Tree Aloe Research Articles

Revisiting the anatomy of the monocot cambium, a novel meristem

Main conclusionThe monocot cambium is semi-storied, and its cells do not undergo rearrangement.The monocot cambium is a lateral meristem responsible for secondary growth in some monocotyledons of Asparagales. It is an unusual meristem, not homologous with the vascular cambia of gymnosperms and non-monocotyledonous angiosperms. Owing to the limited information available on the characteristics of this meristem, the aim of this study was to survey the structure of the monocot cambium in order to clarify the similarities and dissimilarities of this lateral meristem to the vascular cambium of trees. Using the serial sectioning analysis, we have studied the monocot cambium of three species of arborescent monocotyledons, i.e., Quiver Tree Aloe dichotoma, Dragon Tree Dracaena draco, and Joshua Tree Yucca brevifolia, native to different parts of the world. Data showed that in contrast to the vascular cambium, the monocot cambium is composed of a single type of short initials that vary in shape, and in tangential view display a semi-storied pattern. Furthermore, the cells of the monocot cambium do not undergo rearrangement. The criteria used in identifying monocot cambium initial cell are also discussed.

Yet two more firsts for Mary E. Barber (1818–1899), and a new synonym for the largest aloe, Aloidendron barberae (Asphodelaceae subfam. Alooideae)

Walker et al. (2019: 12) suggested that the name Aloe zeyheri (Barber 1870a: 80) (Asphodelaceae subfam. Alooideae) was merely proposed and not validly published for the species of tree aloe today known as Aloidendron barberae (Dyer 1874a: 566) Klopper & Gideon F. Sm. in Grace et al. (2013: 9) (Fig. 1A–C). Aloidendron barberae occurs naturally along the eastern seaboard of South Africa, and slightly inland, and caused quite a stir in botanical circles when it was first and more or less simultaneously collected in South Africa’s Eastern Cape and KwaZulu-Natal provinces.

Museomics Clarifies the Classification of Aloidendron (Asphodelaceae), the Iconic African Tree Aloes.

Arborescent succulent plants are regarded as keystone and indicator species in desert ecosystems due to their large stature and long lifespans. Tree aloes, the genus Aloidendron, are icons of the southern African deserts yet have proved elusive subjects due to the difficulty of obtaining material of known provenance for comparative study. Consequently, evolutionary relationships among representatives of the unusual arborescent life form have remained unclear until now. We used a museomics approach to overcome this challenge. Chloroplast genomes of six Aloidendron species and 12 other members of Asphodelaceae were sequenced from modern living collections and herbarium specimens, including the type specimens of all but two Aloidendron species, the earliest of which was collected 130 years ago. Maximum-likelihood trees estimated from full chloroplast genomes and the nuclear internal transcribed spacer (ITS) region show that Aloidendron sabaeum, from the Arabian Peninsula, is nested within Aloe while the Madagascar endemic Aloestrela suzannae is most closely related to the Somalian Aloidendron eminens. We observed phylogenetic conflicts between the plastid and nuclear topologies, which may be indicative of recurrent hybridisation or incomplete lineage sorting events in Aloe and in Aloidendron. Comparing species ecology in the context provided by our phylogeny suggests that habitat preference to either xeric deserts or humid forests/thickets evolved repeatedly in Aloidendron. Our findings demonstrate the value of botanical collections for the study and classification of taxonomically challenging succulent plants.

Divergence Time Estimation of Aloes and Allies (Xanthorrhoeaceae) Based on Three Marker Genes

Aloes and allies are prominent members of African succulent vegetation and especially of the highly diverse Cape Flora. The main goal of this study was to obtain age estimates for alooids by calibrating a Bayesian phylogenetic analysis based on two chloroplast markers (the trnL-trnF spacer region and rbcL gene) and one gene marker (ITS) using a relaxed molecular clock. Seventy four species from all succulent genera of alooids were analysed with MrBayes to infer species relationships. We discuss the age estimates to address the question whether vicariance or dispersal could account for the diversification of Madagascan alooids. In the combined maximum clade credibility tree obtained from BEAST the succulent alooids have split from asphodeloids around 51.8 Mya in Early Miocene. Divergence time age estimation for succulent drought resistant alooids (late Oligocene to early Miocene) correspond well with dates identified for several other plant lineages in southern Africa and does match with the start of dry period in Miocene which triggered speciation and evolutionary radiation of these genera and families. All climbing aloes and some tree aloes which were recently split into new genera are amongst the early diverged group in alooids and the crown node of this group diverged around 16.82 (15.5–22.4) Mya. The oldest node age estimation for aloes from Madagascar (5.1 Mya) is in early Pliocene and our findings support the hypothesis that the Africa-Madagascan divergence is best explained by oceanic long-distance dispersal rather than vicariance. This study is one of the first to give age estimates for clades of alooids in Xanthorrhoeaceae as a starting point for future studies on the historical biogeography of this family of succulent plants which are important for ethnomedicine, and as ornamental and horticultural plants.

Seed storage and germination in Kumara plicatilis, a tree aloe endemic to mountain fynbos in the Boland, south-western Cape, South Africa

Seed storage under appropriate conditions is a relatively inexpensive means of safeguarding plant genetic material for ex situ conservation. Post-storage germination trials are used to determine the viability of stored seeds, and hence the efficacy of the particular storage treatment. Kumara plicatilis (=Aloe plicatilis) is a tree aloe endemic to mountain fynbos in the Boland, south-western Cape. The viability and germination behaviour of K. plicatilis seeds were assessed for seeds stored for four and nine months at −80°C, 4°C, 25°C and under ambient conditions in a laboratory. Seeds were germinated under controlled conditions and germination rates and percentages determined. Ungerminated seeds were tested for viability using tetrazolium salt. Seed viability was not significantly reduced during storage. Seeds stored at −80°C for four and nine months exhibited the fastest germination rate overall (both 5.9±0.3 weeks, mean±S.E.), and slowest was for seeds stored under ambient conditions for four and nine months (both 7.8±0.4 weeks). All seed lots showed similar percentage germination after four months of storage (78.0–90.4%). The highest percentage germination overall was for seeds stored at −80°C for four months (90.4%) and the lowest was for seeds kept at 4°C and −80°C for nine months (39.2 and 39.6%, respectively). Respective percentage viability for ungerminated seeds in these two treatments was 82% and 87%, respectively, indicating the induction of secondary dormancy. Induced dormancy triggered by protracted cold temperatures may be an adaptation that enables seeds to survive prolonged extreme conditions that are unfavourable for germination. Further research on the long-term storage of aloe seeds would be beneficial for developing long-term seed storage and germination testing protocols for ex situ conservation.

Elucidating patterns in the population size structure and density of Aloe plicatilis, a tree aloe endemic to the Cape fynbos, South Africa

Studies on plant population size structure provide important baseline information for monitoring and conservation. Traditionally, inverse J-shaped size class distributions (SCD) were considered indicative of ‘healthy’, stable plant populations; however, this may not hold true for long-lived, slow-growing species. This study assessed the population size structure of 19 populations of Aloe plicatilis, a Cape fynbos endemic tree aloe, and quantified population size and density. A strong positive linear relationship between stem diameter (SDr) and height was used to align SDr and height SCD bins, and to harmonize stage and size classes. Onset of reproduction occurs at ~15cm SDr and ~0.8m in height. Some A. plicatilis individuals appear to display a ‘bonsai effect’ whereby the growth of individuals in very rocky sites is suppressed. Stunted plants may be non-reproductive ‘suppressed juveniles’ or ‘reproductive dwarfs’. The short, shrub-like growth form of many individuals may also result from height restrictions imposed by persistent strong winds. Population size, extent and density ranged from 31 to >110 000 individuals, 0.05–103ha and 75–3000 plants/ha respectively. Bell-shaped SCDs were the most common (50% of populations). The SDr and height SCDs for all populations combined were also bell-shaped. Seven populations displayed irregular SCDs, but were similar in structure to the bell-shaped SCDs, both of which were attributed to an adult-persistence population survival strategy. This study challenges the suitability of the inverse J as the only SCD indicative of healthy, stable populations for long-lived, slow-growing species. The study provides baseline demographic data on A. plicatilis across its distribution for long-term monitoring, and provides insights useful for examining trends in the demographics of other long-lived, slow-growing species.

Reproductive ecology of Aloe plicatilis, a fynbos tree aloe endemic to the Cape Winelands, South Africa

While the pollination ecology of many Aloe species is well-documented, knowledge on aloe seed ecology, and hence aloe reproductive ecology in its entirety is limited. The aim of this study was to investigate the reproductive ecology of Aloe plicatilis, a Cape fynbos tree aloe endemic to the Cape Winelands, South Africa. Results from a pollinator exclusion experiment conducted at an A. plicatilis population on Paarl mountain suggests pollination primarily by insects, although bird visitation significantly increased seed set/fruit indicating possible co-pollination with insects. The species' long-tubed flowers and production of concentrated nectar, with observations of malachite sunbirds as the most common avian visitors to A. plicatilis flowers indicate the importance of long-billed specialist avian nectarivores as floral visitors. Analysis of the relationship between plant size and inflorescence production for five populations combined revealed a significant, positive linear relationship between plant size and the logarithm of the number of inflorescences/plant. Natural fruit and seed set determined for three populations (1325, 27,930 and 251,616 seeds/population) suggests low reproductive output compared to several other Aloe species. The smallest (31 individuals) and least dense (75plants/ha) A. plicatilis population produced the lowest seed set/plant (128 seeds) and per population (1325 seeds), suggesting an Allee effect. Evaluation of seed dispersal potential showed that potential dispersal distances were approximately three times the canopy height; however, the occurrence of A. plicatilis on mountains isolated from more continuous mountain ranges on which the species also occurs suggests the possibility of long-distance dispersal by strong, gusty, summer winds. Soil seed bank samples collected from 13 populations yielded close to zero seedling emergence, indicating the absence of persistent seed banks. A. plicatilis seeds stored under ambient laboratory conditions for 3, 18 and 24months were germinated in an environmental control chamber and a laboratory. High percentage germination was recorded for 18- and 24-month-seed (86 and 80%, respectively), while germination of 3-month-old seeds was three times lower, suggesting the need for after-ripening. Germination of fresh and one-year-old seed under ambient nursery conditions at the Karoo Desert National Botanical Garden in Worcester yielded emergence percentages of 67 and 44%, respectively, and were therefore less successful than germination under more controlled conditions. This is the first known study to investigate the reproductive ecology of a tree aloe species and that of a Cape fynbos aloe. The study highlights the importance of further studies on aloe seed ecology, particularly for rare and threatened species.

Genesis of the Institute for Aloe Studies

A loe sabaea, it all started with Aloe sabaea (Fig. 1). I was visiting the Ruth Bancroft Garden in Walnut Creek, California for the first time and when I saw that majestic tree aloe with its long twisted and pendulous leaves, I was hooked. It reminded me of the wonderful illustrations in the children’s books written by Dr. Seuss. I knew then that I had to have one and I was lucky enough to meet Brian Kemble, the garden’s curator, who was happy to send my brother and I home with Aloe sabaea plants of our very own. Over the next few years I collected aloes with a passion. I read as much as I could about them, tracked down copies of both aloe books written by G. W. Reynolds and began volunteering at the Ruth Bancroft Garden. My brother and I spent many afternoons driving to the many nurseries in the San Francisco Bay Area in search of new species to add to our collection. Our shared enthusiasm helped us become much closer and is a passion we continue to share. We spent many Wednesday afternoons volunteering at the Ruth Bancroft Garden, which has a large number of aloes in its collection. It has flowering sized plants of species rarely seen in cultivation, such as Aloe sabaea, A. medishiana, A. powysiorum, A. ukambensis, A. sheilae and many others. Repotting seedlings and adult plants, maintaining the greenhouse and

The Tree Aloes of Southern and Eastern Africa

The Tree Aloes of Southern and Eastern Africa

Antimicrobial, anti-inflammatory and mutagenic investigation of the South African tree aloe ( Aloe barberae)

Ethnopharmacology relevance In recent times, many products ranging from aloe drinks to aloe gels, powders, capsules, and creams have appeared on the commercial market prepared from different aloe species including Aloe barberae. These products are used in ethnomedicine to treat various conditions including gastrointestinal disorders, insect bites, skin burns and other skin injuries by traditional communities. Aim of the study This study was aimed at evaluating the antibacterial, antifungal and anti-inflammatory activities as well as genotoxic effects of different extracts of Aloe barberae. Materials and methods Organic and water extracts of the upper stem, young bark, mature bark, leaves and roots of the South African tree aloe ( Aloe barberae) were evaluated for their antimicrobial [Gram-positive ( Bacillus subtilis and Staphylococcus aureus), Gram-negative ( Escherichia coli and Klebsiella pneumoniae) bacteria as well as the fungus Candida albicans], anti-inflammatory (COX-1 and COX-2) and mutagenic properties (Ames test). Thin layer chromatography (TLC) was used to compare the phytochemical profiles of different extracts of Aloe barberae. Results The petroleum ether (PE) and dichloromethane (DCM) extracts of the mature bark, leaves and roots exhibited good activity against all the bacteria and fungus Candida albicans with minimum inhibitory concentrations (MIC) ranging from 0.195 to 1.56 mg/ml. All the PE extracts evaluated showed a high activity (>70%) in both COX-1 and COX-2 assays. Apart from the organic extracts of the root with consistently good activity (>70%), all the remaining extracts showed moderate activity (40–69%) in COX-1 assay. The PE extracts also showed a dose dependent increase in activity. Ultraviolet (UV) spectrum of the leaves and root EtOH extracts indicated the presence of compounds that could absorb UV light (wavelength: 190–820 nm). None of the extracts had a mutagenic effect in the Salmonella/microsome assay against a tester strain, TA98. Conclusion Activity observed in the bark, leaves and roots of Aloe barberae validates its use in commercial herbal products, ethnobotany and ethnoveterinary medicine by South African communities and small scale farmers to treat various conditions.

Pharmacological activities of the South African tree aloe (Aloe barberae)

Pharmacological activities of the South African tree aloe (Aloe barberae)

The Birds and the Bees: Using Selective Exclusion to Identify Effective Pollinators of African Tree Aloes

Bees are frequent visitors to plants that, on the basis of floral traits, would be expected to be specialized for bird pollination. We conducted bird exclusion experiments to establish the contribution that bees make to the female component of pollination success of five “ornithophilous” Aloe species in the succulent‐rich subtropical thicket vegetation in southeastern South Africa. Experiments showed that all five aloes are genetically self‐incompatible and require cross‐pollination for seed set. Seed set in flowers exposed to both bird and bee visitors was generally severalfold higher than in flowers exposed to bee visitors only. The magnitude of this effect varied greatly among species and was affected by floral traits and bee behavior. In three aloes where the relatively short corolla tubes are blocked by exserted filaments, bees made a negligible contribution to seed production. However, in two aloes with long tubular flowers and included filaments, bees often crawled into the perianth tube to obtain nectar and contributed to seed production. Pollen‐collecting honeybees contribute the least to pollination and may even impact negatively on aloe reproductive success by acting as pollen thieves.

Coexistence of succulent tree aloes: partitioning of bird pollinators by floral traits and flowering phenology

Coexistence among species that lack genetic barriers to hybridization usually depends on pre‐mating isolating barriers. It has been difficult to explain coexistence among African Aloe species because they readily hybridize, often flower simultaneously and are mostly bird‐pollinated. Here we show that co‐flowering aloes in a succulent thicket community in South Africa partition the fauna of flower‐visiting birds. Aloe species with small amounts of concentrated nectar in long corolla tubes were pollinated primarily by long‐billed sunbirds. These species co‐flowered with species with large amounts of dilute nectar in short corolla tubes which were pollinated primarily by short‐billed, generalist nectarivores. Aloe species which share pollinators tend to have divergent flowering times and differences in pollen placement on birds. Without these isolating barriers, genetic dissolution of sympatric Aloe species would be likely.

Notes on the phenology, natural geographical distribution range and taxonomy of Aloe dichotoma (Aloaceae)

Summary: Aloe dichotoma Masson is a tree aloe occurring naturally in the arid, northwestern parts of southern Africa. In this paper a detailed description of the distribution range and flowering phenology of the species is given and its taxonomic position with regard to the closely related tree aloes, A. ramosissima Pillans and A. pillansii L.Guthrie is discussed.

Phylogenetic relationships in Asphodelaceae (subfamily Alooideae) inferred from chloroplast DNA sequences (rbcL, matK) and from genomic fingerprinting (ISSR)

AbstractTwo independent lines of molecular evidence have been studied to explore phylogenetic relationships in the family Asphodelaceae. Genomic fingerprinting by ISSR (Inter Simple Sequence Repeats) analysis was compared to sequence data of the chloroplast genes matK and rbcL. Molecular data indicate that some long­established taxonomic concepts would have to be re­evaluated. The subfamily Asphodeloideae clusters as a sister group to a distinctly monophyletic Alooideae. However, several Alooideae genera, including Aloe and Haworthia, are apparently not monophyletic. From a molecular point of view, Haworthia can be divided into two distinct groups that agree closely with the current subgeneric classification: a monophyletic group including species of subgenus Haworthia, and a second polyphyletic group with the subgenera Hexangulares and Robustipedunculares. This second clade includes Poellnitzia, Astroloba, Gasteria and even one Haworthia­like aloe (Aloe aristata). In the polyphyletic assemblage currently classified as Aloe, several smaller clades can be recognised, often reflecting morphological, chemical and geographical discontinuities. The tree aloes (sections Aloidendron and Dracoaloe) and climbing aloes (series Macrifoliae) appear to have separated early in Alooideae, while other groups (e.g., the flavonoid­containing group and a Madagascan group) are embedded within and amongst other genera. Chortolirion clusters with the grass­like aloes (section Graminialoe Reynolds, syn. Leptaloe Berger), A. boylei and A. verecunda , on a well­defined branch. The current taxonomic system clearly does not reflect the phylogenetic affinities and relationships amongst the succulent genera Aloe, Astroloba, Chortolirion, Gasteria, Haworthia, and Poellnitzia.

Dead leaves and fire survival in Southern African tree aloes.

Many aloe species in southern Africa have stems clothed with a layer of persistent dead leaves. The degree of stem coverage is species-specific. The suggestion that persistent dead leaves have an insulatory function and are adaptive in fire-prone habitat was tested on Aloe ferox. Field surveys demonstrated a significant negative relationship between mortality and degree of stem coverage and laboratory studies confirmed the insulating properties of dead leaves. The distribution of southern African tree aloes supprts the prediction that bare-stemmed species would be confined to fire-free habitat whilst fully clothed species would occur in both fire-prone and fire-free habitat. The study suggests that harvesting of Aloe ferox leaves for medicinal purposes could result in significant mortality in fires.