A Close-up View of the

Castor Bean Plant

Ricinus communis

by Brian Johnston   (Canada)

The castor bean plant is a very fast grower.  In one year it can reach a height of more than three metres, and each of its leaves can end up being one metre long!  There are at least two downsides to growing them however.  If you live in the Northern climes where temperatures drop to 0 degrees Celsius, the plant will die. 

If you allow the plant to retain its flowers, and form seeds, these seeds are extraordinarily poisonous – the most deadly on earth!  They contain the toxin ricin, 0.5 mg of which can kill an adult (2 to 3 seeds).  A single seed has enough ricin to kill a small child.  Unfortunately the oval, shiny, spotted seeds are beautiful.  Worse still, when the capsule containing the seeds breaks open, they are ejected with considerable force away from the plant where they may be picked up by human passers-by or by animals.  Some jurisdictions discourage the planting of Ricinus communis for these reasons.  Others recommend that the flowers be removed so that seeds will not form. 

In the back of your mind you may remember being given castor oil (produced from the seeds of the plant) as a child.  No, your parents were not trying to dispose of you!  Procedures are undertaken in the production process of castor oil to prevent ricin from being included.

Ricinus communis is a member of the Spurge family (Euphorbiaceae), and is distantly related to the poinsettia, and the rubber tree.  Like other members of the family, this plant has very unusual flowers which occur in clusters at the ends of some of the stems.  The term monoecious is used to describe plants like this one which have separate male and female flowers.  In this case, the female flowers are always located above their male counterparts.  In the first image in the article, and those that follow, the female flowers can be identified by their reddish pistils.  Male flowers are identified by their clusters of beige stamens.

Castor bean leaves are palmately shaped, with from 5 to 11 pointed lobes.

Let’s look more closely at the plant’s female, or pistillate flowers .  There are no petals, only sepals, and each flower consists of three feathery-appearing, forked stigma lobes connected by short, or non-existent styles to a three-compartment, spiny ovary.

Since a newly opened leaf is shown in these images, notice in the two closer views below, that the edges of the lobes are very irregular, covered with what appear to be tiny, perhaps glandular ‘bumps’.  Interestingly, mature leaves do not have these tiny ‘curls’ along their edges.

In the images that follow we are now close enough to see that beneath the reddish stigmas, there are masses of yellow-green spines.  These spines cover the surface of the ovary connected to the base of the stigmas.

Just to the right of the central pistillate flower in the image on the left below is a female flower bud about to bloom.  Two pink lobes can be seen peeking out of the beige capsule from which it is erupting.  The image on the right shows a more highly magnified view.

The surfaces of the stigmas of most species have projections that help to collect and retain pollen grains.  Usually they are microscopically small.  Here however, they are large enough that even macro-photographs clearly show  them.

Photomicrographs of these structures reveal their cellular structure and variable pigmentation.

Beneath the castor bean’s female flowers are many male, or staminate ones.  Notice in the image that the anthers and their supporting filaments are initially packed within droplet shaped, light beige containers formed by five tightly fitting sepals.

When the staminate flower blooms, the tips of the 5 sepals separate to reveal masses of shiny, bi-lobed anthers.  Their packing is so tight, that their filaments are not visible.

Views of a second staminate flower can be seen below.

Once the sepal container opens, the stamens and their lengthening filaments soon extend out to form a large, branched, shrub-like structure.

When first exposed, the filaments are white, and the anthers light beige.

Within hours however, exposure to the atmosphere causes both anthers and filaments to turn a greenish-brown colour.  Eventually, after several days, they darken to a light brown colour.

The process whereby an anther releases its pollen is called dehiscing.  In this species the bi-lobed anther opens rather like a woman’s purse, revealing the pollen grains within.

These pollen grains are roughly ellipsoidal in shape.

Approximately a week has gone by since the last photographs were taken.  During that time the ovaries at the base of the pistillate flowers have dramatically increased in size and now have a diameter of about 1.5 centimetres.  Approximately spherical in shape, they are covered with what look like yellowish-green spikes with a sharp spine at each tip.

When the sepal ‘container’ first opens in a pistillate flower, these spikes are much more tightly packed, with most of their spines pointing towards the stigma lobes.

As the sepal ‘container’ begins to disintegrate, the spikes begin to separate.

Photomicrographs showing the tip of a green spike with its sharp spine can be seen below.

In order to expose the terminal flower cluster for the purpose of photographing it, I had to remove several leaves.  As soon as this was done, tiny leaves near the base of the plant began to grow at an amazing rate.

Leaves grow from the intersection points of stems.  At first, tiny ‘bumps’ appear at the intersection.

A short time later a ‘bump’ becomes a short stalk with a tiny folded leaflet encased by thin protective bracts.

Notice in the above images that a bamboo-like ring exists at each branch point along the stem.  A closer view of one of these rings can be seen below.

New leaves are lighter green and very glossy.  They have variable numbers of deeply indented lobes.

Older leaves are much duller, and this allows their complex vein pattern to be more easily seen.

The oldest leaves on the plant are greyish-green.  The prominent longitudinal vein can be seen below.

Leaves viewed from below display their intricate three-dimensional vein pattern. 

Notice in the image that follows, the two light green structures at the base of a leaf.  These are believed to be glands which exude secretions attractive to ants.  If a browsing herbivore happens upon the plant, the biting or stinging ants may discourage ingestion.

 The undersides of slightly older leaves are flatter than younger ones, and they are lighter in colour.

Notice the saw-blade-like edge of the tip of one of the leaves.  If you compare this image with the ones above, you will see that there is considerable diversity in edge decoration in the plant.

At the limits of macro-photography, the cellular structure of leaves barely becomes visible.

Much higher magnification photomicrographs reveal the tiny red spots that are present on the surface of a leaf’s primary vein.

The underside of a leaf is dotted with many oval stomata, and their associated guard cells.  These structures control the entry and exit of gases into and out of the leaf’s interior.

The castor bean plant produces huge quantities of dust-like pollen.  Each grain is ellipsoidal in shape.

Over a week has passed, and the terminal flowering cluster has transformed into a cluster of swelling, spiny ovaries.  Notice that the red pistillate flowers’ stigmas have begun to disintegrate, and fall away.

Later still, these ovaries have grown to almost 2.5 centimetres in diameter.  Unfortunately, at this point, I had to consign the plant to the garbage, as I was leaving on a holiday.

Castor oil makes up about half of a castor bean’s mass.  This oil is valued for industrial as well as medicinal purposes.  Most is used in producing lubricants for fine machinery, automobile engines, and liquid products like paints, inks and dyes.

Ricin, on the other hand, is probably best known for its involvement in the 1978 murder of a Bulgarian dissident named Georgi Markov, who worked for the BBC in London.  The actual deed was done by placing a 1 mm ricin containing pellet in the tip of a modified umbrella, and stabbing him in the leg with it!

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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