Close-up View of the Mountain Ash
View of the
by Brian Johnston (Canada)
Few trees present such a spectacularly
colourful display during the months of August and September, as the
Mountain Ash. Plentiful bunches of bright orange berries cover
the tree’s entire crown, and make more mundane green, leafy trees in
the vicinity pale by comparison. The number of berries is so
large, in fact, that many still remain on the tree late into the
winter. This abundant harvest provides the birds with much needed
food when it is most needed. In fact, the berries are the
preferred food source of the American Robin, European Starling, Cedar
Waxwing, and the Pine Grosbeak.
American Mountain Ash trees are members of the Rose family (Rosaceae). The genus name Sorbus comes from the Latin name
for the common European Mountain Ash, Sorbus
domestica. The species name for the North American
relative is obvious – americana.
less common names for Sorbus
americana are the Dogberry, Small Fruited Mountain Ash,
Roundwood, and the very strange “Missey-Mossey!
The buds of the Mountain Ash are borne in cymes – an arrangement in which
pairs of flowers grow above one another on the stem. Buds are
spherical, and range in colour from very pale green, to cream.
When the buds begin to bloom, they open into five-petaled, creamy white
flowers with a diameter of about 6 mm.
Numerous round-petaled flowers are tightly packed into upright clusters.
Each flower has many stamens,
each of which consists of a light brown anther (male pollen producing
organ), and its supporting filament.
One such stamen can be seen below in two different views. The
anther is bi-lobed, and has numerous pollen grains adhering to its
Higher magnification photomicrographs reveal the ellipsoidal shape of
At the centre of the flower, there are four pistils. Note the
white, fibrous material that is associated with the flower’s surfaces.
Two images showing a cluster of pistils
can be seen below. Each stocky style
supports a slightly dome-shaped stigma
(female pollen accepting organ).
When flowers are fertilized by insects, colourful fruit begin to
develop. The many bright orange-red fruit are grouped in large
clusters that are relatively heavy, and therefore sag under the
influence of gravity.
Although only about 6 mm in diameter, the fruit look remarkably like
Notice in the rear view shown below, that the fruit grow in pairs along
When young, the twigs of the Mountain Ash are hairy, and reddish-brown
in colour. As the twig ages, the hairiness disappears first.
Older twigs and branches have a gray-brown colour. Notice the
strange ringed appearance of some of the more mature twigs.
The leaves of the Mountain Ash are lance-shaped (lanceolate), and stalkless.
The edges of leaves are saw-toothed, but the degree appears to be quite
variable. Leaves grow opposite one another on the stem. (Note – The leaves of this plant are
Under the microscope, the exact shape of a saw-tooth is revealed.
Notice the many long, fine hairs that grow from the leaf’s surface.
On the underside of a leaf, these hairs are concentrated on the main
veins (left image), and to a lesser degree, on the subsidiary veins
(surface) cells can be seen in the image below.
Another Mountain Ash species grows in my area – Showy Mountain
Ash, Sorbus decora.
literature suggests that it can grow to 9 m in height, but I have
never observed a tree anywhere near that large. As the species
name decora suggests, the plant is particularly decorative because of
its very large (20 cm diameter) clusters of white flowers.
The bud and flower structures are identical to those of Sorbus americana, but the overall
effect is more striking.
Here too, the leaves are stalkless, but the saw-tooth pattern along the
edges is more complex.
In Southern Ontario where I live, most of the Mountain Ash, and Showy
Mountain Ash trees have been deliberately planted as landscaping for
homes and parks. In this environment they grow into spectacular
showpieces of botanical art!
All of the macro-photographs were taken with an eight megapixel Canon
20D DSLR equipped with a Canon EF 100 mm f 2.8 Macro lens which focuses
to 1:1. A Canon 250D achromatic close-up lens was used to obtain
higher magnifications in several images.
The photomicrographs were taken with a Leitz SM-Pol microscope (using a
dark ground condenser), and the Coolpix 4500.
Little, Elbert L. National Audubon
Society Field Guide to North American Trees - Eastern Region.
2004. Alfred A. Knopf, Inc. (Chanticleer Press, Inc. New York)
American Mountain Ash
Showy Mountain Ash
A Flower Garden of
A complete graphical index of all
of my flower articles can be found here.
The Colourful World of
A complete graphical index of all
of my crystal articles can be found here.
Microscopy UK or their contributors.
Published in the May
2011 edition of Micscape.
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