A Close-up View of a

Stokes' Aster Hybrid

Stokesia laevis 'Peachie's Pick'

by Brian Johnston   (Canada)

Stokes’ Aster was named after Dr. Jonathan Stokes who was an early 19th century Scottish doctor, botanist and writer.  It is a native North American wildflower commonly found in the Southeastern United States.  Since it possesses many desirable traits, including large diameter, showy flowers and remarkable heat tolerance, Stokesia laevis has been the starting point for many cultivars.

Asters belong to the family Asteraceae, (or Compositae), which is one of the largest families of flowering plants when considering only the number of species.  One estimate of this number worldwide is 23 000!  (Amazingly, the orchid family has the most species – more than 25 000 in total!)  Dahlias, dandelions, chrysanthemums, sunflowers, lettuces and thistles are commonly encountered members of the Asteraceae family. 

The subject of this article is the cultivar Stokesia laevis ‘Peachie’s Pick’, a dwarf growing to a height of about 30 centimetres.  Although the literature suggests that the 5 to 8 cm diameter flowers are cornflower-blue, all of the examples that I have seen tend more towards the pink end of the spectrum.  The unusual name ‘Peachie’s Pick’ refers to its discoverer, the Mississippi gardener Peachie Saxton.

Blooms like the ones shown in the images that follow, look like flowers.  This is a misconception however.  Each bloom is actually a flower-head composed of many flowers with those growing in the central area being referred to as ‘disk’ flowers, and those around the periphery being called ‘ray’ flowers.  Notice in the images that the plant’s very immature buds are colourless, and that the final flowers’ colour develops at a much later stage.

If you examine one of the ray flowers in the outer ring of the flower-head shown below, it is evident that its showy petal is divided into five lobes.

The tip and body of the petal are shown in the two photomicrographs that follow.  (Note that the magnifications used for the two images are different.)

Much higher magnification reveals the lumpy, irregularly shaped cells that compose a petal.

The surface of a petal is often disfigured by blemishes.  One such blemish is shown in the photomicrographs below.

Once a flower-head begins to bloom, some of the petals become coated with pollen grains, as can be seen in the last three images.  These pollen grains are roughly spherical in shape, and have complex surface detail.

The image below shows the transformation of a bud into a fully mature flower.

Each composite flower-head, even in the bud stage, is surrounded by several overlapping layers of protective green bracts (modified leaves).

Closer views reveal that the bracts are sharply pointed at their tips, and have spines along their edges.  Unlike those in a thistle however, these are easily bent, and do not puncture the skin.

Looking down on the top of a bud-stage flower-head, one can see the longer outer petals that will eventually become the ray flowers in the composite bloom.  At the centre of the bud are the very tightly packed, immature disk flowers.

Notice in the image on the right below that the ray flower petals have considerably deepened in colour, and that individual petals are more easily seen.

It is this stage that to me, appears the most photogenic.  Each vividly coloured ray flower petal is longitudinally trisected by two deep grooves.  Note that the inner-most whorl of bracts is a pale beige colour.

Still closer views show tiny granular appearing structures that liberally cover each petal’s surface.

If all but one of these petals are removed, it’s easy to see that the central disk flowers have yet to mature.

Often, all of the outer ray flowers develop simultaneously.

Occasionally however, one ray flower petal erupts from the bud long before the others.

In the two views below, notice the extreme length of the spines on the bracts’ edges.

The three images that follow show the next stage in the bud’s opening.  Here, the ray flower petals have lengthened, and have taken up more random positions around the circumference of the flower-head.

Still later, these petals have bent back, and are (almost) all lying in a plane perpendicular to the flower-head’s stalk.

Views from beneath a flower-head show the many whorls of bracts, and the oppositely arranged leaflets (without stalks, thus clasping), that grow from the stem beneath the bloom.  Many of these leaflets have spines along their edges.

A side view of a flower-head reveals that it is difficult to determine an exact location on the stem where bracts end and leaflets begin.

The Stokes’ Aster bloom shown below reveals an inflorescence (bloom) soon after it has fully opened.  An outer ring of ray flower petals largely accounts for the overall size of this composite flower.  Next is a ring of smaller ray flowers, each with a long, distinctively forked pistil.  The disk flowers at the centre of the bloom have yet to open.

A sequence of images taken with increasing magnification shows this central area of the bloom.

If the ray flowers are removed, leaving only the central disk section, a number of columnar structures are visible which have colourless bases, and purplish-pink tips.  These are the corolla tubes of disk flowers.  Eventually, their tips will open and the flowers’ elongating styles will push their stigmas out of the tops of the tubes, where they will be visible.

The following sequence of three images shows a similar central section in which this process has begun to occur.  At this point the stigma forks are still united, and are not visible.  However, both stigmas and styles, (white in the images), are completely pollen covered.  (More about this later.)

Two images follow that show the top of a disk flower’s corolla tube (centre of image).  The higher magnification view on the right reveals that the five petals are temporarily joined, except at the very tip of the tube.

As time passes, the flower’s lengthening style pushes the stigma out of the top of the corolla tube, while simultaneously, the five petals begin to separate.  In this particular flower, the petals have separated near the middle of the tube, resulting in a cage-like structure (left image).  Hours later, the tips of the petals separate (right image).  In the aster, the anthers are fused together around the inner circumference of the base of the corolla tube.  As the style pushes the stigma up the tube, both stigma and style come into contact with the pollen covered anthers, and they become coated with grains.

Eventually the mature disk flower is visible, with its pollen covered stigma.  For some reason the two stigma lobes failed to separate in the central area of my flower-head.  Ray flower stigma lobes separated in all cases.

Here are two additional images taken from above the bloom which show the disk flower’s five petals in their final positions.

Now let’s take a closer look at the reproductive structures of the ray flowers which are positioned around the central disk area.  The closer to the bloom’s outer edge that we look, the larger, and longer are the ray flowers’ petals.

If the stigma of a flower comes into intimate contact with its own pollen during its passage up through the corolla tube, why doesn’t self-pollination occur in every case?  (Self-pollination is detrimental to the long term viability of the species.)  In order to discourage self-pollination, the receptive surfaces of the two stigma lobes are facing one another and are in close contact until the stigma has passed through the danger zone.  Only later, do the two lobes separate to reveal fresh receptive surfaces to visiting insects.  Thus cross-pollination is favoured.

Next we’re going to look at the style of a flower using the microscope.  As you can see in the macro image below, the style beneath the bi-lobed stigma is pink in colour.

In addition to the pollen grains clinging to the style’s surface, notice the strangely shaped, transparent structure at the centre of the right hand image.  Literally thousands of these mystery structures also cover the surfaces of both disk and ray petals.  Earlier in the article these structures were described as granular.  All have the same strange shape, and I have been unsuccessful in determining their function.

Two photomicrographs follow that show the coating of pollen gains on the tip of one stigma lobe (left image), and the bifurcation point (right image).

Higher magnification reveals the many hair-like protuberances that cover the stigma.  These help to collect and retain any pollen grains that come their way.

The strange transparent structures mentioned earlier can be seen again here.  They are as transparent as glass, as evidenced by the fact that they act as lenses, reversing the foreground and background as an actual lens would.

At very high magnification, the complex surface structure of an individual pollen grain is visible (right image).

One of the plant’s pale green bracts, with its white spines, can be seen below.

Images follow that show the spines and their bases.

The prominent longitudinal vein that runs up each bract is visible in the image that follows.

Three images below show one of the plant’s leaflets positioned just under the flower-head.  Unlike the bracts in inner whorls, the spines here become less noticeable near the leaflet’s tip.

Leaves near the base of the plant’s stem have spines only at their bases.

It was mentioned earlier that leaves without stems are referred to as clasping.  Clear evidence of this method of attachment can be seen below.

If the point of connection of leaflet to stem is examined closely, the hairy nature of both is very evident.

A photomicrograph showing the under-surface of a leaflet follows.  Note the many oval stomata and guard cells that control gas entry into and out of the leaflet.  On the right is another photomicrograph showing pollen grains on the leaflet’s surface.

Our strange, transparent mystery structures seem to be present on the surface of leaflets, as well as petals and styles!  The twisted hairs that cover the leaflets’ surfaces are visible as well.

Three photomicrographs follow that show the cellular details of one of a leaflet’s spines, and its connection to the leaflet itself.

Finally, here are several more images showing the blooms of this strikingly beautiful plant.

Photographic Equipment

The low magnification, (to 1:1), macro-photographs were taken using a 13 megapixel Canon 5D full frame DSLR, using a Canon EF 180 mm 1:3.5 L Macro lens.

A 10 megapixel Canon 40D DSLR, equipped with a specialized high magnification (1x to 5x) Canon macro lens, the MP-E 65 mm 1:2.8, was used to take the remainder of the images.

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

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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