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Eucalyptus arborella Brooker & Hopper, Nuytsia 14: 336
(2002).
T: Fitzgerald River National Park, 1.5 km SE of Twertup Field Studies
Centre, 12 Mar. 1989, S.D.Hopper 7131; holo: PERTH; iso: AD,
CANB, MEL, NSW.
Mallet to 5 m tall. Lignotuber absent.
Bark smooth throughout, pale grey and whitish, sometimes pale
pink to pale coppery.
Branchlets lacking oil glands in the pith.
Juvenile growth (coppice or field seedlings to 50 cm):
not seen.
Adult leaves alternate, petioles 0.5-1 cm long; blade narrowly
elliptical to narrowly oblanceolate, 4-7 cm long, 0.7-1.5 cm wide,
base tapering to petiole, margin entire, apex pointed, concolorous,
green, glossy, side veins acute, reticulation moderate to dense and
broken, intramarginal vein remote from margin, oil glands obscurely
island, irregularly shaped.
Inflorescences axillary unbranched, peduncles strap-like, down-turned
at bud maturity, 3.5-7.5 cm long; buds ?11 to 15 or more, syncarpous
(all of the buds in a cluster are completely joined by the hypanthium
only, the upper part of each bud remains free) scar present (outer
operculum lost early) but may be difficult to see at bud maturity,
operculum often curved, 5-8 times the length of the fused part of
the bud, stamens completely erect, anthers narrowly oblong, versatile,
dorsifixed, dehiscing by longitudinal slits, style long and straight,
stigma blunt, small, locules 3, the placentae each with 4 vertical
rows of ovules; flowers greenish.
Fruit on down-turned peduncles, syncarpous, the individual
capsules in the woody mass 0.9-1.5 cm wide, dehiscing by elliptical
holes formed as the 3 valves split along the sutures but remain fused
apically, disc covering the surface of the valves.
Seed black, 1-4 mm long, ovoid to more or less angularly so,
dorsal surface shallowly reticulate, hilum ventral/terminal.
Cultivated seedling (measured at node 10): cotyledons Y-shaped
(bisected), robust; stems at first
triquetrous but soon rounded in cross-section, scabrid; leaves always
petiolate, opposite for 3 to 6 nodes then alternate, deltoid to ovate,
4-5.5 cm long, 2.5-5 cm wide, dull, green to grey-green; stem, petioles
quite scabrid, lamina and margins slightly scabrid, at least until
nodes 12-15, with bristle glands.
NOTES
Eucalyptus arborella (from Latin,
arbor, tree, with diminutive suffix, ella.)
A mallet that branches low, endemic to Western Australia, with southern
near-coastal distribution. It is found only in Fitzgerald River National
Park on rocky sites (breakaways, slopes and creeks) from near Twertup
south along the Fitzgerald River valley to near the estuary. It has
smooth bark and glossy green crown in which the large pendulous bud
and fruit clusters are easily seen.
Eucalyptus arborella belongs in Eucalyptus subgenus
Symphyomyrtus section Bisectae sub-section Hadrotes
because the cotyledons are coarsely bisected, buds have an operculum
scar, a long (inner) operculum and erect stamens, fruit are large,
thick-rimmed and held rigidly. Of the ten species in sub-section Hadrotes,
eight lack oil glands in the pith of the branchlets and have scabrid
seedlings. Together these eight species form series Lehmannianae,
a group further characterized by having fruit with exserted valves
that remain fused at their tips after seed shed, a feature shared
with the more distantly related E. cornuta .
Of the eight species in series Lehmannianae four, E. arborella,
E. lehmannii , E. mcquoidii and E. conferruminata ,
have the buds (and fruit) in each axillary cluster fused basally whilst
the other four species, E. newbeyi , E.
burdettiana , E. talyuberlup , and E.
megacornuta , have buds and fruit free.
E. arborella differs from E. lehmannii in mallet, not
mallee, habit and in having broader deltoid, not ovate, seedling leaves.
Both these species have broadly flattened down-turned peduncles, bud
clusters of similar size (up to 21 buds per cluster) and opercula
that are very long and slender. E. mcquoidii differs in being
a non-lignotuberous shrub (marlock) or stunted mallet with terete
down-turned peduncles, many more buds per cluster (to ca 50) but with
similar graceful slender opercula. E. conferruminata is also
a marlock or mallet but has flattened down-turned peduncles, also
with up to 21 buds per cluster but stout relatively short opercula
(< 5 times as long as wide).
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