A close-up view of two members of the mallow family.

Members of the mallow family (Malvaceae) exist world-wide, but most of the approximately 1000 species are found in South America. The species which has the greatest economic importance is Cotton; many of the other species grow wild, or are cultivated as garden flowers. Both the Hibiscus and Hollyhock belong to this latter group.

The Exotic Hibiscus

The largest group of plants in the Malvaceae family is the genus Hibiscus. Many species possess large, colourful and frankly, spectacular blooms. Approximately 200 different species are known - each with a uniquely coloured or structured flower. The name “Hibiscus” is derived from the Greek hibiskos, and refers to the common “marsh” mallow plant. There is some difference of opinion about whether the subject of this article, Hibiscus rosa-sinensis is a naturally occurring species, or a collection of man-made hybrids. The plant has been cultivated for so long that its origins are in doubt.

As you can see below, Hibiscus leaves are intensely green, ovate in shape and roughly toothed.

An immature bud is shown below. Five to ten bracts (modified leaves) are visible that form a whorl around the calyx enclosing the flower. The calyx is the outermost whorl of a flower, and in the bud stage, it helps protect the petals and other structures as they develop.

When the flower begins to open, the calyx can be seen to have five pointed “teeth”. The green colour of both bracts and calyx indicates that chlorophyll is present, and that food can be synthesized in these structures.

The following images show the elongation of the petals that make up the corolla of the flower. It is the brightly colored corolla that aids in pollination by attracting insects once the bloom is fully open. The calyx and corolla together form the perianth.

Notice the striking sculptural quality possessed by the calyx.

The unopened corolla is also very sculptural, due to its prominently veined structure.

Once the Hibiscus plant begins to bloom, the reason for its great popularity is evident. The large colourful flowers are striking and very identifiable. Unfortunately, they last for only a single day in most species - opening in early morning and wilting by the evening. The flowers studied in this article have a single whorl of five petals making up the corolla, and are referred to as the “single form”. (“Double form” species have a second whorl of petals.) It is interesting to note that a Hibiscus flower, when cut from the plant, will not wilt until its natural closing time!

The unusual reproductive structures of an Hibiscus bloom can be seen below. The pale pink tube arising from the centre of the flower is the “staminal column” or “stamen tube”. Many fine filaments grow out from this tube, and each filament supports a pollen encrusted anther (the male pollen generating organ). The staminal column is actually a thin-walled tube, and it encloses a long, slender white style that branches at the tip. Each style branch tip holds a bright red, round stigma pad (the female pollen accepting organ). The third image shows each stigma pad to be covered in fine “hairs”.

With the petals removed, it is much easier to distinguish the features mentioned above. If you look closely at the two images that follow, you should be able to see the pointed pink teeth at the very top of the staminal column through which the style emerges.

A higher magnification reveals the many filaments topped by anthers, which seem to be arranged randomly over the column’s surface.

A still higher magnification shows the curled shape of an anther, and the coating of bright yellow pollen grains.

Under the microscope, the anther’s curled shape and connection to the filament can be seen. Each pollen grain’s rough surface, and many spikes, are just visible.

Using a shorter focal length objective results in an image which displays a grain’s detail more clearly.

As mentioned earlier, each of the flower’s stigma pads is covered with what looks like fine red “hair”.

Under the microscope, these protuberances can be seen more easily.

Strangely, some of the hairs at the junction of stigma to style are incompletely coloured. (It looks like gaps in a red-liquid filled column.)

If the light is positioned properly, the very three-dimensional character of the surface of each petal is accentuated.

The front surface of the petal is covered with prominent pink veins that radiate out from the centre.

The back surface is more three-dimensionally veined than the front.

This is more evident at a higher magnification.

For comparison purposes, here is a sequence showing the opening of a flower in another hibiscus hybrid. Although very similar to the previous plant, this one has a darker, more solid colouration.

Notice the packing of the many anthers in the image below. In this partially opened bud, the copious pollen grains seen in earlier images are absent.

The Lowly Hollyhock

If you compare the images that follow with those of the Hibiscus, the similarities are very evident. There are however, differences.

Unlike the Hibiscus, the Hollyhock’s leaves are divided and edged with rounded lobes. They are also not as thick and shiny as those of the Hibiscus.

A Hollyhock bud possesses the same outer ring of bracts, and inner calyx, but they are lighter green and more hirsute (hairy).

The calyx in an open flower lacks the interesting sculptured shape of the Hibiscus form. The petals show the same prominent veining.

Hollyhock has very similar reproductive structures. Here, however, the pollen grains are so large and tightly packed, that it is impossible to see the anthers and their filaments hidden beneath. The “clumps” of pollen, do however indicate their location.

It is just possible to resolve individual round white pollen grains in the more highly magnified image shown below.

Under the microscope, the details of a spiked spherical pollen grain can be seen.

Like the Hibiscus, the Hollyhock has five hairy stigma pads. Here however, they are white instead of bright red.

The two photomicrographs that follow show the protuberances on the stigma’s surface.

The purpose of these protuberances is illustrated below. Their form is ideal for the “catching” of pollen.

In my opinion, some flowers are beautiful, but not particularly interesting, because they hide structural details. The Rose and Carnation are examples. The Hibiscus, (and Hollyhock to a lesser degree), are both beautiful and interesting, with their display of unique reproductive structures. (The Passion-flower would be an over-the-top example of this latter type.)

Photographic Equipment

The photographs in the article were taken with an eight megapixel Canon 20D DSLR with Canon EF 100 mm f 2.8 Macro lens. An achromatic close-up lens (Canon 250D) was used to obtain higher magnification in some of the images. This lens screws into the 58 mm filter threads of the EF 100 mm Macro lens. The photomicrographs were taken with a Leitz SM-Pol microscope (using dark-ground condenser), and the Coolpix 4500.

All comments to the author Brian Johnston are welcomed.

Further Information

Hibiscus Flowers
http://www.backyardnature.net/fl_hibsc.htm

Hibiscus
http://darwin.nmsu.edu/~molbio/plant/hibiscus.html

Heavenly Hibiscus
http://www.bbg.org/gar2/topics/plants/2004fa_hibiscus.html

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