Close-up View of Several Members of the Cactus
by Brian Johnston (Canada)
most cacti grow in hot dry
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
of spines. More adaptations will be discussed later in the
A First Cactus
As can be seen in the image
and the two that follow, the surface of the cactus is covered in
variety of spines. Each group of spines grows from a bump
surface called a tubercle.
tubercle bears an areole,
a patch of tissue from which spines are propagated.
in cacti.) These spines protect the plant from predators,
they also help channel water condensed from cool night air down
stem to the plant’s roots. If you look closely at the
image, two groups of spines are visible. Three red brown
project out from the surface at the centre of the areole.
these spines is longer, and has a hook shaped tip. The two
remaining shorter spines are perfectly straight. These
spines are referred to as the “central
group”. Closer to the plant’s surface is
the “radial group”.
white spines grow radially around the circumference of the
apex. The white colour of the radial spines is thought to
partially protect the plant’s surface from the sun’s
Under the microscope, the
tip of the long central group spine is seen to be unpleasantly
sharp. The higher magnification image at right shows the
structure of the same spine. Tiny, irregularly shaped
protuberances cover the surface.
Notice in the two images that
follow, the many rings of white radial group spines that cover
The photomicrograph below
red tips of several of the white radial spines. These too
extremely sharp, although it is unlikely that they contribute to
defense against grazers.
The bases of the radial spines
shown at their points of attachment to the areole. The
the right shows the bulbous bases of the central group
Careful inspection reveals four central spines instead of the
expected. Surprise! Although there is always just
hooked spine, there may be two (usually), or three (unusually)
central spines connected to each areole.
Occasionally a large tissue
appears on the surface of the stem. (The plant
had two such growths.) The arrangement of areoles on the
of the bump appears to be less regular than on the rest of the
A photomicrograph of one of
straight central spines can be seen below right. These
spines form the lethal looking barrier preventing contact with
surface of the plant (left image).
Higher magnification images of
tip and body of one of the spines can be seen below.
The bud stage of an areole can
develop into a flower if conditions are right. In the
below, the tip of a flower bud can be seen pushing up through
mat of fibrous threads which covers the areole.
These fine white fibers are
concentrated both around areoles, and in the depressions between
areoles, but only near the top
cactus. With the UV light from the noon-day sun
most dangerous, this is where the cactus needs the most
Photomicrographs of these
fibers using dark-ground (first two), and phase contrast (last)
illumination can be seen below.
Several days after the flower
has first pushed up through the matted fibers, the pink petals
surrounding greenish-yellow bracts
(modified leaves) become visible. The image on the right
atypical formation. Both images show central groups
three spines and others containing four spines.
Later the petals elongate and
The flowers are particularly
striking at this stage.
Eventually the reproductive
structures at the centre of the flower become visible.
is pale pink, and has a dark pinkish-red band up the middle.
Four white stigmas are
in the middle of the flower.
Under the microscope, the stigma (female pollen
organ) appears to be composed of a number of spherical white
cells. The supporting red style
is also visible. Several pollen grains cling to the stigma
A lower magnification
photomicrograph shows some of the stigma (white) – style (red)
lobes that compose the pistil. All surfaces are
Higher magnification of a
of style reveals the shape of the pollen grains – ellipsoidal
A much higher magnification
phase-contrast photomicrograph shows the rough surface of a
The mature flower has both
and petals, but they are indistinguishable. At this
you can see a large number of stamens overhanging the central
Notice in the close-up below,
some of the petals have a pointed hook at their tips.
The photograph below shows the
of anthers (male pollen
producing organs) and supporting filaments.
Although it appears as though self-fertilization is likely, this
the case. Most cacti are protandrous,
meaning that the anthers shed their pollen before the stigma’s
is receptive. Also, cacti are usually self-sterile.
therefore left for visiting insects to transfer the pollen from
different plant, thus ensuring cross-pollination.
Photomicrographs of the
supporting filament can be seen below.
A higher magnification view of
several filaments shows their cellular structure, and the many
grains adhering to their surfaces.
The retailing of cactus plants
an interesting phenomenon. What strange species has small
flowers, and a single large yellow flower at its top? In
no such species exists! The greenhouse managers have
“we” the buyers want our cacti to be colourful all of the time,
just when they are in bloom. They therefore glue the dead
of an entirely different plant
(- not a cactus!!!) to
of each cactus they sell in order to increase its visual
The mind boggles!
There is no denying that the
glue-on is quite attractive, but it looks very out-of place on
of a cactus. (The stuck on flower is usually a strawflower
(Helichrysum). Many distributers fail to mention that this
value-added flower doesn’t belong. I must give credit to
Depot as the attached label on the containers states
“cactus/strawflower”. (All four cacti photographed in the
had these glue-ons attached. Fortunately, careful use of
and a very sharp blade removed the flowers with no damage to the
Out of curiosity, I
the cellular structure of a dried strawflower petal.
A Second Cactus
Species (Mammillaria matudae)
If you look carefully at the
shown below, it is evident that the shape is similar to the
one. There are differences, however. There are no
white fibers present around the top areoles as there were in the
earlier plant. Although the radial group spines are
central group consists of a single spine instead of the three or
spines seen earlier.
Details of the spine
can be seen more easily in the images that follow. The
central spine in this case is considerably shorter than any of
central spines in the first species.
Notice that in this species
tips of both sepals and petals are more pointed, and less
in the earlier one. Note the large number of shriveled
petals that remain from earlier flowers.
The blooming flowers, with
long tubular base extending to the centre of the areole, are
In this species, the stigmas
green) extend out farther than the many yellow anthers.
be an additional adaptation to decrease the possibility of
A Third Cactus
The last two cacti to be
were not flowering, and so it is the spike arrangement of the
that is of interest. Here, the plant’s overall shape is
to the first two, although the length is greater.
Careful study shows that most
areoles have six central, identical, slightly curved spikes,
few have only five spikes. Around and between the areoles
is a fine dusting of white fiber-like specks.
The view on the right below
the six central spikes and the many radial spikes of a single
A Final Cactus
Species (perhaps a species of Pachycereus or Stenocereus?)
This species is markedly
from the first three studied. Here, the tubercle bumps
vertical parallel rows called ribs.
longest spikes are about three centimetres in length and are
viciously sharp and extremely strong.
Each areole possesses a single
woody central spike, and eight or nine woody radial spikes that
slightly up (and away) from the rib.
When I first got the idea to
this article about cacti, I certainly did not expect to be
the plants. While taking the macro-photographs, it slowly
on me why these unique plants are so sought after and
Their strange forms, intricate detail, and strikingly beautiful
can easily become addictive!
All of the macro-photographs
taken with an eight megapixel Canon 20D DSLR equipped with a
100 mm f 2.8 Macro lens which focuses to 1:1. A Canon 250D
achromatic close-up lens was used to obtain higher
The photomicrographs were
with a Leitz SM-Pol microscope (using dark ground and phase
condensers), and the Coolpix 4500.
Microscopy UK or their contributors.
Published in the
January 2007 edition of Micscape.
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