Editing Eucalyptus (section)

===Leaves===
[[File:Eucalyptus leucoxylon1.jpg|upright|thumb|''[[Eucalyptus leucoxylon]]'' var. 'Rosea' showing flowers and buds with [[Operculum (botany)|operculum]] present]]
[[File:Eucalyptus tetragona - glaucous leaves close.jpg|thumb|''[[Eucalyptus tetragona|E. tetragona]]'', showing glaucous leaves and stems]]

Nearly all ''Eucalyptus'' are [[evergreen]], but some tropical species lose their leaves at the end of the dry season. As in other members of the [[Myrtaceae|myrtle]] family, ''Eucalyptus'' leaves are covered with oil glands. The copious oils produced are an important feature of the genus. Although mature ''Eucalyptus'' trees may be towering and fully leafed, their shade is characteristically patchy because the leaves usually hang downwards.{{Citation needed|reason=This section lacks citations|date=March 2019}}

The leaves on a mature ''Eucalyptus'' plant are commonly [[lanceolate]], [[Leaf#Characteristics of the petiole|petiolate]], apparently [[Alternate leaf|alternate]] and waxy or glossy green. In contrast, the leaves of seedlings are often [[Opposite leaf|opposite]], [[Leaf#Characteristics of the petiole|sessile]] and [[Leaf#Surface of the leaf|glaucous]]. But many exceptions to this pattern exist. Many species such as ''[[Eucalyptus melanophloia|E. melanophloia]]'' and ''E. setosa'' retain the juvenile leaf form even when the plant is reproductively mature. Some species, such as ''[[Eucalyptus macrocarpa|E. macrocarpa]]'', ''[[Eucalyptus rhodantha|E. rhodantha]]'', and ''[[Eucalyptus crucis|E. crucis]]'', are sought-after ornamentals due to this lifelong juvenile leaf form. A few species, such as ''[[Eucalyptus petraea|E. petraea]]'', ''[[Eucalyptus dundasii|E. dundasii]]'', and ''[[Eucalyptus lansdowneana|E. lansdowneana]]'', have shiny green leaves throughout their life cycle. ''[[Eucalyptus caesia]]'' exhibits the opposite pattern of leaf development to most ''Eucalyptus'', with shiny green leaves in the seedling stage and dull, glaucous leaves in mature crowns. The contrast between juvenile and adult leaf phases is valuable in field identification.{{Citation needed|reason=This section lacks citations|date=March 2019}}

Four leaf phases are recognised in the development of a ''Eucalyptus'' plant: the 'seedling', 'juvenile', 'intermediate', and 'adult' phases. However, no definite transitional point occurs between the phases. The intermediate phase, when the largest leaves are often formed, links the juvenile and adult phases.<ref name=FieldGuide>Brooker & Kleinig (2001)</ref>

In all except a few species, the leaves form in pairs on opposite sides of a square stem, consecutive pairs being at right angles to each other (decussate). In some narrow-leaved species, for example ''[[Eucalyptus oleosa|E. oleosa]]'', the seedling leaves after the second leaf pair are often clustered in a detectable [[spiral]] arrangement about a five-sided stem. After the spiral phase, which may last from several to many nodes, the arrangement reverts to decussate by the absorption of some of the leaf-bearing faces of the stem. In those species with opposite adult foliage the leaf pairs, which have been formed opposite at the stem apex, become separated at their bases by unequal elongation of the stem to produce the apparently alternate adult leaves.{{Citation needed|reason=This section lacks citations|date=March 2019}}