Primary and secondary growth
The discussion of secondary growth is essentially the discussion of timber. Timber is the secondary xylem of one of the 30 000 species of tree. It is a low density, polymeric fibre composite. The arrangement of the plant cells and the components of the plant cell wall determine its macroscopic properties. Plant cells differ from animal cells in several important ways: they possess a large vacuole and a thick cell wall (which may have secondary thickening). Many useful plant cells are dead, and devoid of cytoplasm (xylem), others are barely alive (phloem sieve cells). In animals, cells are formed in a great many places, but in plants, new cells are formed at discrete meristems. Plants cells are interconnected cytoplasmically by plasmodesmata, meaning that a plant is, in effect, a gigantic coenocyte, with a continuous cytoplasm (the 'symplast').
Meristems are areas in a plant containing pluripotent cells. They come in several varieties: apical meristems are found at the tip of shoots and roots; axilliary meristems are buds at nodes, which will develop into apical meristems in time. Cambial meristems are those found within the stem, particularly those associated with the secondary growth of vascular bundles.
The apical meristem is found at the tip of a shoot (and root), where it forms new leaves (or roots). It also 'spins out' vascular bundles behind itself as it grows upwards.
The tunica is the epidermis of the meristem (the lighter layer above) and forms the dermal system. The corpus forms the vascular and cortical tissues (the darker ground of the diagram). Some plants, so called 'graft-hybrids', or chimaerae, have genetically different tunicae and corpora. Sometimes the tunica is only slightly different (as in the chlorophyll-free tunica (or vice versa of many variegated plants); sometimes it is derived from an entirely different species, as in +Laburnocytisus adamii, which is formed by grafting of buds (and subsequent cell rearrangement) of Laburnum (golden rain), and Cytisus (broom). After division of cells in the meristem, the cells then expand by turgor pressure pushing against weaker areas of the cell wall. The vacuoles then grow during differentiation into mature, adult cells.
Later in development, the plant cell may develop a thickened secondary cell wall, which is formed within the confines of the old primary wall. It is multilayered (usually with three layers, S1, S2 and S3), and can become heavily lignified. Cells with only a think primary cell wall are generally termed 'parenchyma' (in the broadest sense, this also includes tissues such as phloem and aerenchyma). Those with an unlignified, but thickened cell wall are termed collenchyma, and those with a thick, lignified secondary cell wall (generally dead) are termed sclerenchyma (this includes xylem in the broadest sense).
Vascular bundles are composed of four main tissues: xylem conducts water to leaves; the vascular cambium between xylem and phloem can produce new cells; phloem conducts sugars from sources to sinks such as growing leaves, roots, storage parenchyma in the trunk and specialised storage organs; and sclerenchyma fibres protect the bundle from mechanical damage.
In dicot and gymnosperm trees, the trunk is formed by secondary growth. Different strategies are used by monocot palms, dragon trees and bananas. The trunk acts as a medium for storage of sugars, conduction of water, and support of the canopy. Primary growth is the growth and differentiation of cells produced by the apical meristem. It forms primary xylem and phloem in discrete vascular bundles.
The outermost layer of a plant stem is the epidermis. Immediately below
this is a layer of cells termed the cortex, in which the vascular
bundles of xylem, phloem, cambium and fibres are embedded. The centre
of the stem is termed the pith, and in many plants, is composed of
lightly packed cells, or even a large air space.
Secondary growth occurs if the vascular cambium continues to produce cells. Cambia in different bundles link up to form a (nearly) complete circle. The ring then makes new cells, xylem to the inside, and phloem to the outside. Where the cambia don't quite meet, the pith/cortex form the beginnings of medullary rays. After a few rounds of this, the vascular bundles form a neat ring. Note that the oldest xylem is right at the centre of the trunk, whilst the oldest phloem is just inside the bark. Generally only the a couple of year's worth of phloem will actually be in use: older phloem is obliterated as it is crushed against the inside of the bark. Many trees produce different amounts or sizes of xylem in different seasons. This forms annual rings within the secondary xylem, spawning a whole science of dendrochronology.
Water must also be conducted radially in the trunk, and this is achieved by medullary rays, which run from the centre to the edge of a trunk. They start within the annual ring in which they were first formed: some are created at the beginning of secondary growth, and extra ones are formed in the cambium as the trunk becomes thicker. They are usually composed of tracheids and parenchyma.
The secondary xylem of a trunk may be divided into heartwood and sapwood. Heartwood consists of old, dead xylem no longer used for water conduction; phenolic extractives reduce fungal attack, and the heartwood provides mechanical support to the canopy. The outer layer of sapwood is still alive, and used mostly for conduction and storage. The durability of timber types is in the order live sapwood > heartwood > dead sapwood. This is because live sapwood can mount an 'immune' response to invading fungi. This is why it is the heartwood of old tree that goes rotten. However, once killed and dried, heartwood is far more durable than sapwood, because sapwood contains a lot of easily metabolised sugars, and nowhere near as many toxic phenolic extractives as heartwood.
We can cut wood in three important ways. Note the various ways in which the medullary rays are cut in the different diagrams:
Transverse (cut across trunk).
Longitudinal tangential (skim off bark).
Longitudinal radial (cut down centre).
The outermost layer of a tree is composed of bark. Bark itself is composed of two tissues: an innermost layer of live phloem, and an outer layer of periderm (the bark 'proper'), which has an outermost layer of waterproofing cork (phellum) which protects the wood to some degree from insects, etc. The cork has its own cambium (phellogen) between the phloem and cork layer. Only the outermost layer of a tree is alive (essentially only the phellogen, phloem, cambium, and maturing xylem of the current years growth). Consequently, the majority of the trunk does not require gaseous exchange. The bark is punctuated by lenticels, a sort of giant stoma, which allows the thin outermost living layers of the trunk to 'breathe'.
Wood (and its cut and dried form, timber) is essentially just the secondary xylem of the plant. It is a low density fibre composite formed from fibres (cells), which themselves are formed mostly from polymeric cellulose strands, embedded in a lignin matrix. Fibres (cells) are glued together by a lignified middle lamella. There are two broad categories of wood:
Softwoods are the timber of gymnosperms (larches, firs, pines and spruces). The xylem of softwoods contains tracheids, resin ducts and parenchyma.
Tracheids are the main conductive cells in softwoods. These are longitudinal tracheids, but there are also transverse tracheids (which form most of the medullary rays). Tracheids are formed from a single hollow (dead) spindle-shaped cell, which communicates with neighbouring cells by pits pairs.
In living cells, simple pits are holes in the secondary wall that allow cells to communicate via plasmodesmata. Pits between tracheids have no plasmodesmata (the cell are dead and have no cytoplasm), but they do contain a ball of lignified tissue (the torus) 'stitched' into the pit by fine fibres (the margo), and a thickened border that stretches from around the edge of the pit to nearly occlude the hole containing the torus. These bordered pits acts as 'pressure valves': if the pressure in an element becomes too high, it will force the torus trapped between the two rings of thickened tissue into the hole, blocking it. Pits between tracheids and parenchyma cells in the medullary rays are termed window pits, and allow water to enter the living storage parenchyma.
Gymnosperms often have ducts that conduct sticky resin in the wood, which act like a scab if the trunk is damaged. Amber is a fossilised form of this resin. Like most trees, softwoods show annual rings of growth. In softwoods, these are formed by different sizes of tracheid: the earlywood, which is produced in spring, when water is abundant, has wider tracheids than the latewood, which is produced in summer and later in the year.
In contrast to softwoods, hardwoods are the timber of dicotyledenous angiosperms. Monocots do not produce true wood (they have lost the ability, since they appear to have evolved from water-lily-like herbaceous plants with no need for wood). Bananas are giant herbs (the 'trunk' is made of leaf bases), and the 'trunks' of true palms are produced by primary growth: the stem thickens not from within, but by growing thicker each year - the bottom of a palm would be a rather unstable inverted cone shape if it were not for roots that pull the base into the ground as it does this. This explains why palm trunks are the same diameter all the way up: true secondary growth trunks are thicker at the bottom, and taper towards the top. Dragon trees have 'rediscovered' a form of secondary growth, but it is not organised in the same way as in dicots.
True hardwoods are the wood of trees such as oak, ash, beech, teak and mahogany. It is composed of vessels, (few) tracheids and (lots of) parenchyma.
Vessels are wide tubes composed of several stacked vessel element cells. The thickening patterns of these dead, hollow cells can be characteristic (helical, pitted, etc). The ends of hardwood vessels may be perforate (pure holes), or have perforation plates, which help stop cavitations from spreading from vessel to vessel.
Vessels do not run from top to bottom of the plant. They form networks, making the system redundant, so that if one vessel fails, they others can work around it. The same goes for tracheid networks.
Fibres are very thick walled sclerenchyma cells. They provide strength to hardwoods, whose wood is generally rather 'holier' than softwoods.
The top and bottom of medullary rays (in both hard and softwoods) is composed of radial parenchyma. This is much more extensive in hardwoods (the rays are 'multiseriate', with several layers of parenchyma). The rays are also wider - softwood rays are usually only one cell wide. Hardwoods often also have axial parenchyma, which forms arcs running parallel with the annual rings. This storage parenchyma is used to store sugars and water during the winter months, particularly in (largely deciduous) hardwoods.
Hardwoods may be broadly classified into two groups. The first are termed ring-porous, and include species such as oak and ash. The vessels in these sorts of wood are much wider and more numerous in earlywood (put on in spring) than latewood (put on in summer, and later in the year). They have clear annual rings.
Ash
Diffuse porous hardwoods, on the other hand, such as beech and birch show little variation in the quality and quantity of vessels in diffuse porous woods through the year, so the annual rings are much less obvious.
Beech
The typical composition of wood is shown in the following table:
|
Component |
% w/w approx. |
State |
Made of |
Functions |
|---|---|---|---|---|
|
Cellulose |
50 |
Crystalline |
Glucose
|
Microfibre |
|
Hemicellulose & pectin |
20 |
Semi-crystalline |
Galactose Mannose Xylose
|
Matrix |
|
Lignin |
25 |
Amorphous |
Phenyl- propane
|
Matrix |
|
Extractives |
5 |
Monomeric |
Terpenes Phenolics
|
Toxicity |
Cellulose is a crystalline polymer of glucose. Strong crosslinking between strands forms microfibrils containing c. 100 cellulose molecules.
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Hemicellulose and pectin are small polymers of galactose, sugar-acids, xylose and mannose, which link betweens microfibrils. They are generally semi-crystalline.
Lignin is a massive polymer formed by oxidative crosslinking of phenylpropane alcohols (sinapyl, coumaryl, coniferyl). The molecule shown below is based on a rather old paper, and is probably incorrect: the structure seems to be far less random than was once thought.
Phenolic extractives include tannins, and lignin monomers, which make the wood unpalatable for wood destroying insects, and also give heartwood its colour and smell.
Test yourself
- How do softwoods and hardwoods differ?
- Wood is strong in tension and compression. Account for this by its structure.
Bibliography
- Raven, P. H., Evert, R. F. and Eichhorn, S. E. (1999). Biology of plants. 6th edition. W. H. Freeman, New York. 647-671. "Secondary growth in stems".
- Taiz, L. and Zeiger, E. (2006). Plant Physiology. 4th edition. Sinauer Associates Incorporated, Sunderland, Massachusetts. 377-415. "Growth and development".
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