to a wide distribution across plant families and species. A further example of a common plant steroid present is stigmasterol.

Phytochemical Composition of Vascular Epiphytic Medicinal Plants
Epiphytes belong to a distinctive plant class as they do not survive in soil and this influences the secondary metabolites present. Epiphytes are physically removed from the terrestrial soil nutrient pool and grow upon other plants in canopy habitats, shaping epiphyte morphologies by the method in which they acquire nutrients [30]. Nutrients, such as nitrogen and phosphorus, are obtained from different sources, including canopy debris (through fall) and host tree foliar leaching [30], the latter influencing canopy soil nutrient cycling [31,32]. In the conversion of sunlight into chemical energy, the epiphyte often uses a specific carbon fixation pathway (CAM: Crassulacean acid metabolism) as a result of harsh environmental conditions [33], making them unique and thus worthwhile for scientific studies.
In the early 20th century, laboratory-based research on epiphytes studied the plant's production of alkaloids, cyanogenetic, and organic sulfur compounds, with the plants producing limited quantities of these compounds [34]. Common plant steroids, e.g., β-sitosterol, have been shown to be present in 22 different epiphytic medicinal plants (Figure 7). This is possibly due to the function of the steroids as structural cell wall components, giving rise to a wide distribution across plant families and species. A further example of a common plant steroid present is stigmasterol.  Table 2 lists the secondary metabolites identified in epiphytic medicinal plants and details the species, isolated compounds, and provides references. Currently, only 69 species have been phytochemically studied (23 fern and 46 non-fern epiphytes) and 842 molecules have been isolated from these epiphytic plants. Analysis of the literature showed epiphytes were able to produce a range of secondary metabolites, including terpenes and flavonoids, with no alkaloids being isolated from epiphytic fern medicinal plants thus far. β-Sitosterol, a common phytosterol in higher plants, was reported across fern genera. Interestingly, there is one unique terpene produced, hopane, which is commonly called fern sterol. Common flavonoids, such as kaempferol, quercetin, and flavan-3-ol derivatives (catechin), were also reported across the epiphytic ferns. Epiphytic pteridaceae,  Table 2 lists the secondary metabolites identified in epiphytic medicinal plants and details the species, isolated compounds, and provides references. Currently, only 69 species have been phytochemically studied (23 fern and 46 non-fern epiphytes) and 842 molecules have been isolated from these epiphytic plants. Analysis of the literature showed epiphytes were able to produce a range of secondary metabolites, including terpenes and flavonoids, with no alkaloids being isolated from epiphytic fern medicinal plants thus far. β-Sitosterol, a common phytosterol in higher plants, was reported across fern genera. Interestingly, there is one unique terpene produced, hopane, which is commonly called fern sterol. Common flavonoids, such as kaempferol, quercetin, and flavan-3-ol derivatives (catechin), were also reported across the epiphytic ferns. Epiphytic pteridaceae, Acrostichum aureum Limme, is rich in quercetin [35]. Further analysis showed there were more secondary metabolites reported from non-fern epiphytic medicinal plants than from fern epiphytic medicinal plants, including terpene derivatives, flavonoids, and alkaloids. Included were flavanone, flavone, and flavonol derivatives but no flavan-3-ols were reported in these epiphytes so far. In the non-fern epiphytes, there were more phytochemical studies on orchid genera with additional classes of compounds reported, including penantrene derivatives (flavanthrinin, nudol, fimbriol B) [36,37] from the Bulbophyllum genus and the alkaloid dendrobine from the Dendrobium genus [38].
Therefore, while epiphytes may have limitations in accessing nutrients, adaptation has enabled them to successfully survive these environments. Studies on numerous medicinal epiphytes show that the unique environment does not constrain the plants from producing different types of secondary metabolites. These include terpenes, flavonoids, and alkaloids, especially the non-fern epiphytic medicinal plants.

Pharmacological Activities of Vascular Epiphytic Medicinal Plants
The pharmacological activities of medicinal epiphytes are summarised in Table 1 From the 191 collected records of epiphytic medicinal plants, around 71 species were subjected to bioactivity testing, with 25 of these species using crude extract samples. Although this testing represents almost 50% of the species examined, only a few of the pharmacological tests were related to ethnopharmacological claims. Here, we discuss selected species where the outcomes indicated a coherent relationship between bioactivities and traditional claims.

Infectious Disease Therapy
Research on epiphytes that have been used in infectious disease therapy include in wound healing, dysentery, and skin infections. A study on the methanol extract of Adiantum caudatum L., Mant showed anti-fungal activity against common fungi found in wounds (Aspergilus and Candida species) [39], including Aspergillus flavus, A. spinulosus, A. nidulans, and Candida albicans, with minimum inhibitory concentration (MIC) values of 15.6, 15.6, 31.2, and 3.9 µg/mL, respectively. Gallic acid was one of the bioactive constituents [40]. The methanol extract of Ficus natalensis Hochst (a semi-epiphytic plant) showed anti-malarial activity against Plasmodium falciparum, with an half maximal inhibitory concentration (IC 50 ) value of 41.7 µg/mL, and weak bactericidal activity against Staphylococcus aureus, with an MIC value of 99 µg/mL [41]. These results became preliminary data for confirming its traditional uses as malarial fever therapy and wound healing. Phytochemical studies on Pyrrosia sheareri (Bak.) Ching successfully isolated several compounds and were subjected to anti-oxidant testing. While this was not in line with the plant's ethnomedical uses for dysentery therapy [42], one of the isolated constituents was protocateuchic acid, which is known to possess anti-bacterial activity. It implies that the traditional uses of the epiphyte was for bacillary dysentery therapy.

Non-Infectious/Degenerative Disease-Related Therapy
An exploration on Drynaria species, highly prescribed in bone fracture therapy, successfully isolated flavonoid constituents that induce osteoblast proliferation [43]. Previous studies on Acrostichum aureum Limme failed to show its anti-bacterial activities [44] contrary to its traditional claims in wound management. However, patriscabratine 257 was isolated from the defatted methanol extract of whole plant of A. aureum, and subsequent testing showed it possessed anti-cancer activity in gastric cells and this supprted the traditional use of the plant in peptic ulcer therapy [35]. A decoction from the epiphyte Ficus deltoida has been used to treat diabetes. A study on the hot aqueous extract of this plant revealed anti-hyperglycemic activity by stimulating insulin secretion up to seven-fold. Furthermore, its activity mechanism was related to both the K + ATP -dependant and -non-dependant insulin secretion pathway [45]. However, further studies are required to identify the constituents responsible for the anti-hyperglycaemic activity.
The Indigenous people of Paraguay have used Catasetum barbatum Lindley to topically treat inflammation. Four bioactive compounds were isolated from this species and 2,7-dihydroxy-3,4,8trimethoxyphenanthrene (confusarin) 595 showed the highest anti-inflammatory activity [46]. The study also revealed the compound to be a non-competitive inhibitor of the H 1 -receptor.

Epiphytic Plant-Host Interactions on Secondary Metabolite Tapping
Secondary metabolite tapping has been an interesting study to reveal the molecular interactions between epiphytes and their host. This interaction was more visible when a physical channel between the two were developed. This channel (haustorium) made an epiphytic plant act as a parasite that enabled the plant to harvest molecular components from the host plant. A study on Scurulla oortiana (Korth.) Danser growth in three different host species (Citrus maxima, Persea Americana, and Camellia sinensis) identified three secondary metabolites (quercitrin, isoquercitrin, and rutin) in the S. oortiana (Korth.) Danser epiphyte growing on the three hosts [48]. Interestingly, extensive chromatographic and spectroscopic studies discovered that the flavonoids found in the S. oortiana (Korth.) Danser were independent of the host plants [48]. Secondary metabolite production in a host plant can also be triggered by the existence of a parasite, as discussed in a study on Tapirira guianensis infested by Phoradendron perrottetii, in which infested branches produced more tannin compare to non-infested branches, with infestation inducing a systemic response [48].  [188], anti-cancer [189], hypoglycemic activity (extract, rodents) [45,188] antimicrobial activity (extract) [190], Anti-inflammatory [191] 107 Ficus lacor Buch.-Ham.

Conclusions
Epiphytes are the most beautiful vascular plants and contain interesting phytochemicals and possess exciting pharmacological activities. An analysis of the literature revealed 185 epiphytes that are used in traditional medicine, in which phytochemical studies identified a total of 842 secondary metabolites. Only 71 epiphytic medicinal plants were studied for their pharmacological activities and showed promising pharmacological activities, including anti-inflammatory, antimicrobial, and anticancer. Several species were not investigated for their activities and are worthy of exploration, including epiphytes from the Araceae (P. fragantissimum), Aralliaceae (S. caudata, S. elliptica, S. elliptifoliola, S. oxyphylla, S. simulans), and Asclepidaceae (Asclopidae sp., D. acuminate, D. benghalensis, D. imbricate, D. major, D. nummularia, D. platyphylla, D. purpurea, Toxocarpus sp) families, in which no phytochemical and pharmacological studies had been reported. These species have been used by Indigenous populations to treat both degenerative and nondegenerative diseases. It is known that there are examples of Indigenous populations living in protected forest reserves (e.g., in Indonesia) where epiphytes are used in their medicine, e.g., some species of Dischidia are used to treat fever, eczema, herpes etc.; these plants have not yet been studied. Therefore, the possibility of responsible bioprospecting exists (in compliance with the Nagoya protocol), which would be invaluable in biodiscovery knowledge as well as in mutual benefit sharing agreements.