A Close-up View of the

"Common Crocus"

Crocus vernus

by Brian Johnston   (Canada)

And all the woods are alive with the murmur and sound of Spring,
And the rose-bud breaks into pink on the climbing briar,
And the crocus-bed is a quivering moon of fire
Girdled round with the belt of an amethyst ring.

From:  Magdalen Walks

Oscar Wilde

The blooming of the common crocus has long been a harbinger of spring in cold climates.  This small flowering plant, originally native to Southern Europe and Asia, is a member of the iris family (Iridaceae).  Its popularity is so widespread that, over the years, many colourful cultivars have been developed for the nursery trade.

One particular species, the saffron crocus (Crocus sativus), produces the world’s most expensive spice (by weight) – saffron.  The flowers’ stigmas and styles are dried and used in cooking as a seasoning,  and colouring agent.  It should be kept in mind, however, that most varieties of crocus are extremely poisonous!  The toxic compound in the plant is called colchicine, [(S)-N-(5,6,7,9-tetrahydro-1,2,3,10-tetramethoxy-9-oxobenzo[a] heptalen-7-yl) acetamide], and it produces symptoms similar to arsenic poisoning.  In the worst case situation, a person that has ingested crocus tissues dies of respiratory failure.

Crocus buds have an inverted tear-drop shape formed by six petals in two whorls of three.  The outer whorl’s petals are slightly larger than the inner whorl’s.

When a petal of the purple flower seen in the first image is examined under the microscope, using low magnification, the surface cells appear to be shiny, and slightly iridescent.

Higher magnification reveals details of the petal’s cellular structure.

Near the petal’s base, where it joins to the stalk, purple gives way to green.

The base of one of the patterned flowers is bright red, and this colour extends to the top portion of the stalk.

The bottom of the stem growing from the plant’s bulb is wrapped in several layers of white, cloth-like tissue.  This gives each plant’s base a very distinctive look.  Note that both flower, and leaf stems are shrouded in this wrapping.

A close-up of the tissue shows its unusual cloth-like texture.

Two photomicrographs of the tissue, shown with increasing magnification, can be seen below.

As the buds begin to bloom, the characteristic shape of crocus flowers is revealed.

Although the flowers’ petals eventually open up more than those shown below, they tend to be less symmetrical, (and less photogenic)!

If you look carefully at the narrow leaves of the crocus, you may be able to see a central, lighter coloured stripe on each leaf.

Under the microscope, the explanation for the lighter colour is revealed.  Cells in the centre of the leaf contain less chlorophyll than those along the edges.

Here is the purple crocus flower after its ‘best to view before’ date.  Although the petals are rather ragged, they have opened up enough to allow the flower’s reproductive structures to be seen.

Higher magnification reveals details of the structures.  Surrounding the central style that branches into three feathery stigmas (female pollen accepting organs), are three anthers (pollen producing organs).  The colour differences in the two images are the result of different cameras being used.  On the left is an image taken with the Canon 20D DSLR, and on the right is an image taken with the Sony DSC F-828.  Although the Sony tends to produce images with more ‘punch’, the true colour is given by the Canon DSLR.

Here again, the two cameras were used to image a flower’s three stigma lobes.

A low magnification photomicrograph of the edge of one of the stigma’s ‘fronds’ can be seen below.

Higher magnification reveals the stubby, sticky protuberances that ring the stigma’s edge.  These protuberances aid in the retention of pollen grains.

The three anthers at the base of the flower’s style can be seen below.  They are completely encrusted with yellow pollen grains.  (The right-hand DSLR image shows the true colour.)  The right image also shows the point where the style divides to support the three stigma lobes.

The photomicrograph on the right shows the many pollen grains that cling to one another on the surface of an anther.

Using a higher magnification reveals that the pollen grains are roughly spherical in shape, and that they lack noticeable surface detail.

Finally, here is an image of a new purple crocus bud erupting from its enclosing sheath.

So many crocus cultivars have been developed by botanists, that there are actually rules to judge the desirable traits of each type in horticultural flower shows!  An excellent source of information about the crocus can be found at the web-site mentioned in ‘further information’ below.

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 a dark ground condenser), and the Coolpix 4500.

Further Information

Crocus Pages          http://www.thealpinehouse.fsnet.co.uk/crocus%20pages/

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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Published in the May 2009 edition of Micscape.
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