Above: a computer model of a pine tree, generated using the TomTree add-on to Pov-Ray. Many pines have dark, almost black bark,
but this model was based on the Scot's Pine with its bright reddish bark. Pine trees belong to a group of trees called but they are not
flowers in the true biological sense and conifers belong to the gymnosperms (lit. 'naked seeds') a group of non-flowering plants distinct
from the flowering plants or angiosperms. Most conifers are trees, though some like juniper are woody shrubs. Also within the conifers
are yew trees (Taxus, sometimes not considered to be a conifer as its female structures are hardly 'cones'), larches, firs, cypresses,
cedars, redwoods and sequoias. The only conifers native to the British Isles are the yew tree, the juniper bush and the Scot's Pine. This
article will focus primarily on the Scot's Pine (Pinus sylvestris) which forms part of the ancient Caledonian Forest fragments in Scotland,
along with birch and aspen trees. The cones are modified shoots with compressed growth (such that all the whorls of 'leaves' are
produced close together on a short stem) and the leaves are modified to small brown scales that bear and shield the reproductive
organs. In female cones each scale leaf bears a pair of ovules. These ovules are naked (not enclosed within an ovary as in flowering
plants) and so the seeds they form are 'naked' (not enclosed within fruit). Within each ovule is the nucellus tissue which encloses the
developing embryo and is open to the outside through a tiny pore, the micropyle, which allows sperm entry (the sperm being carried
within the pollen grains). Pine trees have a characteristic resinous organic odour due to the synthesis of turpentine containing aromatic
pinenes. The leaves, stem and branches contain abundant resin canals that carry the resin. This resin is protective, it will seep from
wounds and quickly seal them, trapping and killing any parasites in the wound, and discouraging animals from eating the leaves. These
resins and oils give pine woods a distinctive and very pleasant aroma.
In the British Isles, Pinus sylvestris only grows wild in Scotland, where the mean summer temperature falls below 26.7 degrees C. It is a
light-demanding tree which prefers light, non-calcareous and well-drained soils (e.g. heaths where there is thin peat over coarse sand),
glacial and fluvioglacial deposits and humus iron podsols lacking earthworms. Pine leaf-litter has an acidic pH of 4.0-4.2 which excludes
many competitors from growing beneath their canopies. They form the dominant (climax) species on nutrient-poor soils.
Growth is more-or-less monopodial (with a single trunk formed from a single shoot) with whorls of branches radiating away at a near
horizontal angle (especially lower down the trunk). The leaves consist of pairs of tough needles enclosed about their bases by a
common sheath. On the upper stem the bark is thin, papery, orange or red-brown and lower down the stem the bark is thick, dark
brown and fissured (thinner and grey-brown in older trees). The tree typically reaches 15-30 metres in height (up to 46 metres
recorded) with a girth of 3.3 m at a height of 1.3 m and so is quite a large tree. It is frost resistant.
The roots are typically shallow and form a distinctive root-plate with no distinct tap-root (the tap-root is the main central root which
descends quite deeply in many trees). Symbiotic fungi clothe some of the roots, forming ectomycorrhiza. These mycorrhizae
('fungus-roots') supply the tree with additional soil minerals in return for carbon fixed by photosynthesis. The rate of photosynthesis per
gram weight of needle, is low, but continues for at least part of the winter. Roots and shoots resume growth in early Spring with male
cones developing early followed by the female cones in May/June.
Scot's Pine has a maximum lifespan of 395 years, but averages about 200 years. At six years of age the tree sets seed and the male
cones produce pollen at 10-15 years. Cone production peeks at 20-55 years of age. The trees are monoecious (having separate male
and female cones on the same plant) but occasionally single-sex (with only either male or female cones on the same plant). Shade
branches and older branches tend to be male and sun branches female. The pollen is wind-dispersed and each pollen grain has a pair of
air-sacs that were once thought to aid in buoyancy, allowing the pollen to carry further on the wind. However, experiments have failed to
demonstrate this function of the air sacs. The female cones are open (anthesis) and receptive to pollen in June. The pollen is caught on
a drop of fluid on the ovule micropyle (the fluid is secreted by the ovule) and then drawn toward the nucellus (the inner part of the ovule in
which the embryo develops) as the fluid is absorbed. The air sacs may function to orient the pollen correctly when it is suspended in this
drop of fluid. Cross-pollinated cones are the more fertile. The embryo has 3-10 cotyledons and so is polycotyledonous (compare to the
two main groups of flowering plants - the monocotyledons and the dicotyledons). Pollen tube growth occurs in the Summer, but is very
slow and does not complete in a single season but arrests over the Winter and resumes the following Spring. Development of the female
gametophyte (egg-producing structure that is haploid) actually follows pollination and the slow pollen tube growth allows time for this, with
the pollen tube entering dormancy until the female gametophyte is ready. Pollination is not double, as it is in angiosperms, the two sperm
do enter the egg. At fertilisation the pollen grain delivers the two sperm, the tube nucleus and the sterile cell into the cytoplasm of the egg,
but only the larger leading sperm fertilises the egg, the other sperm along with the pollen tube nucleus and sterile cell degenerate.
(Double and multiple fertilisation are rare in gymnosperms). First year female cones are green and tightly closed, but second year cones
will open in dry conditions for seed dispersal and eventually fall from the tree.
Like most conifers the Pine is evergreen, keeping its leaves all year round with needles shedding individually when they senesce. A few
conifers are deciduous, shedding their leaves in autumn, such as the Larch and the Dawn Redwood.
Photomicrographs of ovuliferous
(ovule-bearing) scales of a female
cone of Pinus sylvestris. Top left and
top centre: showing internal resin
traversing the scale leaf, notice the
spiral wall-thickenings in the xylem
tracheids crossing the middle of the
picture at top right. These are called
Bottom left: a section through an
ovule, showing the nucellus tissue
enclosing the megaspore mother cell
which develops, after meiosis, into the
haploid gametophyte containing the
Structure of the female cone
A stoma with guard cells and subsidiary
cells and substomatal air-space.
The photosynthetic mesophyll cells make-up the bulk of pine needles and are not
differentiated into separate palisade and spongy mesophylls. The endodermis has
either a Casparian strip (when young) or suberin in its radial (tangential) end-walls
creating a barrier to movement of water in the apoplast in a manner similar to the
roots of many plants.
Pine needles have certain xerophytic characteristics (xerophytes are plants adapted
to tolerate a lack of water). They have a small surface area to volume ratio, as they
are needle-like. The stomata, which occur on all sides in vertical rows, are sunken
and the sunken guard cells are over-arched by subsidiary epidermal cells. The
waxy, waterproof cuticle is thick. Beneath the epidermis are one or more layers of
strengthening fibrous sclerenchyma cells forming a hypodermis. These strengthen
the needle and prevent it collapsing when dehydrated or frost-damaged. The
epidermis also tends to be lignified, strengthening and waterproofing it. Lignified
cells with thickened walls are also often found between the two vascular bundles in
the central vein and beneath the phloem. Such characteristics in leaves are usually
variable depending upon the age of the leaf, being more abundant in older tougher
leaves, and how exposed it is. These adaptations allow conifers to tolerate cold
conditions in which liquid water may be scarce.
Above: a cross-section through the bases of a pair of Scot's Pine needles enclosed in their common sheath. Half of one of the needles is
shown. Each needle has a single central vein containing two vessels. The xylem of conifers consists of tracheids and lacks vessel members.
The phloem tubes lack the one-to-one relationship with a companion cell derived from the same progenitor cell. Instead certain nearby
parenchyma cells are modified into albuminous cells which presumably load/unload the phloem in a manner similar to companion cells. The
vessels are enclosed in tranfusion tissue consisting of parenchyma cells and tracheids. Note the resin canals.
The thick cuticle
Structure of the bark
Structure of the wood
Structure of the root
Above: sections through the bark of a
small sapling stem (of 5 mm diameter)
of Pinus sylvestris. The white-red
amorphous layers are the outermost
layers of cork cells (phellem).
Immediately inside this is a layer of
cells, the phellogen, which divide to
produce new cork cells on the outside
and parenchyma cells (phelloderm,
stained blue-green) on the inside. Just
inside this layer of bark are a few cell
layers of large parenchymatous cells,
the cortex, which can also contain
Phelloderm parenchyma cells
Cortex cells (top of picture) inner to the
phelloderm (lower part of picture).
These photomicrographs show regions of a longitudinal section through a young
Scot's Pine stem. The fibrous phloem is a thin layer beneath the bark (and bark).
In Pinus sylvestris, shown here, the phloem consists of neat alternating strands
of parenchyma cellsThese photomicrographs show regions of a longitudinal
section through a young rows of three different cell types: (the slightly (the
slightly elongated darkly staining red-brown cells), elongated darkly staining
red-brown cells), strands of sclerenchyma fibresstrands of (the fine fibrous
bluish cells) for strength and the (the fine fibrous bluish cells) for strength and
the made of made of phloem phloem sieve cells joined together by walls
perforated by porous sieve areas (the large, elongated, empty-looking and
whitish cells). These rows generally run vertically through the stem, and often in
conifers only the very newest layer is fully functional and conducting, and are
crossed by radial plates of parenchyma cells forming rays. The rays are
visible here in cross-section as groups of rounded parenchyma cells (though they
may be elongated in and out of the page). The diameters of the sieve elements is
typically less than 0.8 micrometres in conifers, about ten-times narrower than in
plant parts. This sap contains not only water, but essential minerals absorbed from
the soil by the roots (this mineral uptake is enhanced with the help of mycorrhiza).
The wood of conifers differs from that of angiosperm trees in that the conducting
elements are tracheids only, no wide vessel elements forming well-defined vessels
occur. (See tree wood for details). Notice the prominent circular features, these are
pores with valves (bordered pit-pairs with margo and torus) that connect adjacent
tracheids together and allow xylem sap to pass from one tracheid to the next.
Angiosperm vessels also have pores with valves, but the structure is different.
Tracheids are typically almost ten-times shorter than vessels and have diameters
ranging from 10 to 65 micrometres, compared to 17 to 500 micrometres for
angiosperm vessels. Narrower lumens increase resistance to water flow, however,
this is offset in tracheids by the pore-valve arrangement which is much more
permeable than in angiosperm vessels, such that tracheids have a similar
conductance per unit area to vessels (individual vessels only have larger
conductivity because they are wider, but this does not translate to increased
conductivity in the whole stem). The wood of conifers, containing only tracheids as
conducting (tracheary) elements, is called homoxylous wood. The wood of
angiosperms, with both tracheids and vessels, is called heteroxylous wood. Only the
outermost and newest layers of wood in a stem are conducting, the rest no longer
function in conductance and are converted into dumping grounds for waste materials.
(hardwoods). This lower density is an important factor in construction, allowing conifers to reach great heights (the tallest trees
are conifers and the tallest of these are the Coast Redwoods) by reducing self-loading (the stems have less of their own weight
the severe damage that lightning can do to redwood trees. Another factor is the loading just described when dynamic loading
due to high winds is taken into account. The taper of many tall conifer stems, and the buttresses (wide flanges at the base of
taller a tree, the higher a column of water in the xylem must be lifted up against gravity and against frictional resistance with the
walls of the vessels or tracheids and so the greater the suction pressure required to raise the sap (generated by transpiration).
If these tensions become too great then the water column snaps and cavitation occurs (an air bubble forms at the point of
water column breakage in a process called cavitation, which blocks the flow of water either temporarily or sometimes
irreversibly). Cavitation is more likely for wider vessels and also in low temperatures in which water becomes more brittle.
Conifers have narrow tracheids and so cavitation in cold conditions is less of a problem for them than in angiosperm trees and
conifers are well-adapted to cold conditions. Studies have shown that to reduce the risks of cavitation, the tracheids become
narrower toward the top of the tree (where tensions in the water column are greatest due to the pull of gravity on the water, that
is on the weight of water pulling down) and the valves become more efficient (the margo reduces in size whilst the torus remains
unchanged - see the detailed structure of xylem for details) since one of the key functions of the valves is to close when a
tracheid cavitates, preventing the air from seeding cavitation in neighbouring tracheids. However, these changes to the valves
also reduce the ease with which water can pass through the pores. The problem is, that narrowing the tracheid lumens and
increasing the resistance across the pores make sit harder and harder for sap to move through the tracheids. Calculations
predict that at around 110 m height the resistance becomes so great that sap ascent stops! This is in quite good agreement
with the tallest height of trees, which is around 130 to 140 m.
The bulk of the wood is made-up of elongated cells called tracheids that carry sap from the roots up the stem and also
give the stem strength. These sections are lengthwise (longitudinal) and so you can think of these pictures as representing
the wood in a small branch flowing along the elongated tracheids from left to right. Alternatively, rotate the images so that
the grain is vertical and think of the tracheids as conducting the flow vertically up a stem. Notice the flattened circular
structures - these are pores and their valves (seen edge on) connecting adjacent tracheids together and permitting the
sap to flow from one tracheid to another.
The tracheids are 'non-living' at maturity as they contain no protoplasts and no cytoplasm. Crossing the wood are the rays -
plates of living parenchyma cells that move along a radius of the stem, connecting the inner cylinders of tracheids to the
outermost cylinders. The outer cylinders are the newest, as trees continue to grow in annual increments throughout their
lives by forming new conducting elements as the old ones become dysfunctional. The rays carry waste materials into the
The tracheids are 'non-living' at maturity as they contain no protoplasts and no cytoplasm. Crossing the wood are the wood.
The ray cells also store food reserves. Tracheids (ray tracheids) may also accompany the rays in conifers, allowing some
sap to move across the stem more easily (than simply passing sideways from tracheid to tracheid or through the
parenchyma of the rays). In these images the rays can be seen in cross-section as the small roundish green cells that form
strands (rays) parallel to the plane of the page. Also seen in the image at top right and bottom left are axial parenchyma
cells (also stained green) that follow the grain of the wood and so move vertically up and down the stem (i.e. parallel to the
stem axis). Resin canals may also be found inside the rays, carrying resin across the stem, and also in the axial system
carrying resin up and down the trunk. The walls of these resin canals are formed by thin epithelial cells in pines.
An axial parenchyma strand (green
cells) arranged vertically along the
stem (moving from left to right in this
image as the stem is on its side). It
looks as if these may be the epithelia
lining a resin canal.
The older roots undergo secondary growth with the formation of wood and bark in a
manner similar to the older parts of the shoot system 9the trunk and branches). The
primary xylem is the first-formed xylem that occurs in green non-woody shoots and
root surrounded by a number of resin canals. The bulk of the root is wood
(secondary xylem) consisting of tracheids running along the length of the root and
parenchymatous rays running along a radius across the root in cross-section. As for
stems, new rings of wood are deposited annually. This xylem is surrounded by a
cambium, a thin layer of generative cells that produce new xylem inside and new
phloem outside. The phloem forms a cylinder outside the cambium and xylem. (This
means that the oldest xylem is toward the middle of the root and the oldest phloem
toward the outside, as is the case for stems). Outside the phloem is a cylinder of
parenchyma cells called the cortex and around this is the root bark.
Cross-section through root of Pinus
sylvestris. The wood (xylem) is at the
bottom (cells stained pink) traversed
by parenchyma rays (4 visible). Above
the xylem is the phloem (cells stained
green) and at the top of the view are
parenchymatous cortex cells with
various coloured materials stored
Above: a pair of growth rings in the
root xylem. The smaller tracheids were
formed one autumn (late wood) and
the large tracheids to the left of them
were formed the following spring (early
Above: four resin canals around the
central primary xylem. Resin canals (or
resin ducts) are lined by epithelial
cells, which are flattened in pine, which
secrete the conifer's resin (is this true
of the flattened cells in pine trees?).
Above: a close-up view of one of the
large central resin canal;s in the root.
The thin epithelial cells (barely visible)
are surrounded by sclerenchyma cells
with thickened walls (cells stained red)
as happens in the larger canals.
Above: root xylem tracheids in
A small resin canal in the root wood.
The canal is lined by flattened
epithelial cells and connected to a
parenchyma ray by parenchyma cells.
Sclerenchyma is just beginning to form
around this canal.
Above: another resin canal with
sclerenchyma beginning to form
around it. The sclerenchyma
strengthen the canal and probably
prevent it from collapsing under the
hydrostatic pressure of neighbouring
cells as the root grows.
Bibliography / References
Structure of the Phloem (inner bark)