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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 leaves.
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.

Another 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. mannensis, 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. muelleriana, 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. eremophila, 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.