Close-up View of the Mountain Ash


A Close-up View of the

"Mountain Ash"

Sorbus americana

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.  Other 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 surface.

Higher magnification photomicrographs reveal the ellipsoidal shape of the grains.

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 ripe apples!

Notice in the rear view shown below, that the fruit grow in pairs along the stem.

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 poisonous!)

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 (right image).

Individual epithelial (surface) cells can be seen in the image below.

Another Mountain Ash species grows in my area – Showy Mountain Ash, Sorbus decora.  The 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!

Photographic Equipment

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.

Further Information

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

Mountain Ash

Showy Mountain Ash

A Flower Garden of Macroscopic Delights

A complete graphical index of all of my flower articles can be found here.

The Colourful World of Chemical Crystals

A complete graphical index of all of my crystal articles can be found here.

 All comments to the author Brian Johnston are welcomed.

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