A close-up view of the English White Oak Quercus robus cv Fastigiata

Live thy Life,                Summer-rich            All his leaves
Young and old,             Then; and then        Fall'n at length,
Like yon oak,                Autumn-changed    Look, he stands,
Bright in spring,            Soberer-hued           Trunk and bough
Living gold;                Gold again.              Naked strength.

“The Oak” Alfred Lord Tennyson

The mighty Oak is not only strong, but it also has a very long lifespan – up to 500 or 600 years if left undisturbed! White Oak, the subject of this article grows mainly in the Eastern part of North America, and its wood has been used for many years to produce furniture, floors and veneers. During the colonial period it was much prized for ship building. White Oak is sometimes called Stave Oak, because it makes excellent, tight-fitting barrel staves.

Quercus robus is member of the Fagaceae, or Beech family.

Tree parts photographed for this article were obtained from several young trees growing in my local park. Typically, White Oak trees have wide, irregular crowns when they grow with lots of space around them. In a forest situation, the crowns tend to be upright and oval. For some reason, the park trees behave as though they are in a forest situation! Notice in the image on the left, below, that two of the trees are stunted, and one of them has died. For some inexplicable reason, the park maintenance crews tend to store salt at the bases of these two trees for dispersal onto the slippery park road during winter. The result is not surprising! (Note that the unusual pointed shape of these oak trees is due to the fact that they are English White Oak Quercus robus cv ‘Fastigiata')

Leaves are positioned alternately on the plant’s stems, and have from 7 to 10 rounded, finger-like lobes around their perimeter. Each leaf’s upper surface is smooth, and reflected light results in the white highlights that are visible in the left image. Notice the complex vein structure of the tip of one of the leaves (right image).

When a species has both male and female structures growing on the same plant, it is referred to as monoecious (“in the same house”). The two images that follow show the very beginnings of the English White Oak’s reproductive structures. At the base of new growth shoots (left image), red-brown male staminate catkins begin to develop. At new growth axils, (points where the leaves’ stalks meet the stem), red-brown female pistillate catkins begin to develop. (A catkin is a compact cluster of unisexual flowers.)

Below, in the left image, a group of reddish bracts, (modified leaves), protects each developing staminate catkin. Later, these groups of bracts open at the top, and cone-shaped, or oval-shaped male catkins appear.

When the tightly packed, yellow-green, bi-lobed anthers, (male pollen producing organs) first appear, they are so densely packed that their supporting filaments are not visible (left image). As the catkin’s stalk increases in length, the filaments eventually appear (right image).

In the image that follows, the anthers have not yet begun to produce pollen.

Higher magnification views of these immature anthers can be seen below.

Notice the different appearance of the catkin, once pollen production begins.

Low, and high magnification photomicrographs follow that show an anther.

Ironically, Stave Oak pollen grains are barrel-shaped! Several longitudinal grooves mark the surface of each.

Within a day or two, as the anthers and their filaments dry out, some of the male catkins begin to look untidy, and they turn a light brown colour. Other bract enclosures however, have yet to open.

Quercus robus cv ‘Fastigiata' twigs are red-brown, or slightly gray in colour, and tend to be hairless, and rather shiny. Notice the interesting crater-like depressions on the twigs’ surfaces.

Most descriptions of female English White Oak flowers make the point that they are not visible to the naked eye. How true! Last spring, I made a determined attempt to find such a flower on “my” trees. I failed completely. During a five day period in which I was unable to visit the park, the flowers had bloomed, and those not fertilized had fallen from the branches (abscised). A short time later however, the tiny, swollen fruit that eventually transform into recognizable acorns, became noticeable. The two immature acorns shown below are only 3 to 4 mm in diameter.

Each nearly spherical fruit is initially, almost completely enclosed by a cap which is made up of many layers of overlapping greenish bracts (modified leaves). As the darker green fruit grows, it pushes aside the material near the cap’s opening, increasing the opening’s diameter. Notice in the two images, the remnants of the flower’s style projecting from the top of the fruit.

Although the cap continues to grow, the fruit increases in size even more quickly. This results in the original, almost spherical acorn, taking on an ellipsoidal, or egg-shape.

Notice in the image that follows, that several tiny immature acorns have dried up and turned brown. Also note the strangely coloured dark brown fruit in the upper right corner of the image.

Additional images of two of these tiny “dead” acorns can be seen below. The one shown in the image on the left still has its very prominent green style attached.

The many layers of overlapping bracts that constitute the acorn’s cap can be seen clearly in the high magnification macro-image that follows.

Acorns mature approximately four months after fertilization. The acorn’s cap has reached its maximum size early on, but the fruit itself has continued to grow to an amazing degree, forming a long, thin ellipsoidal structure.

Two examples of these late stage acorns can be seen below. At this point the adhesion between cap and fruit is much weakened. Eventually, the fruit drops out of the cap and falls to the ground. (Sometimes squirrels pull the fruit out of the cap before this happens.)

If a sharp knife is used to cut lengthwise through a fruit, it can be seen to consist of a tough, thick, outer cover that protects its high water content interior. (In the image, the fruit is held in position by the wire seen at its base.)

The outer casing, and inner core of the fruit are shown below.

After the ellipsoidal fruit has fallen from the cap, the cap remains attached to the tree for some time, and then it too falls to the ground.

The placenta-like structure that provides the connection between the acorn’s stalk, and its fruit can be seen in the image that follows.

Oak trees grown in the open, like those in my park, may produce acorns as early as 20 years of age. A particular tree may produce very different numbers of acorns in successive years, and good acorn crops occur only every 4 to 10 years. These acorns are usually disseminated by rodents - mainly squirrels and mice. They are a favourite food for many wildlife species.

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. An eight megapixel Sony CyberShot DSC-F 828 equipped with achromatic close-up lenses (Canon 250D, Nikon 6T, and Sony VCL-M3358 used singly, or in combination), was used to take a few of the images. The tree images were taken with a five megapixel Sony DSC-F 717.

The photomicrographs were taken with a Leitz SM-Pol microscope (using a dark ground condenser), and the Coolpix 4500.

Further Information

White Oak
http://www.na.fs.fed.us/spfo/pubs/silvics_manual/Volume_2/quercus/alba.htm

White Oak
http://en.wikipedia.org/wiki/White_oak

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