Close-up View of a
Barrel Cactus from Argentina
by Brian Johnston (Canada)
Since most cacti grow in hot,
regions, they have developed many unique strategies to enhance
chances for survival under inhospitable conditions. Cacti
“juicy” - lentos
“leaf”) and store water in
their stems. The most basic of these survival adaptations
plant’s shape. In order to minimize water loss from the
the ideal shape is a sphere which has the lowest surface area
given volume. Most cacti are reasonably close to this
spherical, barrel-like or candle-like forms commonly being
observed. A second important survival strategy is the
of an intimidating defense shield against grazers, formed by an
As can be seen in the image
and the one that follows, the surface of the cactus is covered
groups of spines. Each group grows from a bump on the
called a tubercle.
tubercle bears an areole,
patch of tissue from which spines are propagated. (Areoles
found only in
These spines protect the plant from predators, but they also
channel water condensed from cool night air down the stem to the
Examination of the two images
reveals that the number of spines associated with an areole is
Many cacti display two groups
spines at each areole, a central
and a radial
the case of this species, it is difficult to distinguish between
two. It may be that all seven spines associated with the
“perfect” areole seen below are members of the radial
Alternatively, the two pointing up, and the one pointing down
the radial group, while the two pairs seen on the left and right
form the central group.
The image that follows shows a
closer view of one of the plant’s vertical ribs between two
Notice that the two areoles have very different spine
Higher magnifications reveal
surface details. In order to prevent water stored within
of the plant from evaporating, its surface is often covered with
coating to prevent transpiration.
Although cacti normally grow
areas with very little rainfall, it does rain
Certain atmospheric conditions also result in the condensation
vapour as dew or fog. When liquid water is present on the
surface, the dense mat of fibrous threads that is present at the
each areole helps to trap the liquid, and transfer it into the
The fibrous threads, and the
of spines can be seen clearly in the high magnification
macro-photographs below. Note once again that this areole
still another spine configuration. Variability seems to be
particularly high in this species!
It’s not only spines that grow
an areole; below you can see the first stage of bud
Buds sometimes emerge from the uppermost sections of areoles.
Two views of a bud erupting
the mat of fibrous hairs can be seen below.
In many cactus species, the
bud-stage flower-head is positioned closer to the
this species, it is held aloft by an unusually long “stalk”.
Three views of the cactus
that show the position of the developing bud. The
show the plant’s remarkable defensive shield formed by its many
spines. It would take a brave grazer indeed to attempt a
of this tasty specimen!
Protective bracts or sepals
(modified leaves), arranged in an overlapping pattern, protect
stalk, and the bud itself.
If the cactus receives full
this period, the bracts begin to splay outward, revealing the
By about 11:00 am on a sunny
the flower opens to the degree shown below. If the day
overcast, the flower will close again until the next sunny day.
Looking down into the
open flower reveals its overlapping layers of white petals, and
bright yellow, pollen covered anthers with their fragile
By 1:00 pm on a sunny day, the
flower has almost completely opened.
A different angle of view
Gymnocalycium schickendantzii has a very large
number of stamens consisting of fine
white filaments supporting yellow anthers. Deeper into the
the out-of-focus ring of stigma lobes is visible. In order
insect to obtain the flower’s nectar, (stored beneath the stigma
lobes), it must pass, not through a ring of fire, but through a
pollen encrusted anthers. The pollen that adheres to the
can then be transferred to this flower’s stigma (resulting in self-pollination), or later,
another plant’s flower (resulting in the more beneficial cross-pollination).
The removal of many of a
petals allows us to see how the stamens overhang the central
By removing most of the
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
help but pick up pollen grains on its appendages.
It’s easy to see where all of
pollen in the flower’s base originated from.
At the magnification shown
it is possible to resolve individual pollen grains clinging not
each anther, but also to each filament.
So many pollen grains coat
anther that its surface is completely obscured.
Photomicrographs of an anther
reveal the spherical, to slightly ellipsoidal, shape of each
Several grains adhering to a
filament can be seen below.
photomicrographs show that pollen grains have a rough surface,
possess several longitudinal grooves. (The image on the
dark-ground illumination, while that on the right utilizes
Here again we see that pollen
grains are so abundant, that no surface details are visible on a
Under the microscope however,
possible to find a small region free of pollen on the lobe’s
where the typical hair-like protuberances found on most stigmas
visible. These increase the lobe’s surface area, and help
acquire, and retain pollen grains.
Unlike most other flowering
cacti have no leaves. Instead, their grossly enlarged,
green stems carry out photosynthesis. The interior of a
either spongy, or hollow, depending on the species. Their
shape, and multitude of vicious spines have made cacti a
The low magnification, (to
macro-photographs were taken using a 13 megapixel Canon 5D full
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
lens, the MP-E 65 mm 1:2.8, was used to take the remainder of
The photomicrographs were
using a Leitz SM-Pol microscope (using dark ground and
condensers), and the Coolpix 4500.
A Flower Garden of
A complete graphical index of
of my flower articles can be found here.
The Colourful World
A complete graphical index of
of my crystal articles can be found here.
Microscopy UK or their contributors.
Published in the
October 2010 edition of Micscape.
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