 |
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.
BACK TO EUCALYPT
FEATURES
|
