Fruits:on slender tapering pedicels, obconical or pear-shaped, 4-10 x 3-6 mm, sometimes with faint ribs continuing from angular pedicels; rim usually contracted at top; disc of moderate width, steeply descending; valves 3(4), broad based, more or less at rim level (varying from slightly exserted to slightly enclosed); seed yellow-brown.
Derivation of names: botanical - from Greek micros (small), plus corys (helmet, cap), referring to opercula; common - refers to heartwood colour and to its greasy nature.
Taxonomy: Subgenus Alveolata (etymology not known).
No sections, subsections, series, or subseries.
Related species: none; E. microcorys the only species in subgenus; formerly included by Pryor and Johnson in Subgenus Symphyomyrtus, where it was completely anomalous.
General: coastal or subcoastal species, occurring from central New South Wales into southeastern Queensland; major commercial timber tree in eastern Australia. Introduced into Zimbabwe about 1917, planted in a number of localities, espec-ially in Eastern Highlands where grown for timber on small scale; has been planted in Harare; no actual specimens known, but almost certainly present within city limits.
APPENDIX 1. LIST OF EUCALYPTUS SPECIES KNOWN TO HAVE BEEN PLANTED IN AND AROUND HARARE.
Recent research has revealed that far more eucalypts have been planted in Harare, and its immediate environs, than was previously thought. All species known to have been planted in an around Harare are listed below in taxonomic order of subgenus, and alphabetical order of species within subgenus. Those species marked * are known or believed to be still present in the city and its immediate environs, and are the subjects of the species digests.
CORYMBIA SYMPHYOMYRTUS calophylla* mannensis* citriodora* melliodora*
APPENDIX 2. HYBRIDIZATION IN EUCALYPTUS While hybrids are not uncommon in eucalypts in Australia, even in localities free from human disturbance, there are effective genetic and ecological barriers to wholesale and widespread hybridization between species. The subgenera of Eucalyptus are genetically isolated from each other, and interbreeding cannot take place between these major groups. Even within a subgenus, while some sections may be fully interfertile, others will cross rarely or not at all. In their natural environment there is very precise correspondence of Eucalyptus species with habitat, and the boundaries between stands - each occupying its distinct micro-habitat - are generally very sharp. Interfertile species usually do not occur in the same stands, but they may occur in adjoining stands; when they do, hybrids between them may be found in the narrow zone that forms the transition from one stand to another. This zone may be called a "hybrid habitat" (not suitable for either of the parental species), where the hybrids are confined because of their inability to penetrate their parents' domains. Where two interbreeding species are brought into contact by an abrupt change in habitat, eg through a sudden change from one soil type to another, there may be no zone of hybrid habitat,
and probably no hybrids.
Most stands of eucalypts, especially those in southeastern Australia, contain at least two species, but these invariably belong to different subgeneric groups - Corymbia with Symphyo-myrtus, or Eucalyptus with Symphyomyrtus - and so are unable to hybridize. Sometimes two interfertile species of the same subgenus may occur together, but in such cases they would have different flowering times, and would, therefore, be prevented from crossing. Left undisturbed, plant communities remain stable, and species integrity is maintained, but land clearing, cropping, etc, introduce a new dimension that upsets this balance, and creates conditions that break down species stability and favour the appearance of hybrids.
When large numbers of Eucalyptus species are brought together into plantations, arboreta, or ornamental plantings outside their natural habitat it is inevitable that many related species, with overlapping or coinciding flowering periods, will become confined within relatively small areas; with the geographical barriers to hybridization thus removed, a considerable amount of inter-species crossing can then take place. Some species tend to develop peculiarities of behaviour in an exotic environment, and may be in flower, to some extent at any rate, for all 12 months of the year, a situation that permits them to hybridize with several other species. In Zimbabwe E. tereticornis and E. camaldulensis
have been observed to flower to a greater or lesser extent in every week of the year, and are extreme examples of "promiscuous" species; they cross with each other and with most species in the section Latoangulatae. E. grandis and E. robusta have coinciding flowering periods, and it is not uncommon to find grandis capsules that have a strong element of robusta in them. The occasional overlapping of the flowering periods of E. grandis and E. botryoides produces hybrids that usually look like grandis, but have rough bark to a considerable height up the trunk. At Mtao Forest many of the old stands contained a complex of hybrids involving
E. grandis, E. propinqua, E. punctata, E. resinifera, and
E. tereticornis, which came to be known locally as
E. "mtaoensis", but a meticulous breeding programme over the past 36 years has isolated from this hotchpotch a local landrace of E. grandis suited to the dry climate and winter frosts of Mtao, and that is all that is planted there now.
The general rule with hybrids is that they are morphologically intermediate between the parental species, sometimes tending to be more like the one, sometimes more like the other, but always intermediate. The problem with hybrids is in recognizing them for what they are, and very many have in the past been described and named as new species. To the layman hybrids are extremely confusing, in large part due to the great number of species in Eucalyptus and to the impossibility of the layman becoming familiar with all of them; but the leading taxonomists of today are unlikely to be deceived by hybrids, because of the modern facilities at their disposal and the accumulated experience of the past to draw on.
Many eucalypt plantings in Harare consist partly or wholly of hybrids, the commonest being E. grandis x E. tereticornis, and E. grandis x E. camaldulensis, and during the original preparation of this article on eucalypts in Harare many strange-looking trees were examined, only to find grandis, or near-grandis capsules beneath trees that were clothed in some other species' bark! This abundance of hybrids has arisen from seed collections made in the mixed plantings of the early years of the 20th Century, but today there is more awareness of the need to obtain seed of pure species in order to avoid the risk of raising degenerate trees from seed collected from hybrids.
Provided one is reasonably familiar with the distinctive characters of the pure species in Harare, or in the counmtry as a whole, it is not too difficult to recognize hybrid eucalypts, and to arrive at a fairly accurate idea of what the parental species are. For the eucalypts that are known to be present, or might be present, in Harare the possibilities of inter-species hybridization are summarized below:
E. microcorys - no hybridization possible with any species.
E. calophylla x ficifolia - yes.
Series Torellianae x Maculatae - yes.
E. torelliana x citriodora x maculata x henryi - yes, all combinations.
Notiales x Septentrionales - probably not, due to their long isolation from each other in the southwest and east (respectively) of Australia; this isolation has probably led to a degree of evolutionary divergence and the development of genetic barriers to hybridization.
E. grandis x saligna x resinifera x robusta x botryoides x propinqua x punctata - yes, all combinations.
E. longifolia x other Latoangulatae - possible, but less likely.
E. tereticornis x camaldulensis - yes.
Latoangulata x Exsertaria - yes, but less likely with
E. mannensis x bakeri - yes
E. mannensis x bakeri x socialis - possible.
Latoangulata x Bisecta - possible, but none known.
Transversaria x Bisecta - possible, but none known.
E. cinerea x smithii x maidenii x bicostata x nitens x goniocalyx x viminalis - yes, all combinations.
Latoangulata x Maidenaria - yes; E. grandis x cinerea known at Marondera, and E. grandis x nitens has become important for clonal plantations in South Africa; at this stage uncertain whether all species of these two sections are interfertile.
Exsertaria x Maidenaria - possible, but none known.
Bisecta x Maidenaria - not known.
E. intertexta x normantensis x polybractea - possible, all combinations.
E. siderophloia x crebra x staigeriana - possible, all combinations.
Terminales x Apicales - not likely.
E. polyanthemos x Series Rhodoxyla - not likely.
E. paniculata x Series Melliodorae - not likely.
E. sideroxylon x Subseries Leucoxyla - not likely.
Adnataria x Maidenaria, Exsertaria, or Latoangulata not likely.
Symphyomyrtus x Corymbia or Alveolata - no.
Any hybrid progeny arising from unlikely combinations will probably be sterile.