A close-up view of the Scots Pine

Who leaves the pine-tree, leaves his friend,
Unnerves his strength, invites his end.

Ralph Waldo Emerson, from "Woodnotes"

The pine tree seems to listen, the fir tree to wait: and both without
impatience: - they give no thought to the little people beneath them
devoured by their impatience and their curiosity.

Friedrich Nietzsche

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, immature needles.

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.

Photographic Equipment

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.

Further Information

Scots Pine
http://en.wikipedia.org/wiki/Scots_Pine

Plant Reproduction - Pine
http://sps.k12.ar.us/massengale/plant_reproduction.htm

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.

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