Swamp Milkweed Cultivar
by Brian Johnston (Canada)
Although I have seen images of the
native North American plant Swamp Milkweed in wildflower books,
never come upon one in real life. When this plant turned
up at my
local greenhouse it appeared to offer a chance to compare it
ubiquitous Common Milkweed Asclepias
syriaca. The plant studied here of course, is a
and not the actual wildflower. ‘Soul Mate’ grows to more
metre in height, with tall, branching stems. The plant is
but its flowers are certainly not! Flower diameters of
approximately 6 mm are typical.
Swamp Milkweed’s flowers are
fragrant (vanilla-like), and deep rose-pink in colour.
glossy green, lance-shaped (lanceolate), and unlike Common
are not hairy.
Closer views of a group of
reveal each flower’s narrow waist, lower ring of five bent-back
(reflexed) petals, and upper ring composed of what look like
but are actually referred to as corona limbs, or hoods.
upper ring (facing the viewer in the images) is called the
Flowers are connected by their
relatively long stalks to a common point at the top of a
Such an arrangement is called an umbel.
The sequence of images below
the development of extremely early stage buds. As yet
there is no
indication of the colour of the final flowers.
Notice that when a more mature
finally begins to bloom, its five lower petals are bent up to
the crown structure.
This arrangement is easier to
in the first of the two images below.
Strangely, in some blooming
flowers, the top crown structure is not deep pink in colour as
it is in
most blooms. (At this stage the outer ring of petals
above has ‘reflexed’ or bent back to form the flower’s lower
If we look down at the top of
flower, it is obvious that Milkweed is unusual. At its
white disk which in this species is the stigma. It is
as the ‘stigmatic disk’.
it are five red ‘petaloid
appendages’ called ‘hoods’.
of these hoods extends a curved ‘horn’.
The five hoods and
horns collectively form the flower’s ‘corona’.
the five, pointed, bent-back (reflexed)
petals at the base of the flower form its ‘corolla’. In many of
images, a liquid can be seen glistening in the cup-like section
hood. This is the flower’s nectar, used to attract insects
order to accomplish fertilization.
If you look carefully between
hoods, there are obviously structures located just beneath the
stigmatic disk. These structures play a part in the
fertilization, and will be looked at in greater detail later in
If the tip of one of the
reflexed petals is examined under a microscope, it is apparent
cells at its edge have no pigment. The central section
be light pink with many darker pink spots in a random pattern.
Two identical views taken with
higher magnification show one of these large spots. The
the left is true-colour, while that on the right is
because Photoshop’s ‘Levels’ function was utilized to increase
contrast. The third image shows similar area.
Cells near the edge of a petal
If a petal is pulled from the
flower, and its point of connection to the flower is examined
microscope, short hairs can be seen growing from the tissue.
In the images that follow,
the tiny, black, oval structure positioned between the hoods,
under the stigmatic disk. Between this black structure,
the ‘corpusculum’, and
flower’s waist, there runs a very narrow slit called the ‘stigmatic slit’.
the oval, black corpusculum by means of two tiny threads called
‘translators’, are two
flower’s pollen masses. Unlike flowers that produce
pollen, the Milkweed produces sticky elongated lumps of waxy
an investigating insect accidentally has one of its legs slip
stigmatic slit, it may catch the corpusculum, and as the insect
the leg, it may pull away the corpusculum and its two
Later the insect may fly to another flower and the pollinia may
contact with the stigma, resulting in fertilization. Weak
may have a leg permanently trapped in the stigmatic slit, and
remain trapped until they die. Others may pull away from
flower, leaving the leg stuck to the slit.
Photomicrographs follow that
some of the structures mentioned above. The two narrow
that form an inverted V from the bottom of the black corpusculum
the translators. The stigmatic slit can be seen clearly in
second and third images.
Views of the corpusculum and
translators from different angles follow.
Here is an image showing the
of one of a flower’s translators (brown), and its point of
to a pollinium (yellow).
Mature Swamp Milkweed leaves
lance-shaped (lanceolate), and glossy dark green on their upper
Young leaves are lighter in
and their upper surfaces are dotted with very short, fine hairs.
Two views of the underside of
of these younger leaves show the prominent longitudinal vein
Photomicrographs showing a
underside can be seen below.
For the sake of comparison, I
brought in a Common Milkweed plant (Asclepias
syriaca), to photograph. (Note: A
detailed study of this plant can be found in one of my earlier
articles.) Although this plant is a weed, and the one
above is not, I much prefer the Common Milkweed plant.
flowers, and leaves are larger, and the muted colour pallet, is
more elegant. (I guess there’s no accounting for taste!)
Like Swamp Milkweed, Common
Milkweed in the bud-stage has its four ‘real’ petals temporarily
up to protect the flower’s reproductive structures during their
development. Also note the green, pointed sepals at the
base that are missing in Swamp Milkweed.
Structurally, the flowers of
species are identical. They differ only in size (Common
four times larger than Swamp), and colouration.
This is a test. Can you
identify in the images below, the stigmatic disk, hoods, horns,
corpusculum, and a stigmatic slit?
Common Milkweed’s leaves are
shiny on their upper surfaces, and their shape is more oval than
The leaves’ undersides are
hairier than those of the earlier species, and here the hairs
and downy in nature.
Closer views of the prominent
on the underside of a leaf, and nearby tissue explain why the
feels ‘soft’ to the touch.
Although when cut, both plants
exude toxic sap, in the Common Milkweed it is much ‘milkier’
the Swap Milkweed.
While taking the photographs I
noticed two insects investigating the plant. The first was
The second was the strangest
that I have ever seen! In fact, it was so slow moving that
first thought it to be a bit of fluff that had been blown onto
plant by the wind. Only when I saw that it had legs, and
I forced to change my mind. It looks like an alien insect
science fiction film!
If you can identify the
would appreciate hearing from you. It’s appropriate I
such a strange insect should be found on an equally strange
The low magnification, (to
macro-photographs were taken using a 13 megapixel Canon 5D full
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
lens, the MP-E 65 mm 1:2.8, was used to take the remainder of
The photomicrographs were
using a Leitz SM-Pol microscope (using a dark ground condenser),
the Coolpix 4500.
A Flower Garden of
A complete graphical index of
of my flower articles can be found here.
The Colourful World
A complete graphical index of
of my crystal articles can be found here.
Microscopy UK or their contributors.
Published in the
August 2013 edition of Micscape.
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