Sorbus L.

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Credits

Article from Bean's Trees and Shrubs Hardy in the British Isles

Article from New Trees, Ross Bayton & John Grimshaw

Recommended citation
'Sorbus' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/sorbus/). Accessed 2019-12-09.

Family

  • Rosaceae

Common Names

  • Mountain Ashes
  • Rowans

Glossary

apomixis
Reproduction without fertilisation usually by the asexual production of seeds (agamospermy) (as in e.g. Citrus Sorbus). Includes vegetative reproduction (stolons rhizomes suckers etc.) (as in e.g. Ulmus).
apomixis
Reproduction without fertilisation usually by the asexual production of seeds (agamospermy) (as in e.g. Citrus Sorbus). Includes vegetative reproduction (stolons rhizomes suckers etc.) (as in e.g. Ulmus).
autopolyploid
State of increased ploidy resulting from doubling of the normal two sets of chromosomes in a single species.
bud
Immature shoot protected by scales that develops into leaves and/or flowers.
calcifuge
(of a plant) Avoiding soil containing lime (i.e. usually growing on acidic soil).
clone
Organism arising via vegetative or asexual reproduction.
corymbose
In form of corymb.
hermaphrodite
Having both male and female parts in a single flower; bisexual.
hybrid
Plant originating from the cross-fertilisation of genetically distinct individuals (e.g. two species or two subspecies).
imparipinnate
Odd-pinnate; (of a compound leaf) with a central rachis and an uneven number of leaflets due to the presence of a terminal leaflet. (Cf. paripinnate.)
included
(botanical) Contained within another part or organ.
interfertile
Capable of interbreeding with another species or species to give hybrids.
intergeneric
(of hybrids) Formed by fertilisation between species of different genera.
key
(of fruit) Vernacular English term for winged samaras (as in e.g. Acer Fraxinus Ulmus)
microspecies
Species distinguished on the basis of minute differences of morphology. Generally used only for species that reproduce via apomixis (e.g. Sorbus).
monoecious
With male and female flowers on the same plant.
morphology
The visible form of an organism.
ovate
Egg-shaped; broadest towards the stem.
ovule
Structure inside ovary that when fertilised becomes a seed.
imparipinnate
Odd-pinnate; (of a compound leaf) with a central rachis and an uneven number of leaflets due to the presence of a terminal leaflet. (Cf. paripinnate.)
ploidy
Number of chromosomes.
pollen
Small grains that contain the male reproductive cells. Produced in the anther.
pollination
Act of placing pollen on the stigma. Various agents may initiate pollination including animals and the wind.
pome
Fleshy fruit with leathery core. Typical of Rosaceae subfamily Maloideae (e.g. Malus).
section
(sect.) Subdivision of a genus.
synonym
(syn.) (botanical) An alternative or former name for a taxon usually considered to be invalid (often given in brackets). Synonyms arise when a taxon has been described more than once (the prior name usually being the one accepted as correct) or if an article of the International Code of Botanical Nomenclature has been contravened requiring the publishing of a new name. Developments in taxonomic thought may be reflected in an increasing list of synonyms as generic or specific concepts change over time.
taxonomy
Classification usually in a biological sense.

References

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Credits

Article from Bean's Trees and Shrubs Hardy in the British Isles

Article from New Trees, Ross Bayton & John Grimshaw

Recommended citation
'Sorbus' from the website Trees and Shrubs Online (treesandshrubsonline.org/articles/sorbus/). Accessed 2019-12-09.

2019 - A NOTE

The text below is taken direct from New Trees where an imperfect split between Sorbus sensu stricto and the entire-leaved taxa using the generic name Aria was attempted. It is now clear that both European and Asian taxa fall into multiple genera, which is the approach that will be taken when we tackle the group for Trees and Shrubs Online.

JMG, 1/11/19

 

NEW TREES TEXT

The familiar subfamily Maloideae in Rosaceae, containing many important fruiting trees and shrubs, has long been a challenge to taxonomists, and species have moved rather freely between genera over the centuries. Generic concepts in the Rosaceae continue to change, and among the casualties is the well-known and very popular genus Sorbus, which was dismembered by Robertson et al. (1991). More recently the Rosaceae as a whole have been thoroughly investigated by Potter et al. (2007) who have proposed a phylogenetically based infrafamilial classification, with good support from morphological and physiological characters. In their paper the subfamily Maloideae is recognised as subtribe Pyrineae within the supertribe Pyrodae, in subfamily Spiraeoideae. The work of Potter and his co-authors shows that the various entities that made up the broad genus Sorbus are in fact quite distantly related within the subtribe (Potter et al. 2007), making recognition as subgenera impossible. The result of these studies is that Sorbus has been redefined to contain only taxa with pinnate leaves (excluding Cormus domestica). This concept was adopted by Hugh McAllister in his monograph The Genus Sorbus (2005a), which is the foundation for the present account, and should be consulted as the primary source for information on the genus. We are also grateful to Hugh McAllister and Keith Rushforth for extensive assistance.

The remaining taxa fall into four genera: Aria (Pers.) Host (whitebeams, including three distinct groups), the type species becoming Aria nivea Host (Sorbus aria Wimm. ex Nyman); Chamaemespilus Medik., comprising C. alpina (Mill.) K.R. Robertson & J.B. Phipps (Sorbus chamaemespilus (L.) Crantz, False Medlar) and C. sudetica Roem.; Cormus Spach, including only C. domestica Spach (Sorbus domestica L., Service Tree); and Torminaria Medik., containing only T. torminalis Dippel (Sorbus torminalis Crantz, Wild Service Tree). The principal segregation therefore – for the purposes of New Trees at least – is into the pinnate-leaved rowans, Sorbus s.s., and the non-pinnate whitebeams, Aria. This taxonomy has not met with universal acceptance, however, and some recent authors maintain the segregate genera as subgenera of Sorbus (Aldasoro et al. 2004, Aldasoro et al. 2005). Generic accounts in floras usually refer to the familiar broad genus. It is well to recall, in all of this, that the current standard concept of Sorbus is itself relatively recent: in the first edition of his work, Bean (1914) discussed them all under the generic name of Pyrus!

An important factor in accounting for these discrepancies in generic delimitation is hybridisation, which deceptively holds the genera together. Hybridisation between genera is common and widespread in the Rosaceae, and was thought to indicate a close relationship between interfertile genera. The presence in Europe and western Asia of numerous naturally occurring apomictic microspecies of hybrid origin, between Aria, Chamaemespilus, Sorbus and Torminaria (from the Himalaya eastwards the genera are distinct), suggested that they should all be included in one genus for convenience; but this point of view ignores the fact that Sorbus forms hybrids with several other discrete genera in the Rosaceae, including Amelanchier Medik., Aronia Medik. (= Photinia Lindl.), Cotoneaster Medik. and Crataegus L. Robertson et al. (1991) take the view that the high degree of intergeneric hybridisation in Rosaceae is the result of ineffective barriers between genera, rather than an indication of relationships. The principle of the narrow circumscription of Sorbus advocated by Robertson et al. (1991), Potter et al. (2007) and McAllister (2005a) is adopted here.

Principles and practicalities do not always meet, however, and the difficulty of using this approach in a work such as New Trees is that many whitebeams do not have published names in the genus Aria (though see Ohashi & Iketani 1993), and there are no nothogeneric names for the many hybrids between the two genera. Rather than creating a number of new taxonomic combinations without having undertaken the massive study of the complex that is urgently required, possibly introducing unnecessary synonyms thereby, we cover Aria species as an adjunct to this Sorbus account, as either Aria or Sorbus, depending on whether the combination in Aria is available. Much of the introductory material covers both genera. (Within Chamaemespilus, Cormus and Torminaria there are no new introductions to be discussed, so the issue does not arise for these.)

The pinnate-leaved genus Sorbus (s.s.), typified by the familiar Rowan Sorbus aucuparia L., comprises over 70 named species ranging across the temperate northern hemisphere, and many more that are as yet unnnamed. They are typically small trees or shrubs, or rarely dwarf rhizomatous shrubs, and have smooth or flaky bark. Dependent on the growth phase of the tree, the shoots may be long (vegetative extension growth) or short (reproductive short shoots, or vegetative short shoots). Buds are shortly ovate to conical, often asymmetrical, with (usually) four bud scales plus one extra larger scale overlapping these at the side. The leaves are imparipinnate and spirally arranged (although sometimes appearing almost whorled on short shoots). All species are deciduous, and many produce spectacular displays of autumn foliage colour. Sorbus is monoecious; the inflorescences are paniculate, flat and corymbose or rarely pyramidal, appearing in spring, and are followed by fruits that ripen mostly from late summer onwards, though some ripen as early as midsummer. The flowers are hermaphrodite, rather small (usually less than 1 cm diameter), and mostly very similar among the species (not described in detail here), with sepals more or less fleshy and persistent, petals white to pink, 10–20 stamens and (two to) three to five carpels. The fruit is a small pome, which may be orange-red or crimson to white; yellow fruits are extremely rare (McAllister 2005a).

Sorbus is a justly popular and attractive genus of small trees, valued for their foliage and abundant brightly coloured fruits; seed is frequently collected in the wild, by both amateur and professional collectors, and there is a vast diversity of material in cultivation. Some is named, some is nameable, but much remains unnamed. The key to the diversity of Sorbus – both physical and taxonomic – lies in understanding its reproductive systems. Although complicated, this has great implications for horticulture, and is not merely a botanical curiosity. The basic distinction is between those species that reproduce sexually, through recombination of genes from pollen and ovule in the normal way, and those that reproduce by apomixis, the production of seed without recombination of genes, so that seedlings are genetically identical to the parent tree. A full exposition of Sorbus reproduction is given by McAllister (2005a), from which the following notes have been condensed and modified.

Sorbus (and Aria) species that reproduce sexually may be self-compatible, meaning that they do not need to receive pollen from another tree to set viable fruit, or self-incompatible, in which case cross-pollination from another clone is required. The practical consequence for horticulture is that seed of self-incompatible species, unless gathered from a group of several clones of that species, will have been fertilised by pollen from a tree of another species, with the result that the progeny will be of hybrid origin. With some species (for example, S. scalaris and S. esserteauiana), a more or less uniform batch of seedlings hybridised with S. aucuparia is often obtained. Late flowering or isolated individuals of self-incompatible species (for example, S. matsumurana) may fail to produce fruit. Sexual Sorbus are diploids, with a normal number of chromosomes (2n = 34).

Apomixis is the production of a seed without a fertilisation event of any kind: the embryo is created through mitosis (in which the genetic complement is not reduced to the haploid level and then fertilised, as would normally happen in a sexually reproducing species). The result is that the seedlings of apomictic species are genetically identical to their parent and can be regarded as a clone. Populations of these clones can form in the wild, although they may comprise only a few individuals. When recognised and named, such entities are known as ‘microspecies’. Most apomictic microspecies are tetraploids, with a doubled complement of chromosomes.

In the Maloideae (Sorbus, Cotoneaster, Malus, Crataegus, Amelanchier, etc.), apomicts are pseudogamous: that is, they require pollination and fertilisation of the central fusion nucleus in the embryo sac to stimulate the formation of endosperm and so the seed – though there is usually no fertilisation of the egg cell which develops into the embryo. Most tetraploid Sorbus apomicts are pollen-fertile, but triploid apomicts have limited if any pollen fertility and therefore depend on pollen from neighbouring diploids or tetraploids for seed set. Triploids therefore usually set much less fruit than tetraploids and are rarely widespread in the wild (for example, S. arranensis, S. minima).

Fertile pollen from apomictic tetraploids fertilising sexual diploids generates new triploid microspecies. Unreduced female gametes of these triploids (which normally develop into apomictic embryos) may occasionally (up to 10 per cent) be fertilised by normal reduced haploid gametes from diploids and give rise to new tetraploid apomictic microspecies.

Fertile diploid species are thus able to cross with apomictic tetraploids to give further apomicts. The initial crosses are apparently quite rare, but the apomicts that result from such events are able to reproduce themselves and form clonal populations. An example of this that has been fully elucidated is the cross that has arisen on several occasions in the British Isles and continental Europe between the tetra-ploid apomictic whitebeam Sorbus (Aria) rupicola (2n = 68) and the diploid sexual S. aucuparia (2n = 34). One result is the apomictic triploid hybrid microspecies S. arranensis (2n = 51), which exists as a small population on the Isle of Arran, North Ayrshire. Others are S. minima and S. leyana in Wales, and S. lancifolia and S. subpinnata in Norway. All have a rather similar morphology (intermediate between the two parents), but are not identical, and they each have discrete distributions, justifying their recognition as separate microspecies. Some may even cross with a sexual species to produce further new microspecies. Sorbus arranensis, for example, has backcrossed with S. aucuparia to give the apomictic tetraploid S. pseudofennica (the numerous unnamed microspecies of the S. hybrida type have a similar origin), and with Torminaria torminalis (S. torminalis) to give S. intermedia. Such Byzantine relationships contribute to the nomenclatural difficulties of splitting Sorbus s.l. into segregate genera, and the only option at present is to use the generic name Sorbus for all such hybrids.

In Europe and western Asia the overall picture seems to be that the variable Aria nivea (S. aria) has several times given rise to (presumably autopolyploid) tetraploid whitebeams that are apomictic (S. rupicola, S. porrigentiformis, S. graeca, etc.). Crosses between these and the diploid S. aucuparia, Torminaria torminalis, Chamaemespilus alpina, and Aria nivea itself, have given rise to over a hundred named apomictic microspecies (Warburg & Kárpáti 1968). Within the pinnate-leaved rowans (Sorbus s.s.), crossing and backcrossing between sexual and apomictic taxa has occurred freely in the Sino-Himalayan region, producing a multitude of apomictic microspecies, each one often with a very limited range. Most are probably represented in the wild by only a handful of individuals, and very few have been collected more than once. The conservation status of such taxa is almost impossible to define. Some genotypes are possibly now represented solely by cultivated trees. In The Genus Sorbus, McAllister (2005a) took the step of naming only those that are established in cultivation or likely to become so, acknowledging that there are very many more in the wilds of the Sino-Himalaya that could likewise be described (as has been done in Europe, where some native apomictic microspecies have become common in cultivation: for example, S. intermedia, S. hybrida, S. arranensis, S. lancifolia). While it is useful to have a definitive ‘handle’ for cultivated material, such an approach does not clarify the situation in the wild, for which aggregate names covering a range of microspecies with similar assumed parentage, ploidy and appearance would perhaps be more useful – as in the case of S. microphylla agg.

For the gardener, and especially the nurseryman, knowledge as to whether a tree reproduces sexually or by apomixis is very valuable. Apomicts, reproducing themselves perfectly from seed, should always be seed-raised to allow the plant to develop its own natural form, rather than the shape imposed upon it by grafting. To propagate sexual species from cultivated plants it will usually be necessary to resort to grafting onto a suitable stock (unless a breeding group is maintained in isolation) – but genetic diversity will be lost unless several clones are propagated at the same time. For a new collection of an unknown wild Sorbus, practical observation of resulting seedlings will often indicate whether they vary significantly and are therefore probably fertile diploids, or are uniform and probably apomictic. Confirmation can be achieved only via a chromosome count or more advanced DNA research techniques, or alternatively, by raising numbers of seedlings from cultivated plants grown in association with other species (if the seedlings are very uniform the parent is likely to be apomictic; if variable, it is sexual). Identification of microspecies is likely to be a considerable challenge, and collectors’ numbers should always be retained with any material distributed. Where material does not fit any of the recognised taxa it may be permissible to apply a cultivar name – as in the case of the white-fruited ‘Molly Sanderson’ (see McAllister 2005a) – but the collector’s number is preferable.

In addition to the complexities of the breeding systems of these plants, there is an extensive infrageneric classification of Sorbus s.s. McAllister (2005a) recognises two subgenera, easily distinguished by their fruits. In subgenus Sorbus the fruits are orange-or vermilion-red when ripe, with orange flesh, while in subgenus Albocarmesinae the fruits are white, pink or crimson-red, and have white flesh. Each subgenus then has several sections, containing both sexual diploids and apomictic polyploids. A full synopsis is provided by McAllister (2005a).

A number of different species in sub-genus Albocarmesinae section Discolores (characterised by their pyramidal inflorescences, hard fruits, fleshy sepals in fruit and short styles) have been grown under the name Sorbus hupehensis, two of which have become popular – indeed, outstanding – garden trees. These are the taxa generally known to gardeners as S. hupehensis (or S. hupehensis white-fruited form) and S. hupehensis var. obtusa (or pink-fruited form). Both have Awards of Garden Merit from the Royal Horticultural Society. Research by Hugh McAllister has demonstrated, however, that the name S. hupehensis C.K. Schneid., based on Wilson’s collection W 2082B, is a synonym of S. discolor (Maxim.) E. Goetze. Sorbus discolor is a diploid, sexual species, with a wide range in China, and shows considerable variation. It is rare in cultivation, although it was described by Bean (B421) and Krüssmann (K334). The two so-called S. hupehensis forms are tetraploid microspecies, and their horticultural populations are uniform – despite the six or so supposedly different ‘clonal selections’ of the pink-fruited microspecies! As the placement of S. hupehensis into synonymy with S. discolor left them nameless, new names were necessary, and McAllister (2005a) has described them as S. glabriuscula (white fruits) and S. pseudohupehensis (pink fruits). Several other microspecies allied to them are also excellent garden trees and are described below. A key to the group is provided in the account of S. glabriuscula. The similarly named S. glabrescens (Cardot) Hand.-Mazz. is also a member of section Discolores but is not well understood botanically, and was not described by McAllister (2005a) or by Gu et al. (2003); trees in cultivation under this name are S. glabriuscula.

In the garden in northwestern Europe Sorbus are exceptionally useful, ranging in size from small shrubs to small or medium-sized trees, and some are extremely adaptable. Most true Sorbus – and in this they differ from Aria – are not suitable for dry places in warm climates, as they have shallow roots and require an ample supply of moisture through the growing season. (Exceptions to this are species allied to S. cashmiriana, and members of section Discolores – when grown on their own roots!) In dry gardens the careful preparation of a humus-rich planting site can help. Good aeration is also important, and for this reason they usually do not do well on heavy clay soils. Most, however, are happy on both acidic and alkaline soils (a handful are calcifuge). An interesting discovery by Hugh McAllister is the sensitivity of Sorbus to high nutrient concentrations, especially when young and in pots, although normal potting composts are acceptable.

As noted above, Sorbus should wherever possible be raised from seed, as this gives better-looking and thriftier trees than when grafted (though grafting has to be resorted to on occasion). The traditional method in the United Kingdom is to sow seed in pots in autumn and leave them outside in ambient winter temperatures (not in a cold frame, which may warm up too much). This is the usual method for numerous tree species, and works well with many Sorbus. With some, however, and some recent introductions in particular, it tends to give very low germination rates, perhaps because British winters are too short and warm in comparison with those of their native habitat. Hugh McAllister has devised a technique for maximising successful germination, sowing thoroughly cleaned seeds on filter paper in a Petri dish kept in a refrigerator at 0–3 ºC. Regular inspection is needed, and newly germinated seeds are removed and potted up in a low-nutrient medium. Full details are given in The Genus Sorbus.

The worst enemies of Sorbus s.l. are diseases caused by fungi or bacteria. Coral-spot Fungus Nectria galligena is perhaps the most harmful, causing shoot dieback, and in serious cases, death. It is most prevalent if the trees are drought-stressed, or where the soil is rich in nitrates. Honey Fungus Armillaria mellea will exact its toll in similar circumstances, and especially on wet clay soils. Fireblight, caused by the bacterium Erwinia amylovora, occasionally a problem in western Europe, is often devastating in North America (Dirr 1998). Sorbus trees tend to be short-lived, but though the central trunk may succumb to various fungi, a plant grown on its own roots may well coppice itself by producing new shoots from the base. As usual, it seems, there is a much wider range of pests and diseases in North America, where heat and humidity in the eastern and southern states make Sorbus largely ungrowable anyway. Borers are among the numerous insect pests (Dirr 1998). Young Sorbus are also particularly tasty to browsing mammals, from voles to deer, and protection should be provided as appropriate.

The popularity of the genus means that new collections and horticultural selections regularly become available in the nursery trade. For example, a number of Sorbus selections have been made by Glendoick Gardens in Perthshire and are sold by them as grafted specimens (K. Cox, pers. comm. 2006), under names prefaced by the registered ‘Glendoick‘ name (‘Glendoick Glory’, ‘Glendoick Ruby’, etc.); they are not attributed to species, but are all small trees with attractively coloured fruits. For descriptions of these and other new cultivars nursery catalogues must be consulted; well-established cultivars are largely covered by McAllister (2005a) or Hillier & Coombes (2002).

In addition to the principally tree-forming species described below, many smaller shrubby species have been written up by McAllister (2005a), including the following known to be currently in commercial circulation in the United Kingdom: S. apiculata McAll., S. bissetii McAll., S. californica Greene, S. eburnea McAll. (often grown as Harry Smith 12799), S. frutescens McAll. (often grown as Rock 13268, but this is probably in error for Rock 14987), S. monbeigii (Cardot) N.P. Balakr., and S. munda Koehne (often cultivated as S. prattii). Many of these shrubby species are extremely ornamental and deserving of garden space, and a few are particularly worthy of attention. Sorbus gonggashanica McAll. is a multistemmed bush forming a rounded dome up to 3–4 m tall, that in autumn can be covered by abundant clusters of white fruits – a beautiful sight. First introduced by Roy Lancaster (under Lancaster 906, 927, 937 in part, 946 in part, 1007, 1008) from the Gongga Shan (Minya Konka) in 1981, though also collected on other occasions, it is now quite widely grown and much admired. Very closely related to the familiar S. cashmiriana, and sharing its upright shrubby habit, but with pink fruits, is S. rosea McAll. This was introduced from Gilgit, northern Pakistan by the Swedish Expedition to Pakistan of 1983 (SEP 492), and has been widely distributed as ‘Rosiness’ (originally ‘Rosy-Ness’). Unusually in the genus, the flowers are also a soft pink. It is tolerant of dry conditions, and an excellent choice for small gardens. Sorbus macallisteri Rushforth from Arunachal Pradesh is remarkable in several ways – perhaps foremost for being an epiphyte on Abies densa. The leaves have only one or two pairs of virtually untoothed leaflets, set on very slender shoots little more than a millimetre in diameter, and the flowers have only two styles. It shows some similarity to S. kurzii (see below). First collected by Kingdon-Ward in 1938, it has recently been introduced from the Mande La above Dirang Dzong, near to the Bhutan border. It has white fruits and red autumn colour.

Most of the species covered below were relatively recently described and are not widely grown (at least under these names), so the horticultural commentary given is often shorter than usual. Most arboreta and many botanical gardens have a good range of Sorbus. In England there are two National Plant Collections for the genus, at Ness Botanic Gardens and in the Granada Arboretum surrounding the Jodrell Bank Science Centre, both in Cheshire. A particularly wide range is maintained in Scotland, in the collections of the Royal Botanic Garden Edinburgh and its satellites, especially at Dawyck. In continental Europe there are good collections at the Arboretum Lenoir, Rendeux, Belgium; the Arboretum of Wageningen University in the Netherlands; the Gothenburg Botanic Garden in Sweden; and the Store Milden Arboretum, Bergen, Norway. In North America Sorbus cultivation is effectively limited to the cooler, moister areas and it is not such a popular genus as it is in Europe, but there are reasonable collections at the Arnold Arboretum and Washington Park Arboretum. An interesting selection of Sorbus s.l. is maintained at the National Clonal Germplasm Repository at Corvallis, Oregon, as part of the collection there of potentially edible plants.

An interesting range of well-grown specimens from recent collections is to be found in the Reykjavik and Akureyri Botanic Gardens in Iceland, where many species grow much better than in warmer climates. Reykjavik is probably the only city in the world in which more unusual species can be seen growing commonly in private and public gardens (S. bissetii, S. cashmiriana, S. decora, S. frutescens, for example). This is no doubt partly because in Iceland there is a tradition of propagating trees from seed rather than by grafting, producing much more attractively shaped and longer-lived specimens of these naturally multistemmed species (H. McAllister, pers. comm. 2007).

Bean's Trees and Shrubs

Sorbus

A genus of about 120 species (nearer 200 if European microspecies are included in the total). They are deciduous trees or shrubs, natives of the mountains and moister regions of the northern hemisphere, mostly in the Old World and with a centre in the Sino-Himalayan region.

The leaves are alternate, simple or pinnate (semi-pinnate in some species of hybrid origin); if simple, then toothed or sometimes lobed. Stipules present, usually deciduous, but persistent and forming a conspicuous feature of the flowering shoots in a few species of the section Sorbus. Inflorescence corymbose, more rarely paniculate, terminating a short leafy shoot, the (usually) two uppermost leaves subtending the outer branches of the inflorescence. Flowers up to 1 in. wide in section Aria but more commonly less than [1/2] in. wide. The body of the flower consists of a concave cone-shaped or urn-shaped receptacle (hypanthium, calyx-tube). Calyx-lobes (sepals) five, triangular or sometimes lanceolate. Petals five. Stamens most frequently twenty, in three whorls. Carpels two to five, partly or wholly enclosed within the tissues of the receptacle and more or less connate, each with two ovules. Styles as many as the carpels, free or partly connate. Fruit composed of the thin-walled carpels, each with usually only one seed, embedded in a dryish or juicy flesh developed from the wall of the receptacle. In the section Sorbus (Aucuparia) both the free upper part of the receptacle and the calyx-lobes persist in the fruiting stage and thicken, becoming in effect an integral part of the fruit; at the other extreme, in section Micromeles, the rim of the receptacle with its lobes is shed. In the other sections the calyx-lobes usually persist but do not become fleshy.

The genus is a polymorphic one, held together mainly by technical characters of the flower and fruit, but extraordinarily diverse in outward appearance. Quite an extensive area could be devoted exclusively to Sorbus without the planting becoming monotonous – except at flowering time, the genus having no great claim to floral beauty. But by summer its ally Malus, which vastly excels it in its flowers, has nothing to offer, while Sorbus comes into its own, with its endless variations of habit and foliage. Not many of the simple-leaved species are outstanding either in their fruits, which are no more ornamental than those of the hawthorns, or in autumn colour, and from August onwards the section Sorbus becomes the standard-bearer of the genus. Only a few in this section do not give autumn colour, and in some, such as S. sargentiana, S. commixta and 'Joseph Rock' it is outstanding; all have ornamental fruits, ranging in colour from scarlet as in the common rowan to yellow, orange, pink, crimson or pure white.

The cultivars and hybrids are propagated by grafting. Seeds are used by commercial growers only to raise the necessary stocks, but many species are apomictic and come true when raised in this way, while the others should do so unless the seeds are gathered in large and mixed collections. The seed is best sown in pans as soon as ripe, and overwintered in a frame or cold greenhouse.

Most members of the genus are undemanding, needing only a reasonably good soil and an open position, but the rowan group as a whole are sensitive to drought and excessive heat and may suffer in dry summers if planted in full sun and in a shallow, light soil. Only S. insignis is tender. The worst enemy of Sorbus, as of all members of the sub-family Pomoideae, is Fire Blight, for which see Vol. I, p. 730.

CLASSIFICATION

sect. Sorbus (Aucuparia). – This section, which comprises all the fully pinnate-leaved species except S. domestica, is the only one to occur in North America (see S. americana, S. decora and S. sitchensis) and in western Eurasia is represented only by the variable S. aucuparia. Although the type-species of the section, it is not really representative of the group as a whole. Its nearest allies in eastern Asia would seem to be S. esserteauiana, S. pohuashanensis and S. amurensis, which resemble S. aucuparia in their red fruits and in having white hairs on the bud-scales, on the under-surface of the leaflets and in the inflorescence. Despite the white hairs of its leaflets and its red fruits in broad trusses, the Chinese S. scalaris is not closely related to S. aucuparia and belongs to the same group as the Himalayan S. foliolosa (wallichii). S. americana and S. commixta are both distinct from S. aucuparia in being almost glabrous, and the hairs on the latter, when present, are brown. In their very different ways S. sargentiana of Szechwan and S. gracilis of Japan are unique in the genus.

In the Sino-Himalayan region, where the section Sorbus is at its most varied, red-fruited species are uncommon, and very rare or even absent from Yunnan, their place being taken by what might be called the 'pernettya-fruited rowans', whose fruits ripen from green to porcelain white or from dark brownish red to crimson, light pink or rose-tinged white. The flowers of these are mostly borne in laxer and less branched inflorescences than in the red-fruited species, and are often pink or even crimson. Forrest made extensive collections of these interesting rowans and sent seeds on many occasions, but few of his introductions have survived. He himself did not care for them: 'In all my years in Yunnan, I haven't seen a member of the group to equal our own mountain ash, when well grown' (field-note under F.26547). Yet S. hupehensis, whose presence in gardens we largely owe to Forrest, is considered by some judges to be one of the most ornamental species. It is the tallest of the white- or pink-fruited rowans in cultivation. Mostly they are small trees or shrubs, or even dwarf shrublets (S. reducta and S. poteriifolia). Although introduced early this century, S. vilmorinii is rarely seen outside collections, though it is a delightful species, with its fern-like leaves and handsome fruits, and the same is true of S. prattii. A latecomer to gardens is S. cashmiriana, less ornamental in foliage than the others mentioned, but with large rosy flowers and white fruits the size of marbles.

sect. Cormus. – The one species in this very distinct section, S. domestica, is confined to southwestern Eurasia. Its leading characters are the pinnate foliage, the five completely united carpels, and the large fruits with numerous stone-cells in the flesh.

sect. Aria. – The members of this section range in size from large trees to shrubs, with simple, toothed or sometimes lobed leaves, which in most species have a thin to dense felt or a woolly tomentum beneath. The flowers range from [1/2] to 1 in. in width, white. Styles two to five, free, more rarely partly connate. Fruits mostly [1/2] to 1 in. long, sparsely to densely dotted with lenticels, red, brown or yellowish. Calyx-lobes usually persistent (if deciduous, then falling late), not becoming fleshy in fruit.

This section is mainly represented in cultivation by S. aria, the European whitebeam, and its cultivars. This has numerous relatives in Europe and southwest Asia, some of them microspecies of often very limited distribution, others, mainly in southwest Asia, of wider range. But these have scarcely been tested in gardens.

The Himalayan and Chinese members of the section Aria differ from most of the European species in having fruits that never become an honest red when ripe, but mostly they have fine foliage. See S. cuspidata, S. megalocarpa, S. pallescens and S. thibetica.

sect. Chamaemespilus. – This section has one species, S. chamaemespilus, endemic to the mountains of Europe. The flowers are pink, with erect sepals and petals.

sect. Torminaria. – Like the preceding, and the section Cormus, this is monotypic, with one species, S. torminalis, in Europe and southwest Asia. Leaves sharply lobed or lobulate. Styles, two, united for more than half their length. Fruits brownish, densely lenticellate, with two chambers, which are surrounded by a dense layer of stone-cells.

sect. Sorbus/sect. Aria. – Intermediate between these two sections there is a group of species, which are believed to derive from hybridisation between S. aucuparia and tetraploid members of the section Aria such as S. rupicola. Some are known to be tetraploid or triploid, and to reproduce themselves apomictically. S. hybrida, the first of this group to be described, occupies a central position between S. aucuparia and S. rupicola and shows the influence of the former in the frequent occurrence of free leaflets at the base of the leaves, while in S. meinichii, as yet scarcely known in British gardens, the influence of S. aucuparia predominates. In the most ornamental of the group, the British endemics S. anglica, S. arranensis and S. minima, the influence of S. aucuparia is less obvious, and the leaves are merely deeply lobed. The status of S. intermedia is uncertain, some botanists holding that the deep lobing of the leaves derives from S. torminalis and not, as others believe, from S. aucuparia.

These species of hybrid origin should not be confused with the casual hybrids between S. aucuparia and S. aria, for which see S. × thuringiaca.

sect. Aria/sect. Torminaria. – This group is a counterpart to the preceding, but the genes of the section Aria are here united with those of S. torminalis. The senior species of this group is S. latifolia, which makes a good specimen tree and has interesting though not showy brown fruits. But in other species, notably the British endemic S. bristoliensis, the influence of S. torminalis has brightened the colour of the fruits to orange-red.

Casual hybrids between S. aria and S. torminalis occur; see S. × vagensis under S. latifolia.

sect. Micromeles. – In this polymorphic Asiatic section the leaves are simple, toothed, the lateral veins either straight and running out to double teeth or lobules, or curving. Flowers in most species very small, to about [3/8] in. wide, in mostly rather small often paniculate inflorescences. Carpels two to five, connate and enclosed in the receptacle. Styles two to five, free or connate in the lower part. Fruits rarely much more than [1/2] in. long, lenticellate or smooth. Calyx-lobes and top of receptacle usually deciduous, leaving a conspicuous scar on the top of the fruit.

This section has its headquarters in the rainier parts of the Sino-Himalayan region, with eight species in the eastern Himalaya and northeast India, one of which extends as far southeast as Sumatra. The species best known in gardens are northern outliers of the group and are unusual in having red fruits. These are S. alnifolia, S. folgneri (the two most ornamental species) and S. japonica. Resembling these in the straight lateral leaf-veins, but with fruits of a nondescript colour, are S. caloneura and S. meliosmifolia. The species with curved lateral veins are represented in cultivation, though very rarely, by S. keissleri and S. epidendron.

SELECT BIBLIOGRAPHY

Fox, W. – Unpublished notes on Sorbus.

At the time of his death in 1962 Dr Wilfrid Fox had brought near to completion a horticultural monograph on Sorbus, a genus of which he had planted a comprehensive collection in his arboretum at Winkworth near Godalming in Surrey, now the property of the National Trust. Dr Fox had been assisted in his work by the late Mrs Madeline Spitta, to whom these notes passed on his death. During the closing months of her life Mrs Spitta put the material in order, and entrusted it to the chief editor for use in preparing the present revised account of the genus, and it has been frequently consulted.

Gabrielian, E. – The Genus Sorbus in Eastern Asia and the Himalayas. Erevan, 1978 (in Russian).

– – 'The Genus Sorbus in Turkey', Notes Roy. Bot. Gard. Edin., Vol. 23 (1961), pp. 483-96.

– – Sorbus, in Davis, P. H., ed., Flora of Turkey, Vol. 4 (1972), pp. 147-46.

Gilham, C. M., and McAllister, H. A. – 'Tree Genera – 6. Sorbus sect. Aucuparia', Arboricultural Journal, Vol. 3 (1977), pp. 85-95.

Handel-Mazzetti, H. – Symbolae Sinicae, Part VII 3 (1933), pp. 465-75.

Hitchcock, C. L., Cronquist, A., et al., Vascular Plants of the Pacific Northwest, Part 3 (1961), pp. 188-90.

Hedlund, T. – 'Monographie der Gattung Sorbus', Kongl. Svensk. Vet.-Akad. Handl., Vol. 35, no. 1 (1901), pp. 1-147.

Hensen, K. J. W. – 'The Sorbus-collection in the Botanical Gardens and Belmonte Arboretum of the Agricultural University at Wageningen'. Parts I-III in Jaarb. Nederl. Dendr. Ver., 20, pp. 121-39; 21, pp. 180-7; 22, pp. 48-56 (1956-62). Part IV in Dendroflora No. 3 (1966), pp. 60-72 (in Dutch).

– – – – 'Intermediate taxa between Sorbus aria (L.) Crantz and Sorbus aucuparia L. cultivated in the Netherlands', Jaarb. Nederl. Dendr. Ver. 21, pp. 189-204 (1958) (in Dutch).

– – – – 'Het Sorbus latifolia-Complex', Belmontia, fasc. 13 (1970), pp. 181-94.

Hutchinson, J. – Unpublished Notes on Sorbus.

Early in 1943, Dr. John Hutchinson, of the Royal Botanic Gardens, Kew, undertook at the request of the Royal Horticultural Society to write a botanical account of the genus Sorbus. After drafting descriptions of most species of the section Sorbus (Aucuparia) he abandoned the task, which he is said to have found uncongenial, and handed over the completed material to Dr Wilfrid Fox (q.v.), of whose papers on Sorbus it forms a part.

Jones, G. N. – 'A Synopsis of the North American Species of Sorbus', Journ. Arn. Arb., Vol. 20 (1939), pp. 1-43.

Karpati, Z. E. – 'Die Sorbus-Arten Ungarns und der angrenzenden Gebiete', Fedd. Repert., Vol. 62 (1963), pp. 71-334.

Koehne, E. – Sorbus, in Plantae Wilsonianae, Vol. I (1913), pp. 457-83 (treatment of sect. Sorbus (Aucuparia)).

Kovanda, M. – 'Flower and Fruit Morphology of Sorbus . . .', Preslia, Vol. 33 (1961), pp. 1-16.

– – – – 'Taxonomic Studies in Sorbus subgen. Aria', Act. Dendr. Čech., Vol. 3 (1961), pp. 41-83.

– – – – 'Spontaneous hybrids of Sorbus in Czechslovakia', Act. Univ. Carol., 1961, pp. 41-83 (in English).

Liljefors, A. – 'Cytological Studies in Sorbus', Act. Hort. Berg., Vol. 17 (1955), pp. 47-113.

Mcallister, H. A., and Williams, C. M. 'Sorbus aucuparia in Town and Country' (notes on the section Sorbus, privately circulated, 1975).

Rehder, A. – Sorbus, in Plantae Wilsonianae, Vol. II (1915), pp. 266-79 (sections Aria and Micromeles).

Regelingscommissie Sierbomen N.A.K.-B. – Sorbus, in Dendroflora No. 2 (1965), pp. 28-44.

Richards, A. J., Sorbus, in Stace, C. A., ed. Hybridisation and the Flora of the British Isles (1975), pp. 233-8.

Schneider, C. K. – Illustrierte Handbuch der Laubgehölze, Vol. I (1906), pp. 667-98 and pp. 700-4.

Yü, T. T. – Sorbus, in Flora Reipublicae Popularis Sinicae, Vol. 36 (1974), pp. 283-344.

– – and Kuan, K. C. – 'Taxa Nova Rosacearum Sinicarum', with a classification of the Chinese species of Sorbus by Dr T. T. Yü, Act. Phytotax. Sin., Vol. 8 (1963,), pp. 207-210, 221-225.

Warburg, E., F. – Sorbus, in Clapham, Tutin and Warburg, Flora of the British Isles, ed. 2 (1962), pp. 423-37.

– – and Karpati, Z., E. – Sorbus, in Flora Europaea, Vol. 2 (1968), pp. 67-71.

From the Supplement (Vol.V)

No mention was made in the main work of any species belonging to the group which the Russian botanist Eleanora Gabrielian separates from sect. Aria as sect. Lobatae. Most have simple leaves as in S. aria but these are always more or less pinnately lobed and a few may have one to three free leaflets at the base. They differ from sect. Aria in many other characters and are almost certainly of hybrid origin. Their main distribution is in Anatolia, Armenia, the Caucasus and northern Iran, though the first to be described – S. turkestanica (Franch.) Hedl. – extends into Central Asia. Two species of this group have been introduced recently – S. takhtajanii and S. tamamschjanae – both described by Gabrielian. Some older species in this group are S. persica Hedl., S. armeniaca Hedl. and S. caucasica Zinserling. The last-named was at one time confused with S. intermedia. All are shrubs or small trees.

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