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Fruit

The fruits of eucalypts, commonly called the gumnuts, are a compound structure of supporting tissue, the hypanthium, and the ovary. In bud, the ovary is sunk into the expanded, invaginated top of the pedicel (individual bud stalk) known as the hypanthium. The side walls of the ovary are usually fused to the inner wall of the hypanthium such that they appear as one structure. Following fertilisation, the stamens fall from the flower, the style surmounting the ovary usually sheds, and the remaining structure becomes woody and matures into the fruit. The rim of the fruit comprises the scar or circular "platform" where the operculum was attached, then on the inner side, the narrow or broad ring of tissue that bore the stamens, and finally a band of tissue that links the rim with the ovary roof. This last tissue is the disc. It may descend vertically to the ovary and line the inner wall of the hypanthium as in the bloodwoods, or cross horizontally to the ovary roof, e.g. E. regnans, or be raised and ascend to an uplifted ovary roof, e.g. E tereticornis. Some western species have a further development of the disc, e.g. in E. cornuta and related species and E. coronata and related species, in which the disc extends over the valves such that only the extreme tips of the valves are exposed.

 
Throughout the genus fruit shape is difficult to categorise with certainty. One very distinctive fruit form, however, is seen in a few species endemic to southern Western Australia. In these, the numerous individual fruits in a single cluster are fused by the walls of the hypanthium from the time of bud formation onwards. The fused buds mature into a large, hard, woody cluster that is instantly recognisable, as in E. lehmannii.

 

These fruits were originally considered to be so distinctive that on the discovery of the species, E. lehmannii was thought to belong to a different genus and was given this status in the newly coined name, Symphyomyrtus, meaning 'fused myrtle'. Later the fused character was considered to be somewhat superficial and the species was placed in the genus Eucalyptus. Fusion of organs is easily recognised and of great value in species recognition. It occurs elsewhere in the genus in other organs, e.g. the juvenile leaves of E. uncinata and the stamens of E. synandra.

For western species another useful aid to identification is found in part of the subgenus Eudesmia. The buds and fruit of many of the Eudesmia species are square in cross-section, the sepals being conspicuous on the rim at the tips of the sides of the square. 'Square' fruit are also seen in the widespread E. calycogona, and E. prolixa, which is endemic to the goldfields of Western Australia. This is an interesting convergent character as the two groups are quite unrelated. Curiously the square fruit is also seen in one box species, viz. E.froggattii, and one ironbark, E. tetrapleura. Perhaps the large urceolate fruits of the bloodwoods (e.g. E. calophylla) are the other quite distinctive fruits, but a great variety of the fruit shapes seen throughout the eucalypts makes fruit shape a character for which words are rarely ideally descriptive. Therefore the shape of the fruit, and of most organs, is of doubtful reliability in identification.

The roof of the ovary is "free" and exposed and breaks into valves which spread and allow the seeds to shed. The mature woody ovary may be deeply sunk in the fruit and not actually be visible below the rim; be more or less level with the rim; or in other species, the roof of the ovary may be raised above the rim. This latter character is seen most conspicuously in E. coolabah and the ovary is scarcely inferior, i.e. it is not well sunk into the hypanthium as it is in the vast majority of eucalypts.

Of considerable value in identification are the valves of the fruit. Their number and exsertion can be characteristic of species and species groups, e.g. the red gums in which the ovary splits into 3 or 4 valves which are usually strongly exserted. Valve numbers are usually 3 or 4 for the species from this version of EUCLID, with a few exceptions where the numbers are up to 6 .

There is one valve character that requires qualification. In the large series Subulatae and to a lesser extent the series Falcatae, the ovary is sunk well below the rim of the hypanthium. The style surmounting the ovary splits into three or four needle-like structures (the number of the ovary chambers and therefore the valves). Despite their fragility they persist as the valves spread in dehiscence, and are conspicuously emergent above the rim of the fruit. Ultimately they break off but their early persistence is a feature of these two taxonomic series and may be regarded as a character of medium to high reliability bearing in mind that the 'valves' are finally lost from the fruit.

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