A Close-up View of a Yellow Pincushion Protea


A Close-up View of a Yellow

"Pincushion Protea"


Leucospermum cordifolium

by Brian Johnston   (Canada)

For an earlier Micscape article, I photographed a brilliant orange-red pincushion protea.  Recently, I found a similar, but not identical example that has a yellow colour.  Much of the text of this article is identical to the earlier one, but differences have been noted.

How extraordinary it must be to live in South Africa, or Australia, and be able to look out the window to see Pincushion Protea bushes in full bloom.  These brilliantly coloured flowers are striking, not only because of their appearance, but also for their unusual structure and pollination sequence.  In Canada, where I live, the plant is available only as a cut-flower which is used in the making of elaborate floral arrangements.

All members of the Protea family are woody shrubs or trees.  The leaves, which can be seen in the image above, are hard and leathery (sclerophyllous).  The worldwide popularity of the Proteas has resulted in many hybrids and cultivars being produced to supply the cut-flower market.  Most are grown in nurseries in Israel, California and Australia.

The following images show a pincushion protea bloom.  What you are looking at appears to be a single flower, but it is not!  In fact, this is a composite bloom, made up of many small flowers and colourful bracts (modified leaves).  The bright yellow stalks that emanate from the flower-head, (the pins in the pincushion), are actually the pistils of individual flowers.  Each stalk consists of an yellow column called the style, which supports an identically coloured stigma (the female organ of the flower).  In this plant the pistil’s function is more complicated than normal.  This will be discussed later in the article.

The photographs that follow show the flower-head from different points of view.

At the base of the flower-head, there are multiple rings of black-tipped green bracts.  In the right-hand image, the fine white hairs covering each bract are visible.

A photomicrograph showing several of these hairs can be seen below.

It is not only the flowers of the Pincushion Protea plant that are unusual.  Take a look below, at the unusual shape of the plant’s leaves!  (It is strange that these leaves are completely different in shape than those of the red plant in the earlier article.)

The photomicrograph that follows shows a stoma and associated guard cells that control gas entry into the underside of the leaf.

Notice, in the two images that follow, the yellow ribbons that are associated with each pistil column.  Each of these ribbons is formed from four abutting narrower ribbons.  These are the specialized petals (called tepals) that enclose the brown stamens that are visible in the image on the right.  The tepals are much less hairy than those on the red pincushion protea.  Before a flower “blooms”, the pistil is curved, and the stigma end is buried back within the bundle of tepals.

A cross-section through an entire flower-head reveals its unusual and complex structure.  Many individual flowers are tightly packed together on the surface of the roughly egg-shaped base structure.

When looking a the entire flower-head, it is impossible to distinguish a single Protea flower.  Although difficult, it is possible to dissect one flower from the head.  A single such flower can be seen in the image that follows. The ovary, (in which the seeds develop), is below the area shown in the photograph.  The long curved structure is the style that holds an enlarged stigma at its tip.  At the base of the style, several tepals curve up to the roughly egg-shaped structure that encloses the flower’s stamens.

Front and side views of the strange structure holding the stamens of a flower can be seen below.  The sepals and petals, referred to as tepals, form the perianth. (Literally - “around the anthers”.)  The perianth segments can be seen in the images.  While in bud, these segments abut one another, but don’t overlap.  As the bud blooms, the segments separate to expose the stigma which is buried within the cup formed by the segments.  The third image shows three orange-brown anthers at the tips of perianth segments.

A microscope allows a closer look at the fine hairs attached to a perianth segment.

An immature anther can be seen on the left below.  On the right is a mature anther with some adhering pollen grains.

The cellular structure of the lower portion of the anther can be seen below.

Protea flowers normally remain closed, (with the style curved, and the stigma within the mass of tepals), until an insect, bird, or rodent disturbs the flower.  At that point, the flower snaps open, the style straightens and the stigma is held some distance above the bloom.  During this process, the stigma rubs against the stamens, and pollen becomes stuck to it.  This implies that the Protea is self-pollinating.  The assumption is incorrect.  Proteas are protandrous; the male organs mature before the female ones.  When the immature stigma picks up pollen from the stamens, fertilization doesn’t occur.  The stigma simply serves as an organ of pollen transfer, and is therefore called the “pollen presenter”.  A few hours after being exposed to the air, the pollen falls from the structure.  From 24 to 36 hours after the flower blooms, grooves (called stigmatic grooves) open in the tip of the stigma which can accept pollen from another plant.  Fertilization can then occur.

The three images below show pollen presenters.  The curved styles indicate newly opened flowers that can be seen to have some pollen adhering to their immature stigmas.

The following image shows the tip of the stigma, with its coating of yellow-brown pollen grains.

At much higher magnifications, the curved triangular shape of each pollen grain is visible.

Three types of illumination reveal the detail of a single pincushion protea pollen grain.  The first image uses dark-ground illumination, the second transmitted light illumination, and the third uses phase-contrast illumination.  Note that the “auto-level” function in Photoshop was used to increase contrast in the phase-contrast image.

In their natural habitat, the seeds produced by the Pincushion Protea are gathered up by ants and buried in the soil.  Only after a fire has killed the overgrowing plants, and returned their nutrients to the soil, do the seeds germinate to produce more of these unusual blooms!

Photographic Equipment

Most of the macro-photographs were taken with an eight megapixel Canon 20D DSLR equipped with a Canon EF 100 mm f 2.8 Macro lens which focuses to 1:1.  A Canon 250D achromatic close-up lens was used to obtain higher magnifications in several images.

A few photographs were taken with an eight megapixel Sony CyberShot DSC-F 828 equipped with achromatic close-up lenses (Canon 250D, Nikon 5T, 6T, Sony VCL-M3358, and shorter focal length achromat) used singly or in combination. The lenses screw into the 58 mm filter threads of the camera lens.

The photomicrographs were taken with a Leitz SM-Pol microscope (using dark ground and phase-contrast condensers), 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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