The mature crown consists of a branched leafy canopy
in which flower buds, flowers, fruits and seed are formed. The leaves
of a mature crown are adult in most species but in many others, leaf
advance is arrested at the juvenile phase and the tree is reproductively
mature when in juvenile, not adult leaf. In the development of any
eucalypt there is no distinct point at which the juvenile stage changes
to the intermediate and the intermediate leaves become adult. The
stages are useful although imprecise reference points.
Every leaf begins as a minute bundle of cells,
whether it is on a seedling or a grown plant. The ultimate functional
structure is a mature leaf which can be on a eucalypt plant at any
growth stage. This means that there are mature seedling leaves,
mature juvenile leaves, mature adult leaves etc. and the term "mature"
must not be used interchangeably with the word "adult".
In the great majority of eucalypts, the leaves
are formed in the following sequence. The first recognizable organ
to emerge from a germinating seed is the root which pierces the
seedcoat and penetrates downwards. It is usually white and covered
with fine hairs. Then an aerial shoot appears and a pair of cotyledons
soon unfold. These are situated on the opposite sides of a "square"
stem (a seedlot will occasionally produce seedlings with cotyledons
in threes placed symmetrically around a six-sided stem, but this
condition changes to the normal four-sided stem after a few nodes).
Above the cotyledons, the true leaves are formed
in opposite pairs (see exceptions next paragraph), each succeeding
pair being at right angles to the pair below. While the leaves in
most species continue to be formed in opposite pairs for the whole
life of the tree (this can be checked at the growing tips on a mature
crown), from the late seedling to the adult stage the leaves become
displaced at their point of attachment on the stem such that they
appear to be alternate. In many species, however, the leaf development
does not advance to the adult stage, and the crown is composed of
opposite, (broad, usually glaucous juvenile) leaves for the life
of the tree. In only a few species is the mature crown composed
exclusively of opposite, apparently adult (lanceolate or falcate,
green) leaves e.g. E. doratoxylon, E. erythrocorys.
In a small group of species, after the first
two or three pairs of leaves, the stem becomes five-sided and the
subsequent leaves form in a 2/5 spiral (e.g. E. oleosa).
This is detected by examining the seedling closely. No leaves will
be opposite and any two leaves appearing consecutively, one above
the other on any leaf-bearing face, will be separated vertically
by four other leaves distributed around the other four vertical
faces (e.g. E.
longicornis). Vertically adjacent leaves will occur on the
next leaf-bearing face but one, never the adjacent face. This produces
a spiral arrangement
of leaves that occurs often in seedlings with very narrow seedling
A different spiral formation is seen in small
group of Western Australian eucalypts. In these the stem is three-sided
and a three-leaved spiral forms in the seedling and persists throughout
the life of the tree (e.g. E. lehmannii).
Adult leaf shape is not much use in identification
as most species have lanceolate
or falcate (curved) leaves. Leaf
shape is a character of low reliability. Leaf size is useful if
all the leaves in an adult crown are unusually small or large.
Most eucalypt species have adult leaves that are more or less
the same colour on both sides. But if an adult leaf is distinctly
discolorous (the upper face is
darker and greener than the lower), then this is a fairly powerful
tool in the discrimination of species. The discoloured appearance
of the leaf is a factor of internal structure. The green photosynthetic
tissue (composed of cells with chlorophyll-bearing chloroplasts)
is near the upper surface of the leaf and is lacking towards the
lower surface in this type of leaf. The discoloured appearance is
sometimes maintained on fallen dead leaves although somewhat faded,
and the assessment can reliably be made before sampling the living
crown. Juvenile leaves in all species are usually slightly to distinctly
discoloured, so care must be taken in assessment of colouration.
It is thought that the discolorous (or dorsiventral) leaf is an
atavism (a reversion to an earlier ancestor), maintained in species
of humid or high rainfall regions that most resemble the probable
environment of the rain forest precursors of the eucalypts. It is
seen in E. intermedia in eastern Australia and in E. diversicolor
of the far south-west of Western Australia. E. cladocalyx
of South Australia with its very discolorous leaves is probably
a curious survivor of the ancient forests.
character not influenced by the environment is the leaf
venation and this can be characteristic of certain groups, e.g.
the bloodwoods have many parallel side veins at a wide angle in
a regularly pinnate (feathery) pattern.
Other species have generally fewer side
veins at more acute angles, the extreme being the Snow Gums
(E. pauciflora) and Black Sally (E. stellulata) which
have side veins more or less parallel
to the midrib. While the angle of the side veins is highly diagnostic
for the wide-angled and for the parallel-veined species, it is of
little value for angle states between the extremes.
The midrib of a leaf is the primary
vein, the side veins are the secondary
veins. When these are the only veins apparently present or visible
as in E. suberea, there
is no reticulation, a strong character
in assessing leaves for identification. Tertiary veining links the
side veins and forms a reticulum.
Some species have quaternary veining and the reticulum is consequently
very fine. There is no absolute distinction between these categories
and we use the terms no visible reticulation,
sparse reticulation, moderate
reticulation, dense and very
dense reticulation to describe them.
Eucalypts are notable for their oil glands in the leaves. In a
dried specimen the glands can only be seen with reflected light
and appear as black dots on the undifferentiated surface. But if
a fresh leaf is held up towards the sun and inspected with oblique
light through the leaf, the glands will be seen as white or yellowish
or green structures, obviously within the tissue of the leaf. This
inspection should always be done on the upper surface of the leaf
(i.e. holding the lower leaf face towards the sun). This is to ensure
comparability between specimens. The leaves of some species look
the same when viewed through either face, but most show far more
features when viewed with the underside towards the light source.
Many species will show quite different patterns between top-side
or under-side viewing. Because most eucalypt leaves turn on their
stalks and hang down in the crown, some experience is needed to
determine which are the upper and lower faces. This decision is
easier to make if the petiole is
flattened on the upper surface, as it is in many species. Difficulty
will be experienced in other species in determining the upper and
lower surfaces of a leaf if the leaf stalk is slender and not flattened.
In these instances both sides should be examined and the image with
clearer reticulation and glands assessed, as this is the upper surface.
Then comparable assessment can be made.
Leaf oil gland categories
are usually strong aids to identification as related species tend
to have similar patterns.
The oil glands may be positioned either
at the intersections of
the veinlets, e.g. E. squamosa, and E.
where they appear to be star-shaped, being connected from
the points by a linear chain of cells (appearing as veinlets)
to the tertiary veins.
In sharp contrast, the glands may appear
as 'islands', e.g. E.
loxophleba, and E. marginata, within the un-veined
areas (areoles). 'Island' glands usually appear round although
in some species as in the gimlets, e.g. E.
salubris, they are very irregular.
In some species the oil glands are obscure,
e.g. E. baxteri
which is probably a result of their appearance through thick
leaf tissue. In a few species the glands are apparently absent,
e.g. E. ovata,
and E. todtiana.
Presence or absence may be variable within a species and although
rare, is seen in E. rigidula whose leaves in southern
populations are clearly glandular while populations in more
arid regions of the species distribution to the north appear
to be glandless.
||While oil glands in the leaves
are mostly described as intersectional, island, absent or obscure,
another category confined to Western Australian species is defined
as 'abundant' or 'crowded'.
In these species, e.g. E.
annulata, E. brachycorys and their related species,
the oil glands are extremely numerous, round, crowded, often
obscuring any venation apart from the midrib. The abundant category
of glands is a character of high reliability being confined
to the series as represented by the species named above.
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