A Close-up View of Several

Dahlia Hybrids

Dahlia hortensis

by Brian Johnston   (Canada)

The continuing popularity of the dahlia is not surprising.  Dahlias come in an amazing variety of colours, shapes and sizes.  This member of the Aster family, (Asteraceae), reproduces vegetatively.  Onions, hyacinths, and tulips produce bulbs, crocuses produce a bulb-like structure called a corm, but dahlias (and potatoes) produce tubers.

Dahlias are named after the Swedish 18th century botanist Anders Dahl.  Sixteenth century Spanish conquistadors came upon the ancestors of modern dahlias while conquering the Aztecs in the mountains of Mexico and Guatemala.  (All thirty-five species of the genus Dahlia are found in this relatively small region.)  Several hundred years later, seeds and tubers were located at the Royal Botanical Gardens in Madrid, Spain, from which they were distributed throughout western Europe.  Since then, plant breeders have been actively breeding dahlias to produce thousands of hybrids.  The tremendous variety of these cultivars is due to the fact that most plants have two sets of homologous chromosomes, whereas dahlias have eight sets!  Dahlias are therefore referred to as octoploids.

Several dahlia hybrids are investigated in this article, however photomicrographs of a flower’s reproductive structures are shown only for the last hybrid (near the end of the article).

Dahlia hortensis ‘Patricia’

The first image in the article shows this spectacularly coloured cultivar.  Notice in the image below that a dahlia bud is ringed by five oval, bright green leaflets.  It’s hard to believe that the flower shown in the image on the right had such an unspectacular origin.

Closer views of the centre of a flower show many columnar, pollen encrusted pistils emerging from the disk flowers of the bloom.  Careful inspection of the right-hand image reveals the five pointed, star-like shape made by the fused petals of each disk flower.

Several petals (better called ray flowers) have been removed to better show the yellow forest of pistils (stigmas and styles) at the centre of a flowerhead.  A bloom is cupped by a ring of light green sepals (modified leaves) that make up the flower’s involucre.

The two images that follow show the bi-lobed stigmas of disk flowers.  Notice the fringe of hair-like protuberances on each lobe.

I must confess that I do not have a “green thumb”.  Many times, after photographing a flower, I have attempted to keep the plant alive for a protracted period of time.  Usually the attempt is unsuccessful!  In this case, flowers continued to bloom, but they were smaller, and much less spectacular than the original blooms.  Here is an example of a “second generation” flower to prove my point.

Dahlia hortensis ‘Apricot Sunrise’

This second cultivar has a less complicated petal decoration.  Notice how the innermost petals have a rolled-into-a-tube appearance.  Also note that the bud shown in the image on the right has two small leaflets beneath it, as well as a ring of striated smaller leaflets at its base.

A short time later, the leaflets have separated from the pumpkin-shaped bud.

If a blooming flower is viewed from behind, the ring of leaflets, and a second ring of larger, paler green sepals (closest to the petals) can be seen clearly.

When the flowerhead first blooms, it is the outer ray flowers that predominate.  A dome of yet-to-bloom disk flowers can be seen at the centre of the flowerhead.

Somewhat later, the ray flowers have ‘opened up’ to reveal the central disk flowers with their projecting, pollen covered pistils.

The higher magnification view below shows that not all of the disk flowers have bloomed.

Dahlia hortensis ‘Emily’

This third hybrid has solid pink colouration.  The bloom has just opened, and it too has ray flowers with petals that are curled into tubes near the central disk.  Notice in the image at right, that none of the disk flowers have opened.

Notice the striking contrast between the opening bud at left and the blooming one at right. 

The same strangely angular bud is shown twelve hours later in the image on the right below.

Dahlia hortensis ‘Linda’

Here is another hybrid, this time with a solid yellow-orange colouration.  Early stage blooms tend to be more orange than yellow (far right).  The bloom in the centre has the outermost disk flowers open, while the one on the left is almost mature.

Notice the change in appearance of the bud as it grows.  Unopened yellow disk flowers are just visible at the centre of the bud in the image at right.

As the ray flowers bend away from the centre, the central disk flowers, in bud stage, become visible .  It appears that they are all temporarily covered by a thin transparent membrane.  In the image at right, if you look closely, you can see that some of the outer disk flowers have bloomed, and the pollen coated stigmas extend out from the distorted fused petal assemblies.

A mature dahlia flower of this hybrid can be seen below.  Several dozen stigmas extend an appreciable distance out of the disk flowers.

If you look just below centre in the right-hand image, you can see one of the disk flowers with its five tiny, pointed, yellow petals fused together.  Also note the very stocky style supporting an elongated, orange stigma coated with pollen.

Dahlia hortensis ‘Unknown’

Unfortunately, the supplier of this dahlia hybrid neglected to insert the usual identification tags into the pots.  I have been unable to identify it.

Strangely, this cultivar has ray flower petals with a solid pinkish-beige colour on the upper surface, and brilliant red and white longitudinal stripes on the under surface.  In addition, even when a bloom is completely mature, it displays no disk flowers.

The striped under-surfaces can be seen perfectly in the opening bud.  Here again, there is a lower ring of five, bright green leaflets, and another ring of lighter green sepals beneath the outer ray flowers.

An hour later, the ray flowers have begun to open out towards their final positions.

Notice the extreme contrast between the upper, and what will eventually be the lower surfaces of ray flower petals.

The pale, translucent, green sepals are in contact with the ray flower petals at this stage.  Notice the fine, white, longitudinal striations on each of the sepals.

If one of these sepals is examined under the microscope, the oval stoma and guard cells that regulate gas entry into and out of the sepal can be seen.

Here are front and side views of an almost mature bloom.

Why look at the back-side of a flower?  In the case of this hybrid, the back is the “best side”!

A low magnification photomicrograph of the underside of a ray flower petal can be seen on the right below.

Higher magnifications reveal the almost spherical cells that form the surface layer of a petal.  Note that the three images have had Photoshop’s “auto-levels” applied in order to increase contrast.  The images are thus not true-colour.

The two photomicrographs below show a typical dahlia disk flower.  Notice the five, pointed petals, that are fused together below their tips.  The first image reveals the style protruding out from the ring of petals.  Where are the stamens with their anthers, and supporting filaments?  They never protrude far enough out of the disk flower to be visible to an observer.  If you are sharp-eyed however, you can just make out a darker shadow behind each petal, (the anther), and a number of particles surrounding each shadow, (the pollen grains).  As the stigma is pushed out of the petal tube by the style, it comes into contact with the anthers, and copious amounts of pollen become stuck to its many hair-like protuberances.  Self-pollination is discouraged (but not entirely prevented), by having the stigma become receptive only after its passage through its own ring of anthers.

The cellular structure of a disk flower’s petal can be seen on the left below.  On the right is a photomicrograph that better shows hints of the anthers and their pollen, positioned behind the disk flower’s petals.

Hair-like projections on the surface of the stigma increase both its surface area, and the probability of its capturing pollen grains from the body of a visiting insect.

Higher magnification reveals these projections more clearly.

Finally, a close-up of a section of a disk flower’s style shows a different type of hair-like projection.  Notice that it is segmented.

When dahlias were first brought to Europe, breeders were initially interested in the possibility of using the plants’ tubers as a food source.  (The native dahlias’ flowers were not particularly striking.)  Early experiments using tubers as food were disappointing, and so breeders set to work to develop more visually appealing hybrids.  They certainly succeeded!

Photographic Equipment

Most of the photographs in the article were taken with an eight megapixel Canon 20D DSLR and Canon EF 100 mm f 2.8 Macro lens.  An eight megapixel Sony CyberShot DSC-F 828 equipped with achromatic close-up lenses (Canon 250D, Nikon 6T, and Sony VCL-M3358 used singly, or in combination), was used to take a few of the images.

The photomicrographs were taken with a Leitz SM-Pol microscope (using a dark ground condenser), and the Coolpix 4500.

Further Information

Everything you ever wanted to know about Dahlias, (and much more), can be found at the following link.


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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