Close-up View of a Member of the
(Family: Cucurbitaceae Genus: Cucurbita)
by Brian Johnston (Canada)
Walking in the steady drizzle on a
windy, cold October day, I certainly didn’t expect to come upon
the buds and flowers of this solitary plant. It was growing quite
successfully between the tracks of two railway lines, one seldom used,
and one deserted for decades. How it came to be in that location
is a mystery. Why it was blooming so late in the season, with
only a couple of weeks before the first frost, is also a mystery.
Although it had no chance of producing a mature vine crop, the plant
was growing vibrantly as if to contest its approaching doom.
Members of the pumpkin, squash,
gourd and cucumber family are often referred to as ‘cucurbits’. This name is
derived from their botanical genus classification of Cucurbita.
These were some of the first plants to be cultivated in Europe, Asia
and North America. The family is distinct morphologically and
biochemically from other families. It is therefore called monophyletic.
Most of the cucurbits have separate
male and female flowers growing on a long vine which is supported by
spiraling tendrils attached to adjacent objects (which may be other
plants). Having flowers of both sexes on the same plant is
referred to as monoecious.
Female flowers and buds, such as the one shown above, have a clearly
visible ovary which eventually develops into the cucumber, squash,
etc. This makes them easy to identify. Male flowers and
buds have no such miniature fruit attached to their base, and look like
the two shown below. Notice the many extremely fine hairs that
cover both bud and stems.
The first flowers to open on the
long stem are almost always male. Bees are alerted to the
location of the vine by the pollen on these male flowers. By the
time the first female flowers open, the bees have an established route
to the plant, and they transfer pollen from male to female flower
resulting in fertilization.
The buds of male flowers are quite
beautiful, with their yellow-orange furled tops and ring of five green sepals (modified leaves) at the
base. Notice in the second image, the green lines that radiate
out from the stem. There is one aligned with each sepal, and a
fainter one aligned with each primary vein of a petal.
If one of the buds is cut in half
longitudinally, the main veins of the petals are clearly visible in the
The ‘inside’ view reveals a dense
mass of folded orange tissue with many very fine hairs covering all
The bud feels strangely spongy when
handled. The microscopic views which follow explain why.
Between the surfaces of the petals, there can be seen to be many
tubular structures which bend and collapse as the bud is compressed.
The photomicrograph which follows
shows the granular cellular material of the unopened bud’s petals.
At higher magnification, the base
of one of the tubes seen earlier seems to be composed of a series of
Again at high magnification, there
are many reddish chain-like structures which seem to bridge the gaps
between the layers of tissue.
As can be seen below, the green
sepals just above the bulbous base of the bud, are covered by many fine
hairs. (The water droplets seen in these and other images are the
result of the photography of the specimens obtained during a fine
rain. Images were obtained immediately because of the tendency of
the specimens to wilt, even when placed in water.)
In my opinion, the most photogenic
stage in the development of this plant, is the bud form. Some of
my favourite images follow.
Eventually, the bud opens to reveal
the bright yellow-orange flower, which unfortunately lasts for only a
day or less. The flower consists of five triangular petals which
are fused together at the base. (The ring of petals is called the
corolla.) At the tip of
each petal there is a tiny, green, needle-like spike which is about 10
mm long (right hand image).
The two closer views that follow
reveal the stamen at the center of the flower. The cone-shaped
yellow anther is connected to the rest of the flower by a sturdy white
filament. The upper surface of the petals is covered by a
multitude of fine hairs, many of which have clumps of pollen stuck to
If the petals are removed, the stamen, consisting of an anther, (male pollen producing
organ) and supporting filament,
can be seen more clearly. (There is just a hint in the first
image that there may be more than one stamen, but if so, the multiple
stamens have fused to form one structure.) If you look closely,
individual spherical pollen grains are just visible in the images.
The pollen grains are clearly
visible in the image on the right below. On the left you can see
the hairs which help distribute these grains to investigating bees.
Photomicrographs of pollen taken
using dark-ground illumination reveal that the surface of each grain is
covered with many tiny bumps, and a few larger diameter, higher
The use of a phase contrast
condenser with a non-phase objective provides a different view of a
Two ‘side-view’ photographs of a
partially open flower follow.
Notice how the tiny hairs on a
sepal collect a variety of different diameter rain droplets.
The outer edges of the petals of a
flower are always curled inwards. (The left image was taken from
the outside of a flower, and the right from the inside.) As with
all of the surfaces of the plant, the petals are intensely hairy.
A much higher magnification shows
some of these tiny hairs.
As was mentioned earlier, the
female flowers begin development after the male flowers. In the
case of my plant, the female buds failed to open; they were killed by
the first hard frost of the season. Two images of a very early
stage bud can be seen below. The spherical ovary is beneath the
developing petals and is therefore referred to as inferior.
At a later stage the developing
petals take on a characteristic orange tint. Although the bud
lacks the colour of the open flower, it more than makes up for this
deficiency with its striking architectural form.
When the petals of a female flower
are cut away, three yellow, unusually shaped stigma lobes, (female pollen
accepting organs), are visible. In the second image, notice the
ring of tiny green hairy protuberances that surround the base of the
petals. The last image reveals the furry nature of the three
The ovary beneath the female flower
is almost perfectly spherical, and is held by a thick ribbed stalk
called a peduncle. One
of the main characteristics of the cucurbit family is how the fruit is
formed. The ovary (fruit) is fused with the outer protective
covering of the ovary (receptacle
tissue) to form a hard rind. Botanically, this type of fruit is
called a pepo. In
the right-hand photograph, the fleshy white tissue that will develop
into the inner portion of the fruit, (if the flower is successfully
pollinated), can be seen. Surrounding this is the dark green
protective tissue that will form the rind.
The front and back surface of a
leaf can be seen below. The leaves are positioned alternately on
the stem and are palmately shaped with five irregular lobes. The
back surface of each leaf is prominently veined.
Close-ups of the front surface of a
leaf are shown below. The photomicrograph on the right shows the
hairs at the edges of the chlorophyll filled blocks.
The back of the leaf has a series
of protruding light green veins. Under the microscope, some of
the hairs covering a vein are visible.
A much higher magnification reveals
the cellular structure of the vein, and the fact that some of the
protuberances are not hair-like, but round in shape (glandular).
The edge of a leaf can be seen
below. The image at right shows the variation that exists in the
size of the attached hairs.
Occasionally a pollen grain, or
clump of grains becomes attached to one of the hairs on the leaf’s
The base of a hair can be seen to
be segmented in the image below.
Immature, small diameter stalks,
(left image), are almost round in cross-section, and have pale yellow
striations. Mature stalks, (right image), are darker green, five
sided, and hollow.
The heavy plant is supported by
many coiled tendrils,
(modified shoots), like the one below. They grow quickly, and
entangle any other plant or object in the vicinity.
Under the microscope, the tendrils
can be seen to have hairs growing from their surfaces. The occasional
adhering pollen grain is visible as well.
What is it?
Is the solitary cucurbit that I
found on that rainy October day a cucumber, gourd, muskmelon, pumpkin,
squash or watermelon? I’m not certain! Since the frost
killed the plant before the fruit had a chance to fully mature,
identification is difficult. The largest fruit, (seen above), was
3.25 cm in diameter. Perhaps the final answer is not
important. This article is about an entire botanical family, not
just one member!
An eight megapixel Sony CyberShot
DSC-F 828 equipped with achromatic close-up lenses, (Nikon 5T, 6T, Sony
VCL-M3358, and shorter focal length achromat used singly, or in
combination), was used to take all of the macro images. The lenses
screw into the 58 mm filter threads of the camera lens. Still higher
magnifications were obtained by using a macro coupler (which has two
male threads) to attach a reversed
50 mm focal length f 1.4 Olympus SLR lens to the F 828. The
photomicrographs were taken with a Leitz SM-Pol microscope (using a
dark-ground condenser), and a Nikon Coolpix 4500 camera.
Pumpkins, squashes and gourds -
The Cucurbit Network
Microscopy UK or their contributors.
Published in the
October 2006 edition of Micscape.
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