Close-up View of the
by Brian Johnston (Canada)
leaves the pine-tree, leaves his friend,
his strength, invites his end.
Ralph Waldo Emerson, from
pine tree seems to listen, the fir tree to wait: and both without
- they give no thought to the little people beneath them
by their impatience and their curiosity.
Pine trees, (family Pinaceae),
constitute a large genus that contains over one hundred species – about
one third of all of the Conifers. It is believed that early in
the planet’s history, pines made up a significantly greater proportion
of the Earth’s vegetation than they do today. Widely planted as a
decorative plant in lawns and parks, the pines have, in some
locations, become an invasive species.
Pinus sylvestris, the subject
of this article, is commonly called the Scots Pine. (If the truth
be told, in my area, it is referred to by the politically incorrect
name “Scotch Pine”.) In the distant past, it was also called the
Scots Fir. The genus name Pinus
was derived from the Greek pitus
which referred to a pine or fir tree.
My local park contains many Scots Pine trees. One five metre high
example is shown in the image below. In young trees like this
one, the main (or leader) stem elongates rapidly to form the typical
cone-shape. Older trees eventually stop growing up, and the
top-most branches increase in length to form a more cylindrical
outline. When many of the lower branches decay, and fall off, the
tree is left with a flat topped, or parasol-like shape.
A closer view of the tree’s branches reveals the many brown, female
cones, and vertical new growth shoots that populate their end sections.
One of the identifying characteristics of Pinus sylvestris is that its
leaves, called needles, occur in pairs. The two, twisted, ridged,
blue-green needles form a structure called the fascicle. Several growing,
immature stems can also be seen in the images. At first, the
needles are covered by a brown, papery, protective sheath that falls
off as time progresses.
As the protective sheaths are shed, the light green, grooved structure
of the needles is revealed.
The two images that follow show the pointed, brown, bract-like
structures covering the growing aggregations of needles.
Eventually, most of the brown sheathing falls off to reveal the short,
These needles, over time, lengthen to form a new section of stem.
The photomicrographs that follow show the surface of one of the tree’s
needles. The tiny black specks visible in the higher
magnification image on the right are tiny particles of carbon soot
released by the exhaust of thousands of diesel trucks that pass within
a kilometre of the park every day. Unfortunately, these same
particles are collected by the lungs of all of the human inhabitants
who share the city with the pines!
Male Reproductive Structures
Both male and female reproductive structures are found on the same tree
(monoecious), but they have a
very different appearance. Both are referred to as “cones”, or
more properly, strobili.
The familiar woody cone is the seed producing female structure.
Below, you can see the male pollen producing “cone” which is called the
microstrobilus, or pollen cone.
At an early stage of development, before pollen is actually released, a
number of brown, modified leaves, called microsporophylls can be seen
extending out from the central stem. Under each microsporophyll
are many pollen sacs called microsporangia.
The two images that follow show male cones at the stage when they begin
to release pollen. Each of the greenish-yellow structures is a
male (staminate) flower.
These flowers are collected into an egg-shaped catkin. Strangely, at the
catkin’s tip, there is a section of ordinary needles. (In the
images, these needles are still at a very early stage of development.)
Higher magnification images of a section of the catkin reveal the
microsporangia with their dusty coatings of pollen. The
red-brown, pointed microsporophyll at the base of each staminate flower
can be seen in the images.
No plant that I have photographed to this point has produced such a
huge quantity of pollen. The slightest movement of the stem
results in clouds of grains rising into the air. (I was concerned
about the fine particles entering my DSLR, and getting onto the
camera’s sensor, but most of the pollen falls quickly to the tabletop.)
Images of pollen grains can be seen below. The first shows pollen
on the surface of a needle, the second on a microsporangium, and the
third dusted onto a microscope slide.
Higher magnification, using phase-contrast illumination, reveals the
bumpy surface of a pollen grain.
Female Reproductive Structures
The female cone of a pine tree, called a megastrobilus, or ovulate cone, contains deep within
its structure, ovules, which when fertilized by pollen, become
seeds. Two types of scale form the cone: bract scales derived from modified
leaves, and seed scales, or ovuliferous scales, derived from
highly modified branchlets. Two images of very immature, red,
female cones can be seen below.
Later, the female cone becomes green in colour, and its stalk often
bends at a right-angle to the stem from which it grows. At this
early stage, all of the scales are of the bract type. The seed
scales develop much later, after fertilization.
As a female cone matures, the tips of the bract scales become brown in
colour, and the connecting stalk lengthens. Cones are coated with
a very sticky gum, (or resin) which helps to trap pollen grains.
The Latin word for a tree with such a gum is strobus – hence “gum yielding”.
Some cones are egg-shaped, while others are cone-shaped.
Eventually, the cones display a greyish-brown colouration.
The scales of a female cone open temporarily to receive pollen, and
then close during fertilization. They open once again at maturity
in order to allow the escape of seeds. The image on the left,
below, shows a cone with slightly opened scales.
The final images show two views of a female cone in which the bract
scales have opened fully to allow the seeds to fall to the
ground. Note their reddish-brown interiors. It should be
noted that the process of female cone development takes from two to
three years. The greyish-brown cones shown in the last few images
are therefore at least two years old. (The tube from a pollen
grain to an ovule takes a year to grow!)
Centuries ago, in several parts of the world, Pinus sylvestris was over-exploited
for its useful wood, which resulted in its extinction. The tree
has been re-introduced into these locations, including the British
Isles, Denmark, and the Netherlands. This is not surprising,
since its gnarled appearance, and pleasant scent make it a appealing
addition to the landscape.
Most of the photographs in the article were taken with an eight
megapixel Canon 20D DSLR and Canon EF 100 mm f 2.8 Macro lens. In
several images, a Canon 250D achromatic close-up lens was used to
obtain higher magnification. A five megapixel Sony CyberShot
DSC-F 717 was used to take a few of the images.
The photomicrographs were taken with a Leitz SM-Pol microscope (using
dark ground and phase contrast condensers), and the Coolpix 4500.
Plant Reproduction - Pine
A Flower Garden of
A complete graphical index of all
of my flower articles can be found here.
The Colourful World of
A complete graphical index of all
of my crystal articles can be found here.
Microscopy UK or their contributors.
Published in the April
2009 edition of Micscape.
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