Close-up View of a
Stokes' Aster Hybrid
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
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
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
Often, all of the outer ray flowers
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
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
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
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
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
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.
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
The photomicrographs were taken
using a Leitz SM-Pol microscope (using a dark ground condenser), and
the Coolpix 4500.
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
November 2009 edition of Micscape.
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