A close up view of a Black-Eyed Susan cultivar.


A Close-up View of a

Black-Eyed Susan Cultivar

Rudbeckia hirta 'Indian Summer'

by Brian Johnston   (Canada)

This member of the Asteraceae family is a cultivar of the North American wildflower commonly called the ‘Black-Eyed Susan’, ‘Gloriosa Daisy’ or ‘Yellow Oxeye Daisy’.  Rudbeckia hirta ‘Indian Summer’ produces large 6 to 9 inch diameter flower-heads with orange–yellow ray flowers and purplish-brown centres.  Large flower cultivars are usually produced by treating ‘normal’ plants with DNA altering radiation, or DNA altering compounds such as colchicine.

The genus name Rudbeckia was chosen in the 18th century by the developer of modern taxonomy, Carolus Linneaus, to honour one of his botany teachers, Professor Olof Rudbeck the Younger, and his teacher’s father, Olof Rudbeck the Elder.

The buds of this Black-Eyed Susan are not particularly colourful, but they are unusual, mainly due to their extreme hairiness.

As you can see below, it is not only the whorl of sepals, called the calyx, that is hairy, but also the leaflets that form a background to the bud.  At this stage the sepals completely enclose the bud, and hide it from view.

In order to make the hairs on sepals stand out more, I have used Photoshop’s ‘Levels’ function to increase contrast.  This results in the false colour image that can be seen below.

A sequence of true colour images taken with increasing magnification follows.  The bud itself is still hidden within the sepal cage.

A side view reveals that there are several overlapping rings of sepals forming the calyx.

As the bud swells, the flower’s disk section becomes visible, with a colour mixture of green, beige, and light brown.  Later, this central section appears a purplish-brown colour.

All parts of this plant feel furry to the touch.  The reason is obvious!  Fine, dense hairs coat every surface.  Near the growing tips of stalks and stems, all surfaces are a uniform green colour.

Lower sections of the plant’s stems and stalks possess red accents.

Near the base of the plant, the number and size of these red areas increases.  Notice that the main stem of the plant has longitudinal grooves and ridges on its surface, while the branching stalks have an almost perfectly circular cross-section.

Three closer views of these ridged stems can be seen below.  The density of hairs on the ridges appears greater than in the grooves.

Here are two views showing the surface, and edge of a leaf.  It’s no wonder that leaves feel soft to the touch!

Photomicrographs of the hairs show that they are segmented, and have spots on their surfaces.

Higher magnifications show these spots more clearly.

The point of connection of a hair to the edge of the leaf is an amazingly complex structure with cells of various shapes and sizes contributing to the engineering.

Hairs shown in the previous image were growing from the edge of a leaf.  The ones shown below grow from the veins on the underside of a leaf.

Blooming Black-Eyed Susan flowers possess a central purple disk, a surrounding yellowish-green ring, and finally, a ragged perimeter containing pointed ray petals.  Members of the Asteraceae family have composite flower-heads.  Although what you see in the images that follow appear to be individual flowers, this is not correct.  Each flower-head contains many central, small disk flowers, and an outer ring of larger ray flowers.

The central purple disk is not composed of disk flowers at this point.  They are too short at this early stage to stick up beyond the purple, hair-like structures. (These structures will be examined in more detail later in the article.)

Views of a flower-head from different angles show the ray-flower petals.  They too feel soft to the touch.  What is the reason for this phenomenon?

It should not be a surprise to find that these too are covered with masses of fine hairs.

Hairs grow along the edge of petals.

They also grow from both the upper and lower surfaces of petals.

In order to increase the contrast, and thus provide a clearer view of these hairs, I used Photoshop’s ‘Levels’ function.  This accounts for the false-colours in the following images.  In the first and last images, I suspect that the roughly spherical purplish-brown objects are pollen grains.

Although the circular mat of purplish-brown hairs at a flower-head’s centre appears impenetrable; looks can be deceiving.  The hairs surround each of the disk flowers, and as the disk flowers grow longer, they eventually get taller than the hairs and hide them from view.  In the first two of the images below, there are hairs between the nearby green disk flower buds, but they are too short to be visible.  Flower-heads of the Asteraceae family usually have a pappus associated with each individual ray and disk floret which surrounds the base of the petals (corolla).  The pappus may look like tiny hairs, as in this species, or like teeth or scales.  The name is derived from the Latin pappus which refers to a (whiskery) old man.

If many ray and disk florets are removed from the flower-head, these reddish pappi, (plural of pappus), are clearly visible surrounding each floret.

The pappi surrounding the outer ray florets appear to be smaller than those associated with disk florets.  Notice the tiny sucking insect in the third image.

Closer views of the insect can be seen below.

More highly magnified views of pappi follow.

Under the microscope, more details of these pappi are revealed.  The dark-ground illumination used shows clearly their intense red colour.

Surprisingly, at the point where the pappi are joined to the disk floret,  there are stomata and guard cells that are usually associated with the underside of leaves.  (These control gas entry and egress into and out of the leaf’s interior.)

For comparison, here are several images showing the appearance of pappi in a bud-stage flower-head.

The hair-like structures associated with a pappus in the immature flower-head are generally shorter than those in a mature pappus.

Cellular structure details in the lower segment of a pappus can be seen in the image below.

I have chosen not to include a study of the reproductive structures in this plant because many of my other articles on other members of the Aster family have already covered the topic.

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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