Book Review: Rhododendrons of subgenus Vireya
Dr. George Argent
Royal Horticultural Society and Royal Botanic Garden, Edinburgh, London, 2006
Reviewed by Lyn Craven, Canberra, Australia
For nearly 50 years, Vireya rhododendrons have been a focus for gardeners, attracted by the poise and elegance of the shrubs when in flower, plus the vibrant colours and diversity of form of their flowers. The starting point for our modern knowledge of Vireya rhododendrons is the floristic account by Sleumer (1966) of the Malesian species. These number about 300 and comprise the great majority of the vireyas, classified by Sleumer (1966) in Rhododendron section Vireya. In the decades following, there has been intense interest in the section with investigations resulting in the description of many new species, in the elucidation of many aspects of their reproductive biology, in the establishment of appropriate cultivation techniques, and in hybridising. Accompanying this activity, there have been some contributions on the infrasectional classification and the classification of various species and/or species complexes. More recently, DNA technology has been used to investigate the evolutionary relationships of the section and its species to test the morphologically-based classifications that have been proposed by various workers. Having to deal with the kilograms of diverse publications that I have accumulated during the 40 years has been a considerable nuisance and I, along with other researchers and hobbyists who are involved with Vireya, have been eagerly anticipating the publication of Argent's book as it was hoped this could provide a fresh starting point for work on Vireya.
The volume comprises an introduction to Vireya rhododendrons in which the group is characterised on morphological and anatomical data, its classification is discussed, and notes given on the descriptions that comprise the bulk of the volume. This is followed by a chapter on the history of vireyas, largely dealing with their exploration and introduction to cultivation, with some mention of taxonomic work. Then follow two short chapters giving, respectively, a conspectus of classification of subgenus Vireya sensu Argent (2006), and keys to the sections and subsections sensu Argent (2006). The essential reason behind the publication of the volume is found in chapter five wherein are accounts of the sections and subsections with identification keys and detailed descriptions of each species recognised by Argent. This chapter has many colour photographs which add significantly to both the information content and maintenance of interest for those who find reading botanical descriptions heavy going. The descriptions are a combination of those in Sleumer (1966) with additional data added from Argent's own observations. In the case of taxa described since 1966, presumably the descriptions are based on the information given in the protologues with additions by Argent. The final three chapters deal with collecting and conservation, cultivation and propagation, and pests, diseases and disorders, the last two chapters being written by D. Mitchell and L. Galloway, and S. Helfer, respectively.
How does the volume match up to expectations? On an initial perusal it looked very good indeed, being the first work in which are treated all the species that Argent recognises of Vireya sensu both Sleumer (1966) and Argent (2006). Several new names are published and notes are given under each taxon on aspects ranging from the original discovery of the taxon, to its distinction from similar taxa, its introduction to cultivation and/or notes on its performance in cultivation. The identification keys will be of great benefit to those needing to identify wild-collected material, although the identification of cultivated plants must be treated with caution as there are now many hybrids in cultivation, some of which have erroneously been labelled with the names of species. The chapter on cultivation and propagation is also welcome, as is that on pests and diseases.
With a more detailed reading, however, several major deficiencies were noted. Curious as to what taxonomic status Argent had given particular taxa, I discovered that several names had been omitted from the work. Series Dendrolepidon Argent et al. (1984) is not accounted for, nor is series Citrina Sleumer (1960), series Stenophylla Sleumer (1960), series Taxifolia Sleumer (1960), R. leptanthum var. warianum (Schltr.) Argent (1995) and R. brookeanum subsp. brookeanum var. kinabaluense (Argent et al.) Argent (1995). Even more curious is the lack of discussion concerning R. javanicum and R. brookeanum and their respective infrataxa. Sleumer (1966) recognised both as distinct species (with infraspecific taxa). Argent (1982) combined the two species under R. javanicum and this was maintained in a 1988 publication. In 1989, Argent flagged that the two species might however be distinct, and in 1995 "resurrected" R. brookeanum and its infraspecific entities. In the present volume Argent has returned to his 1982 position without comment. In 40 years, these two species have had four taxonomic dispositions! Given that three of the four viewpoints have been put forward by him, Argent should have given us some detailed explanation as to why he has settled on his 1982 position. Analysis of molecular sequence data (Hall et al., unpublished) derived from one accession each of the R. javanicum and R. brookeanum species complexes places each accession in a different clade and it would appear that Sleumer (1966), using morphological data, had the two species justifiably separated. Argent in fact gives some excellent argument for their separate recognition in this volume. In the key to Bornean species of Euvireya on pp. 235-236, the five included subspecies of R. javanicum sensu Argent (including subspp. brookeanum and kinabaluense both of which are "brookeanum" taxa) key out in widely separated places. If these were forms of the one species one would expect that, even with artificial keys, some of them would key out at the same couplet (following which they would then be further discriminated). I did not read the volume for typographical errors but the spelling "bryophyllum" for the epithet of R. bryophilum did stand out. I suspect that this may have been an editorial glitch by someone with an interest in the crassulacean genus Bryophyllum.
The classification of vireya species in subgenus Vireya with seven formally established sections is rather surprising given that Argent has previously (1985, 1988) and more to the point, presciently in the light of contemporary molecular data, indicated that the sections (as subsections) were mostly not worth maintaining. While Argent's aim "to present a practical way of dividing this large group of species into subunits so that species can be identified" is applauded, this could have been achieved with a system of informal groups that avoid the constraints of the formal nomenclatural system. In fact, it is mystifying as to why he has presented an avowedly artificial classification so formally, given that it will not stand once the significance of the DNA data has been appreciated. The classification adopted by Argent is essentially the same as that of Sleumer (1966) with some minor shifts (separation of Asian mainland Pseudovireyas from the island Pseudovireyas, etc), but with the ranks inflated. The crux of the matter is that molecular data (Hall et al. 2006, unpublished) show that some of the species groupings are placed outside Vireya sensu stricto and do not belong within section (or subgenus) Vireya. The evolutionary relationships of Vireya sensu stricto,and the classification of the whole group, will be discussed in other flora.
Argent's concern that the Papua New Guinean species R. retrorsipilum may be extinct (p. 329) may be unwarranted. By chance, I happened to travel past Markham Point in June 2006 and the rainforest on the whole ridge seemed to be in fine shape with the canopy unbroken. Rhododendron retrorsipilum has been recorded at 600-900 m asl, and only at the very base of the ridge did there appear to be human-induced disturbance; probably this is much the same as it was 100 or more years ago. However, concern is warranted for northern and western Malesia species that occur in upland areas prized for end uses such as tea plantations and housing estates for the upper classes; from these the forest will never return.
The chapter on cultivation and propagation is largely based upon experiences with cultivating vireyas in greenhouses at high latitudes, especially at Edinburgh. It overlooks the fact that the future for vireyas in cultivation lies not in European greenhouses but in those equable regions of the world in which the species and their hybrids can be grown in the garden, or at least under shadehouse equivalence. The Pacific coast of North America, Florida, Hawaii, New Zealand's North Island, and the east and southeast regions of Australia have proved to be ideal for outdoors cultivation. Cultivation in lower montane regions of the tropics will certainly be feasible as well. It would have been a simple matter to have enlisted the collaboration of growers skilled in outdoor cultivation in writing a globally relevant essay. The chapter on pests and diseases is quite comprehensive although Botrytis seems to have been overlooked (unless it has had a name change, that is). Botrytis can be a serious pathogen in poorly ventilated greenhouses, killing such species as R. multicolor and R. stenophyllum and killing branches on species such as R. brookeanum and R. zoelleri. The reported occurrence of some virus-induced diseases such as necrotic ringspot disease (Craven 1994) was overlooked. The insidious nature of viruses is that they can be present in plants without showing symptoms and then show up in cutting- or seed-grown plants derived from these. Not even the Edinburgh Vireya collection is free of virus. Helfer (2006) gives excellent advice when he recommends the destruction of virus-affected plants.
While the book format is user friendly, to my mind the authors and publishers could have taken advantage of some of the opportunities offered by the new publishing technologies. Had the descriptions been published in searchable format on a CD packaged in a sleeve at the back of the volume, a great deal of space would have been freed up. This space could then have been used for a proper synthesis of the Vireya literature, e.g., that on classification, biogeography, reproductive biology, phytochemistry, anatomy, micromorphology, and molecular systematics. More imaginative contributions on Vireya cultivation sensu lato, e.g., their use in landscape plantings in public parks and private gardens, their cultivation in such gardens, their potential as potted flowering plants, their commercial production, could have been included. More, and larger format, photographs could have been added, and the book would still have been about 300 pages or less. The absence of maps giving the species' distributions is a disadvantage but use of a CD would have enabled distribution maps for each taxon to be located with the description of that taxon. Argent could well have taken the opportunity presented by this book to promote further research into these fascinating plants. There are many exciting projects that researchers, and postgraduate students in particular, could get their teeth into. In particular, a great challenge would be to unravel the evolutionary relationships of the New Guinea species of Euvireya so that this incredible radiation could be more thoroughly documented.
In closing, Argent's large number of publications and oral presentations are clear evidence of his excellent knowledge of the morphological variation and natural history of vireyas. It is regrettable, however, that the format of the book, with its emphasis on the species' descriptions, has prevented his passing this on other than in brief notes after each description. Argent, in fact, was in an ideal position to have undertaken a synthesis of the whole of the voluminous Vireya literature for us and his book would have been the ideal place in which to publish it. Instead, one must review the work of the past 40 years and earlier oneself before moving forward. My 10+ kg of pre-2006 literature is still safe from the recyclers.