The distinctive appearance of cacti has made them popular as houseplants, and where conditions are favourable, ornamental landscaping plants.  Since most are native to extremely hot, dry regions of the Americas, they have developed many unique strategies to enhance their chances of survival under these inhospitable conditions.  Cacti are succulents (succus ‘juicy’ – lentos ‘leaf’), and store water in their stems.  Since most species have no leaves in the normal sense, these having evolved into their characteristic spines, their stems have become photosynthetic, as evidenced by their green colouration.  In order to minimize water loss from the stem’s 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.

While searching for a subject for a Micscape article at my local greenhouse, I came upon the cactus shown below.  Since this plant was in the process of blooming, it seemed like an ideal choice.  Approximately 15 centimetres in height, its flowers had diameters of a little over a centimetre.  The plant was unlabelled, and since cacti are notoriously difficult to identify, I will simply describe it as a typical example.


The main plant is roughly barrel shaped, and near its base are rings of much smaller, almost spherical ‘offsets’.


At the tip of this main plant are several flowers in full bloom.  Each is radially symmetrical, and contains both male and female reproductive structures.  Its petals are shiny, and the lighter highlights are visible in the image that follows.


The images below show many stages in the blooming process.  Later we will see these stages in more detail.




As can be seen below, the surface of the cactus is covered with a variety of spines.  Each group of spines grows from a bump on the surface called a tubercle.  Each tubercle bears an areole, a patch of tissue from which spines are propagated.  (In the plant world, areoles are found only in cacti.)  These spines protect the plant from predators, and they also help to channel dew, condensed from cool night air, down the stem to the plant’s roots.  If you look closely at the following images, you will see that there are two different sets of spines arising from each tubercle.  First, two red-brown spines project from the tubercle’s centre forming a ‘V’ shape above its surface.  These two spines are referred to as the ‘central group’.  Closer to the plant’s surface is the ‘radial group’.  These more flexible white spines grow radially around the circumference of the areole’s apex.  The white colour of these radial spines is thought to partially protect the plant’s surface from the sun’s ultra-violet radiation.




A still closer view shows the tip of an areole, and both its central and radial groups of spines.  Notice at the point of connection of the two red-brown central spines that there is a very small patch of bright white fibrous material.  (In some cactus species this patch is much larger in size, and contributes much more to the ultra-violet filtration affect mentioned above.)


Each cactus areole is formed of two sections, an upper and a lower.  From the upper section, either a bud or a side shoot may form.  The lower section is where the spines are found.  In the image below, a bud has projected from an areole’s upper section.  As it increases in size it must push aside the flexible radial spines blocking its path.


Two unusually shaped buds with depressed tips can be seen in the images that follow.


Images showing typically shaped buds growing up through the maze of spines on the plant’s surface can be seen below.


Notice in this image that the bright red petals of the bud are surrounded, and protected by a ring of light pink, rounded sepals or bracts.


Some of the enlarging buds have a bright red colour (first image), while others are much lighter pink (remaining Images).




Closer views of a bud about to bloom reveal its strikingly beautiful structure and colouration.


As buds begin to open, the prominent, bright yellow pistils that project from their bases become visible.



While taking photomicrographs of the light coloured base of one of the sepals surrounding a bud, I noticed movement through the microscope.  Closer inspection revealed the presence of an extremely tiny sucking insect on the sepal’s outer surface.


Notice the overlapping rings of petals that form the flower’s corolla.  Each petal has a deep pink longitudinal strip along its centre line.


In the images that follow, note the many pale yellow male stamens that cluster at the base of a flower’s female pistil.




These reproductive structures can be seen more clearly in the image below.  Notice also the texture of the flower’s petals.


The photomicrographs that follow show the rectangular cells that create the petal’s structure.



Photomicrographs showing the less colourful outer sections of a petal reveal similarly shaped cells.


Each male stamen is composed of a white, hair-like supporting filament, and a yellow, pollen encrusted anther.


The first two images below reveal roughly spherical pollen grains on an anther’s surface.  The third image shows similar pollen grains clinging to the surface of the filament. Notice that each pollen grain appears to be bisected by a shallow groove.



A sequence of images showing a flower’s five-lobbed stigma, taken with increasing magnification, can be seen below.  The observer is close enough in the last image to be able to distinguish the small lobes that cover its surface.



Photomicrographs of a stigma follow which show these lobes even more clearly.  The lobes increase the stigma’s surface area, and thus improve its ability to capture and retain pollen grains deposited by wind, insects or birds.  Many of these pollen grains are clearly visible in the images.



Although most cacti are considered to be ornamental plants, there are some that have commercial uses.  The fruit of the prickly pear and Hylocereus (Dragon fruit) are edible, and many farmers in suitable regions grow such plants as crops.  The Opuntia cactus is used as the host for cochineal bugs in the cochineal dye industry in Central America.  Although the cactus studied in this article has no such important uses, it can be valued highly simply for its colourful appearance!



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.