A Close-up View of a

Barrel Cactus from Argentina

Gymnocalycium schickendantzii

by Brian Johnston   (Canada)

Since most cacti grow in hot, dry regions, they have developed many unique strategies to enhance their chances for survival under inhospitable conditions.  Cacti are succulents (succus “juicy” - lentos “leaf”) and store water in their stems.  The most basic of these survival adaptations is the plant’s shape.  In order to minimize water loss from the surface, the ideal shape is a sphere which has the lowest surface area for a given volume.  Most cacti are reasonably close to this ideal, with spherical, barrel-like or candle-like forms commonly being observed.  A second important survival strategy is the development of an intimidating defense shield against grazers, formed by an array of spines.

As can be seen in the image above, and the one that follows, the surface of the cactus is covered with groups of spines.  Each group grows from a bump on the surface called a tubercle.  Each tubercle bears an areole, a patch of tissue from which spines are propagated.  (Areoles are found only in cacti.)  These spines protect the plant from predators, but they also help channel water condensed from cool night air down the stem to the plant’s roots.

Examination of the two images below reveals that the number of spines associated with an areole is remarkably variable.

Many cacti display two groups of spines at each areole, a central group, and a radial group.  In the case of this species, it is difficult to distinguish between the two.  It may be that all seven spines associated with the “perfect” areole seen below are members of the radial group.  Alternatively, the two pointing up, and the one pointing down may form the radial group, while the two pairs seen on the left and right may form the central group.

The image that follows shows a closer view of one of the plant’s vertical ribs between two tubercles.  Notice that the two areoles have very different spine configurations.

Higher magnifications reveal surface details.  In order to prevent water stored within the body of the plant from evaporating, its surface is often covered with a waxy coating to prevent transpiration.

Although cacti normally grow in areas with very little rainfall, it does rain occasionally.  Certain atmospheric conditions also result in the condensation of water vapour as dew or fog.  When liquid water is present on the surface, the dense mat of fibrous threads that is present at the top of each areole helps to trap the liquid, and transfer it into the body of the cactus.

The fibrous threads, and the bases of spines can be seen clearly in the high magnification macro-photographs below.  Note once again that this areole has still another spine configuration.  Variability seems to be particularly high in this species!

It’s not only spines that grow from an areole; below you can see the first stage of bud formation.  Buds sometimes emerge from the uppermost sections of areoles.

Two views of a bud erupting from the mat of fibrous hairs can be seen below.

In many cactus species, the bud-stage flower-head is positioned closer to the tubercle.  In this species, it is held aloft by an unusually long “stalk”.

Three views of the cactus follow that show the position of the developing bud.  The photographs show the plant’s remarkable defensive shield formed by its many spines.  It would take a brave grazer indeed to attempt a bite out of this tasty specimen!

Protective bracts or sepals (modified leaves), arranged in an overlapping pattern, protect both the stalk, and the bud itself.

If the cactus receives full sun at this period, the bracts begin to splay outward, revealing the flower’s white petals. 

By about 11:00 am on a sunny day, the flower opens to the degree shown below.  If the day becomes overcast, the flower will close again until the next sunny day.

Looking down into the partially open flower reveals its overlapping layers of white petals, and many bright yellow, pollen covered anthers with their fragile supporting filaments.

By 1:00 pm on a sunny day, the flower has almost completely opened.

A different angle of view shows the flower’s stamens.

Gymnocalycium schickendantzii has a very large number of stamens consisting of fine white filaments supporting yellow anthers.  Deeper into the flower the out-of-focus ring of stigma lobes is visible.  In order for an insect to obtain the flower’s nectar, (stored beneath the stigma lobes), it must pass, not through a ring of fire, but through a ring of pollen encrusted anthers.  The pollen that adheres to the insect can then be transferred to this flower’s stigma (resulting in self-pollination), or later, to another plant’s flower (resulting in the more beneficial cross-pollination).

The removal of many of a flower’s petals allows us to see how the stamens overhang the central stigma lobes.

By removing most of the stamens, it becomes possible to see these lobes more clearly.  In the cup-shaped bottom of the flower, a copious amount of pollen has accumulated.  Any insect crawling around in this area could not help but pick up pollen grains on its appendages.

It’s easy to see where all of the pollen in the flower’s base originated from.

At the magnification shown below, it is possible to resolve individual pollen grains clinging not only to each anther, but also to each filament.

So many pollen grains coat each anther that its surface is completely obscured.

Photomicrographs of an anther reveal the spherical, to slightly ellipsoidal, shape of each pollen grain.

Several grains adhering to a filament can be seen below.

Higher magnification photomicrographs show that pollen grains have a rough surface, and possess several longitudinal grooves.  (The image on the left uses dark-ground illumination, while that on the right utilizes phase-contrast illumination.)

Here again we see that pollen grains are so abundant, that no surface details are visible on a stigma lobe’s surface.

Under the microscope however, it’s possible to find a small region free of pollen on the lobe’s surface where the typical hair-like protuberances found on most stigmas are visible.  These increase the lobe’s surface area, and help it to acquire, and retain pollen grains.

Unlike most other flowering plants, cacti have no leaves.  Instead, their grossly enlarged, fleshy green stems carry out photosynthesis.  The interior of a stem is either spongy, or hollow, depending on the species.  Their unusual shape, and multitude of vicious spines have made cacti a favourite of plant collectors.

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.

An 8 megapixel Canon 20D 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 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.

Microscopy UK Front Page
Micscape Magazine
Article Library

© Microscopy UK or their contributors.

Published in the October 2010 edition of Micscape.
Please report any Web problems or offer general comments to the Micscape Editor.
Micscape is the on-line monthly magazine of the Microscopy UK web
site at Microscopy-UK  

© Onview.net Ltd, Microscopy-UK, and all contributors 1995 onwards. All rights reserved. Main site is at www.microscopy-uk.org.uk with full mirror at www.microscopy-uk.net .