BASIC TYPES OF CELLS AND TISSUES
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BASIC TYPES OF CELLS AND TISSUES
EXTERNAL ORGANIZATION OF STEMS
INTERNAL ORGANIZATION OF STEMS
STEM GROWTH AND DIFFERENTIATION
SECONDARY GROWTH
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BASIC TYPES OF CELLS AND TISSUESThe protoplasm of a cell is surrounded by a primary cell wall which is laid down as the cell begins to grow. It is thin and composed of cellulose, hemicellulose and pectic substances. Once growth has ceased a secondary cell wall may form inside the primary cell wall. Secondary cell walls tend to be thicker and contain substances such as lignin, suberin and chitin. Differentiated cells can be divided into three basic types: parenchyma, collenchyma and scelenchyma:
The most basic type of differentiated cell. The most common cell type found throughout the plant and named according to its location, eg. cortex, pith. Sometimes called ground tissue. Features of parenchyma are: thin walled; living protoplasts; vacuoles; varying shapes - may be rounded or elongate; intercellular spaces present. Parenchyma cells are important functionally: photosynthesis; respiration; production of secretions; storage of food reserves. Hence parenchyma cells are important to man as they form most of the bits of plants which we eat.
Parenchyma cells may be specialized in some way, often given slightly different names, eg. chlorenchyma - contain many chloroplasts, aerenchyma - these cells are stellate (star-shaped) with large intercellular spaces allowing movement of air between the cells. They are found in plants such as rushes which live in waterlogged soil - they are in the centre of the stem and allow the movement of air down to the roots.
Features: living protoplasts; unevenly thickened walls; no intercellular spaces; plastic support.
Collenchyma cells also have living protoplasts, but the cell walls tend to be thickened with cellulose. This thickening generally occurs unevenly with more material deposited in cell corners than in other areas. The water content of the protoplast is high and the cells extensible, so they are usually elongate in shape. Their function is usually one of providing plastic support so they tend to be tightly packed together and be found in peripheral positions in stems and petioles. In stems, for example, they may form a complete ring towards the outside, or they may form discrete strands round outside - these can be seen quite clearly in celery where it is collenchyma that form those stringy ridges. It does seem that the amount of thickening may be affected by the degree of mechanical shaking of a plant - so grasses next to motorways - blown by wind - have much thicker walls to collenchyma cells. Tissue consisting of collenchyma cells constitutes a living, flexible tissue which possesses considerable tensile strength. More expensive to make than parenchyma - therefore only found where is it needed.
These cells typically have thick lignified secondary walls. At maturity they generally contain no living protoplast.
Sub-divisions of schlerenchyma cells: mechanical (non-conducting) and conducting -
a) Sclereids
Sometimes called stone cells because they have very hard walls. Usually isodiametric (rounded). Function - Inflexible strength and resistant protection e.g. seed coats - coconut shell, cherry stone. - They are found throughout the plant - form bands wherever plant needs either within other tissues or forming layers of hard material. They grow very rapidly initially and can push between other cells to become star shaped, as it matures. The thick secondary wall is laid down and the cell content degenerate - leaving a hole or "lumen". Individual cells (idioblasts) are often found in leaves - thought that this may make the tissues inedible to predators.
b) Fibres
Long narrow cells generally tapered at the ends. In transverse section, small cells with thick walls (lignified) and extremely narrow lumen. Functions = elasticity, flexible strength. These parts of plants are also commercially important being used to make twine or rope from sisal, while fibres with more cellulose thickening and less lignin provide softer fibres are e.g. used to make linen out of flax (Linum). Wood of most flowering plants contains many fibres - support and elasticity, bark -fibre rich important for resisting insects, fungi, other pests.
Found in vascular tissue
EXTERNAL ORGANIZATION OF STEMSThe aerial part of a plant must do three things:
1. gain height and generally maintain stability,
2. produce a canopy, occupying space to capture light - competing with other plants,
3. produce reproductive structures.
Shoot architecture firstly depends on the positioning and behaviour of the meristems - growing points.
Branching: The simplest way to create a branch is to split the meristem in two daughter meristems, initially of equal growth potential. This is usually known as dichotomous branching. 1st = division of meristem then apical bud divides more or less equally. Examples of dichotomy in angiosperms are few and specialised. Lateral or axillary meristems usually follow the same arrangement as leaves, as branches arise from axillary buds at the base of each leaf = axillary branching.
Continuous growth: The branch and the main stem (parent axis) are of the same age. ie as the axillary meristem is formed it immediately continues growing to produce a stem. Continuous growth is seen in many herbaceous plants and tropical trees.
Discontinuous growth: In most temperate trees, the bud meristems have a period of dormancy before producing a branch - often to avoid the harsh season. The lateral branches are therefore usually a year younger than the main shoot and growth could be termed rhythmical or discontinuous.
The form of the branch: Morphology of branching is further complicated by the form of branch expression. In monopodial branching the terminal shoot grows upright (has a permanent/ long-lived terminal meristem) and produces lateral branch meristems by one of the systems already described. Sympodial branching occurs when a lateral branch develops and becomes dominant - taking over from the short-lived terminal shoot. This will itself be superseded by another lateral branch at some point in the future. This leads to a zigzag formation. It may happen because the terminal shoot becomes reproductive, developing a flower or inflorescence or is aborted (eg. in temperate trees at the end of a season). This relates to
Positioning of reproductive structures: If terminal on axis - determinate growth. If lateral reproductive structures - indeterminate growth. A whole plant may be determinate or indeterminate (creeper) An indeterminate plant may itself have some branches determinate and some indeterminate.
Responses to gravity: Often, if you look at a tree (easier to see in trees), you find distinct tiers or layers of branching. This is a result of a clear differentiation between orthotropic and plagiotropic shoots -vertical and horizontal. An orthotropic shoot is negatively geotropic , so it moves away from gravity, it is upright and radially symmetrical (leaves are often spirally arranged). Clearly adapted to gain height (to get to the light). A plagiotropic shoot is horizontally arranged and will be flattened. Adapted to occupy light space (to harvest the light). Plants may consist of just one sort of branch eg creeping plants - all plagiotropic, most show a combination of branch types, and of course, these two descriptions are the two ends of the spectrum - many intermediate exist.
Long or short shoots: The final factor which has a major influence on plant architecture relates to the length of the shoots. Short shoots have short internodes and limited growth. Long shoots have longer internodes. Long and short shoots may occur on the same plant. The length of the internode has a functional aspect. Long shoots tend to have an exploratory capacity, extending the framework of the plant into new territory. for example, in silver birch (Betula pubescens) - very long internodes, slender branches. It is a pioneering species, developing fast, taking up lots of room, so it can take advantage of openings in an established canopy or colonize new areas. Short shoots will function to maximize light capture from the new space, thus, produce the leaves and flowers
Effects of environment: Mechanical forces, water stress, temperature all affect plant growth and form.
The arrangement of leaves on the stem is known as Phyllotaxis. The relative positions of leaves on a plant must affect the interception of light, and more importantly, the position of a leaf usually fixes the position of its subtended axial bud. Thus, the phyllotaxis of a plant can play a considerable role in determining the branching pattern of a plant. In addition, the positioning of leaves on a stem remains constant within a species and sometimes within a family. It is therefore a useful guide to plant identification.
There are 6 leaf patterns possible:
Alternate - one leaf per node
Opposite -two leaves per node
Decussate - Leaves located in 4 rows
Whorled - three or more leaves per node
Spiral - leaves not aligned with their nearest neighbours
Distichous - leaves located in two rows only
The arrangement of leaves is directly determined by the length of time between the production of the leaf primordia in the apical meristem.
Spiral formations are an interesting arrangement as the angles between the primordia being produced happen to relate to the Fibonacci series.
This series is obtained by adding the last two numbers together to achieve the next. These are the same numbers which can be used to describe the spiral arrangements of leaves. If we calculate the number of turns around the stem and the number of leaves we pass before a leaf is produced in the same place as the first, we get some interesting results. In some species, we get 2 turns round the stem and 5 leaves produced, in others 3/8, also 5/13, 8/21, 13, 34. All of these give approximations to the Fibonacci angle: 3/8 X 360 = 135 (golden section= 137.5). A function of the need to minimize shading and the packing of the apical meristem, this is one of the few areas of botany that can be described by developmental formulae.
INTERNAL ORGANIZATION OF STEMSA tissue is an aggregation of similar cells forming a unit. Simple tissue is composed of one cell type while complex tissue is made up of 2 or more cell types. Within a plant tissues can be divided into 3 tissue systems types - derma, vascular and ground tissue.
1) Derma is Greek for skin - so that is the tissue which covers the plant - looking at later.
2) Ground tissue - this forms the basic mass of the plant - the cortex or pith for example with parenchyma cells being most common.
3) The vascular system is the conducting system of plants made up of xylem and phloem and sometimes a vascular cambium.
In fact the xylem tissue is a complex tissue and so contains a number of cell types: conducting elements (tracheids + perhaps vessels), xylary fibres and parenchyma. The parenchyma cells also tend to be arranged in vertical or horizontal files - their function being storage of food and water with some short distance transfer of those materials. Phloem tissue may also contain fibres, sclereids and parenchyma.
The surface of a plant is made up of a number of layers, which are of extreme importance. They form the boundary between the plant body and the environment.
A layer of cells, easily be distinguished, known as the epidermis. ( Greek - upon skin.). No epidermis forms over the root cap or shoot meristem, but otherwise the layer is ubiquitous - found everywhere. Cells are usually undifferentiated - usually tabular in shape, fitting tightly to neighbours with no intercellular spaces. In monocots tend to be elongate. Many plants retain their epidermis as long as they live - particularly annuals. In woody plants the epidermis in the stem and roots is replaced by secondary growth which forms bark. Therefore in woody plants the main function of epidermis is protection of delicate organs - leaves. Normally, the only breaks in the epidermis are the stomata (more rarely, hydrathodes - structures adapted for water secretion). The ultrastructure of the cells is undistinguished - the organelles are small and few in no. Cells are virtually colourless and rarely contain starch. Usually the outer (tangential) wall is thickest, and in xerophytes it may be lignified. In some species. may accumulating calcium carbonate and silica (eg grasses).
The layer next to the atmosphere and this is attached to the epidermis by a layer of pectin (or similar). This is also the substance that sticks cells together - forms a glue. In addition, it functions as a cushioning layer, or a biological sponge, to help as the leaf flexes. Cutin is a fatty substance which makes the outer walls of the epidermis cell impermeable to water. The cuticle itself is made up of several different layers. It is non-cellular and covers all the aerial parts of a plant. This may finally be coated with epicuticular wax. This is a layer of wax which is embedded in and sometimes exuded over the surface of the cuticle. Doesn't cover whole plant - usually get more on the upper surface of leaves for example. The surface of the wax can be sculpted into different shapes which affects the function.
1. Trichomes are hairs formed solely from epidermal cells and adapted to a variety of functions. May be single celled as in most plant hairs, or form complex structures such as secretary or absorptive glands - they may persist or be ephemeral. Extreme examples are the hairs of cotton which are rich in cellulose and up to 7 cm long.
2. Emergences are larger structures are derived from the layers underneath the epidermis. They form all sorts of different structures: hairs, scales, glands, thorns.
3. Stomata are the most major modification of the surface a plant -stomata are pores formed between two modified epidermal cells, known as guard cells.
Functions of plant surfaces:
click for more details Two kinds: tracheids and vessel members -
a) TRACHEIDS originate from single cells, and are pointed at both ends. The primary cell wall remains complete and the secondary wall is laid down on the interior surface. The movement of materials is facilitated by thin oblique end walls which tend to overlap. The secondary walls are impermeable to water, composed of cellulose, hemicellulose and lignin and they are laid down in patterns of rings, bands or helices (see below). Tracheids were the first type of sclerenchyma cell to evolve and are found in all the groups of vascular plants. In angiosperms tracheids found mostly in fine veins of leaves but also in wood of some species. In gymnosperms and ferns xylem is composed exclusively of tracheids. The connections between tracheids are through pit pairs i.e. through primary cell wall. Pitting is particularly seen in tracheids and allow the movement of water through walls. Because pits are small relative to the whole surface, they don't let much water through so need to be efficient as possible - but the larger the hole/pore, the less strength is conferred. This is why we see the evolution of bordered pits - the pore is made larger without being weaker.
b) VESSEL MEMBERS/ELEMENTS generally only found in angiosperms. These cells are shorter and larger in diameter and there is no cytoplasm. In fact, the secondary walls are composed of the same material as tracheids, but in this case the primary cell wall also dissolves at maturity in the region of the end walls. Instead of the materials having to pass through pits, they can just pass through holes or perforations. The vessel members therefore line up end to end to from a "vessel" and the end wall of each 'member' becomes a perforation plate'. The evolution of vessels occurred in angiosperms and allows a two-phase conduction system. When water is readily available it can move up the stem in high volume through vessels. In Angiosperms the tracheids tend to be narrower, stronger and more resistant to collapse - they function in dry periods when the water is being transported by great tension.
There are a number of patterns which the secondary wall of both tracheids and vessel elements can adopt and these are known as annular, helical, sclariform, reticulate or pitted. The 'protoxylem' in elongating tissues has an annular or helical structure which allows the element to extend as the surrounding living cells grow and expand. Eventually, protoxylem becomes so extended that it tears, at which point the majority of water conduction will take place through the 'metaxylem' formed later in non-elongating tissues. These vessel elements have secondary walls which are far more extensive and are better able to resist collapse - but only occur where primary elongation of the organ has ceased. These tend to have scalariform (ladder) or reticulate (net-like) patterning. Thickening in different patterns leaves different amounts of primary wall free for water movement and gives different degrees of strength.
Vessels must transfer water from parenchyma, tracheids or other vessels. A vessel = a stack of vessel elements; may be very long but not entire length of plant so some lateral transfer is necessary.
Only primary walls , no secondary thickening, no lignin. Must remain alive to conduct nuclei degenerate, nuclear control by associated cells. As with the xylem, there are two types of conducting cell: sieve cells and sieve tubes.
a) SIEVE CELLS are found in the seedless vascular plants and in gymnosperms. They are long, narrow cells and when they are mature they still contain a living protoplast. Shaped like a tracheid. Unlike xylem there is no lignified secondary wall. They just have primary walls with many enlarged plasmadesmata (dia = > 1 micrometre) called sieve pores. Plasmadesmata occur in groups (primary pit fields) and sieve pores also occur clustered together in groups called sieve areas in their lateral walls, and sometimes in their terminal walls, connecting adjacent cells. Associated with albuminous cells.
b) SIEVE TUBES occur in angiosperms and have a more specialized structure. Sieve tubes are longitudinal files of cells made up of individual sieve tube members. At the ends of the cells, sieve plates are formed by a number of pores which have a special connecting strands passing through. Although the living protoplast is retained the cells lack a nucleus. Instead they are associated with a 'companion cell'. Plasmadesmata extend between the sieve tube and companion and control is by the companion cell. Companion cells (and albuminous cells) are usually smaller than the conducting cell, have a prominent nucleus, dense cytoplasm many ribosomes. Companion cells involved in loading and unloading sugars in and out of sieve tube members.
In dicots - arranged in a ring surrounding the pith (parenchyma). In monocots - "scattered" in the stem - not random - complex arrangement. All vascular bundles collateral = contain both xylem and phloem running parallel to each other. Xylem of vascular bundle is primary xylem - contains conductive tracheids and/or vessel elements, usually a large proportion of xylem parenchyma and even mechanical schlerenchyma in the form of xylem fibres. Phloem of vascular bundle is the primary phloem contains sieve elements associated cells (albuminous or companion) plus phloem parenchyma and mechanical schlerenchyma usually as phloem fibres although phloem sclereids also occur.
Vascular tissue and surrounding sheath/ ring of parenchymatous endodermis. The structure of the stele within the stem often relates to the evolution of the particular species. There are two main types.
1) PROTOSTELE found in the shoots of the more primitive vascular plants such as lycopods. They are also found in the roots of more specialized vascular plants. Here the xylem forms a solid mass in the centre with no pith and the phloem surrounds this basic core.
2) SIPHONOSTELE where the xylem is still interior to the phloem, but it is not a solid mass, but has parenchyma -(known as pith) at the centre. Siphonosteles evolved later and only occur regularly in the shoots of ferns and seed plants.
STEM GROWTH AND DIFFERENTIATIONThere are two means by which an organism can organize its growth
Animals use diffuse growth ----- whole organism grows in all parts simultaneously.
Plants exhibit localized growth ----in which distinct regions are responsible for cell division and cell enlargement (these regions are the meristems)
This localized type of growth has 2 main consequences:
1) A plant can have some fully functional, fully developed organs and tissues whilst other parts of it are still young and growing. Because the meristems are sources of undifferentiated, immature cells, plants show developmental plasticity, so the plant can respond selectively, should the environment change. This is possible because all cells have a complete genome -(a set of genes) and differentiation occurs by differential activation and repression of contain specific genes. Therefore the pattern of differentiation of young meristematic cells can be easily altered to give a new pattern of growth.
2) Another consequence of localized growth is that growth can be indeterminate. As long as conditions are favourable many plants can continue to grow indefinitely. However, growth is usually limited by the environment. Plants may be susceptible to the wind, drought or disease. It seems that few trees die of old age - except for the few species. which die immediately after they flower e.g. bamboo, agave. Therefore under exceptionally favorable conditions particularly large individuals result - not the case with animals.
We can consider the system of growth as a meristem with initial cell divisions where cells are mitotically young and the meristematic region where there are additional divisions and growth. The meristem contains cells which are traditionally said to be undifferentiated. This isn't entirely precise as they are in fact highly specialized for their role of cell division, specialized in terms of their structure, ultrastructure and physiology. It is the cells they produce that are, at first, undifferentiated. The main activities of the meristem are two-fold:
1) to establish patterns for the tissues which will develop later.
2) to provide a reservoir of cells that are genetically sound and have gone through very few mitotic cycles.
The meristematic region contains the meristem and also a region where cell division and cell expansion occurs. The region is difficult to define as it extends from the meristem back to where cell division and elongation are occurring "reasonably rapidly". As with many biological systems we see a gradation - from the growing meristematic region to mature regions. Every time a cell divides the process of mitosis exposes the DNA to the possibility of damage through increased sensitivity to thermal damage and to mispairing of bases. In animals - with diffuse growth, the adult body size can be achieved without any cell line undergoing very many replication - division cycles (A cell line is one cell and its descendants.) Even so, the initiators of the sex cells are set aside while the embryo contains only a few hundred cells - reducing the possibility of errors by replication. If plants were to only grow by their tiny meristems so that each plant cell was a result of the division of a meristem cell then each meristem cell would have to undergo millions of replications - and the genome would soon be filled with errors. To avoid this we find an adaptation which is referred to as the "multistep meristem". Here the important cells of the meristem which have to last for the lifetime of the plant, only divide occasionally. They pass cells to the meristematic region and it is then these new daughter cells which will divide possibly many times, elongate and be differentiated into tissues. The cells which are dividing in this meristematic region only need to last for a short period - usually not more than 1 growing season, before being replaced by an additional division in the original meristerm. Any errors occurring are now not likely to be important. Therefore it is because the meristem itself does not grow and divide rapidly, that the meristematic region can be supplied with genetically healthy cells and it is this that allows plants to grow indefinitely e.g. the Bristle-cone pines with an age of at least 11,000 years!
Meristems can be classified in a number of different ways, perhaps the most useful is based on the position of the meristem relative to the cells it produces.
1) Apical meristems. Located at apex of the organ that they produce (shoot, root, gland).
2) Basal meristems. Located at the base of the organ - pushes organ out from itself e.g. prickly pear spine, grass leaf.
3) Intercalary meristems. Located between their derivatives (i.e. cells produced) - contribute to cells on both sides.
4) Lateral meristems. Located on the periphery of an organ - the 2 main ones are the vascular and cork cambium. (Vascular cambium is also an intercalary meristem.)
5) Axillary meristems. Are the apical meristems of buds located in the axils of leaves.
Shoot and root apical meristems are present in the embryo so are termed primary meristems. These permit the first of two aspects of growth known as primary growth which is essentially growth in the length of the plant.
A close consideration of the apical meristems is worthwhile because they don't only determine the anatomies of the stem and root but they are also involved in responsible for the initiation and control of appendages in shoots. In turn, the type and position of appendages determines the form or morphology of a shoot. To understand plant morphology, we must understand the nature of the shoot apical meristem.
This is obviously present in all plants, but its form varies with the group of plants being considered. In fact there are 3 distinct types associated with different taxonomic groups:
This group shows a structurally simple apical meristem: a large, unmistakable cell shaped like an inverted pyramid. The base at the plant surface and 3 or 4 faces pointing downwards. The apical cell divides in an orderly fashion cutting off a narrow first cell from each source in turn anticlinally The daughter cells may then undergo successive divisions periclinally which result in a pattern being established. This can be seen in structurally simple mosses, the leaves are clearly seen to be formed in 3 rows, each being derived initially from 1 segment of the 3-sided apical cell. In many ferns there is a slight variation - instead of 1 apical cell there is a small cluster. These function in the same way, but they avoid cutting off daughter cells along faces that are in contact with other initials.
The apical meristems of seed plants are rather more complex and in gymnosperms there is a range of structures which appear to relate to evolutionary trends. It is however possible to give a general description. There is a layer of apical initials which give rise to both a surface layer of cells (mantle layer) and a group of cells below called the central mother cells these occupy the central mother cell zone, larger and vacuolate irregular in shape. The central mother cells give rise to the rib meristem (in the centre of the shoot) and the peripheral zone. It is the cells in the peripheral zone which are most actively dividing they are small and have dense cytoplasm. It is possible to generalize about the ultimate fate of the cells in the various regions of the apex. Cells of the surface layers give rise to the epidermis and the leaf primordia. The peripheral zone gives rise to most of the cortex and the procambium while the rib meristem matures into pith.
The apical meristems of Angiosperms show a different configuration. They consist of between 1 & 5 surface layers. The cells in these layers only divide anticlinally so only add to the surface layers of the plant epidermis in some cases cortex and leaf primordia. The surface layers are known as the tunica layer. Underneath this there is a core of cells known as the corpus and here cells divide in many planes. The corpus is derived from a layer of corpus initials which are just inside the tunica. Clearly, it is the corpus which is responsible for production much of the cortex and the stele/vascular tissue. Similar zonation to that seen in the Gymnosperms but with extra zones of cells can be seen in the Angiosperms but function is less clear.
So far we've been discussing features of primary growth, and in particular, the role played by the apical meristems. Next we turn to secondary growth.
SECONDARY GROWTHSecondary growth the ability of a plant to extend its girth as well as its length to a feature which is rather advanced and only seen in gymnosperms and in dicots among the angiosperms. It results in the formation of woody tissues:
Secondary growth results from the activities of the lateral meristems and from the vascular cambium in particular. The vascular cambium is a meristematic region between the xylem and phloem which gives rise to xylem on its inside and new phloem on its outside.
The cambium itself consists of several layers of cells with just one single layer being the initials. These initials can produce derivatives in both direction, periclinal division which then become either xylem mother cells or phloem mother cells depending on which direction they were produced in. As a result the production of both 2 xylem and phloem are approximately mirror images of each other There are several layers of xylem-phloem mother cells - also said to be part of the cambial region. During periods of activity and growth the cambial region contains many layers. However, during periods of environmental stress, the vascular cambium becomes dormant and cells stops dividing. However, many of the phloem and xylem mother fells will continue to differentiate or mature so the number of cambial layers reduces dramatically. Usually at least some of the xylem and phloem mother cells become quiescent while only partially differentiated. After overwintering, they can quickly complete differentiating, so providing the plant very quickly with new conducting tissues.
There are two types of cell found within the vascular cambium - known as fusiform initials and ray initials.
The fusiform initials usually produce the v. elongated conducting elements of xylem (= wood) and phloem (= inner bark). Fusiform initials are v. long cells with tapering ends. Those found in gymnosperms are generally much longer than those in dicots, probably because of the types of cell they produce. In gymnosperms the secondary xylem only contains tracheids + secondary phloem contain sieve cells - for both of these the optimal structure is to be as long as possible therefore it makes sense to have the initials as long the derivatives so that new cells don't have to try and elongate/push through tissues at either end. In dicots, the vessel elements and parenchyma which are produced are short, but as there are also long tracheids and fibres, the initials are still fairly elongated and the precursors of the shorter elements divide with transverse walls to create cells of the correct length.
The ray initials are much smaller than the fusiform initials and the isodiametric in shape. They divide in both direction to form a series of parenchymatous cells which create a "passage". Through the wood known as a "ray" (medullary ray) They connect the living cells of the cortex and the pith and facilitate transport of sugars, water and oxygen between vascular tissue and sites of starch storage (parenchyma). Rays in gymnosperms are more primitive - always being only 1 cell thick while in dicots they are often a number of cells thick.
The pattern of fusiform and ray initials set in the young plant is not set for the rest of its life. e.g. fusiform initials will often divide by a series of cross walls to create new ray initials. Patterns also alter slightly as a plant increases in age because as the xylem matures interior to the vascular cambium it expands. The vascular cambium is pushed outwards and its circumference is constantly increased. To keep up with this increase the cambial initials also divide with anticlinal, longitudinal walls to produce more initials rather than derivatives.
The majority of wood is made up of conducting elements and fibres. A feature which is usually associated with trees growing in seasonal climates is growth rings or annual rings, which can often easily be seen in the cross section of a woody stem or tree trunk. These rings are created by varying activity of the vascular cambium through the season . Particularly visible in gymnosperms "early wood" is produced in spring with characteristically large tracheids, with the denser, smaller tracheids called "late wood" formed later in the year. The age of a tree can therefore be calculated simply by counting the rings formed known as Dendrochronology.
A study was done in America which shows that the production of growth rings is not simply a physiological phenomenon but is also taxonomic, relating to the species concerned. Tomlinson and Craighead studied the formation of growth rings in the native trees of subtropical Florida. Most of the trees are from tropical genera while some are from temperate genera - a cross over zone. Whereas the weather conditions were the same for all the trees studied their responses were not. From the 87 species, three types of trees were identified:
Wood without growth rings
51 species fell into this category and these were mostly evergreen tropical trees.
Wood with annual growth rings
Of the fifteen species which produced annual growth rings, most were temperate species, although 4 were tropical
Wood with growth rings, but not annually produced
21 species, various types of species were found in this group; many were tropical evergreen trees, but there were also deciduous tropical trees and temperate evergreen and deciduous trees
The researchers concluded that
a) the ability to produce growth rings is genetically determined
b) formation is not consistently correlated with deciduousness
c) there is no simple correlation between the production of growth rings and climate.
All tissue layers exterior to the vascular cambium including the phloem constitute the BARK.
New phloem cells are cut off to the outside of the vascular meristem although this happens less frequently than the production of new xylem cells. Phloem is a delicate, thin-walled tissue - not thickened with lignin. The old tissue therefore collapses as it becomes compressed at the outer edges, so only a relatively thin layer remains intact closest to the vascular cambium.
The cortex and epidermis which surround the vascular tissue in a young stem are also gradually replaced by the formation of CORK. A second type of lateral cambium called the cork cambium (also called phellogen), is produced through the division of cells just below the epidermis. As the width of the tree expands this new cambial layer cuts off cells along its inner surface which provide the expanding layer of parenchyma cells. It also cuts off cells on its outer surface and it is these which become cork (also called phellem) cells. These are dead cells of regular shape which fit together like bricks and have walls which are lined internally with layers of suberin and wax. This means that the cells are impervious to water, O2 and CO2 and resistant to attack by micro-organisms and to mechanical damage. Some cork cells complementary cells - are loosely fitting together - produced in patches - called lenticels act as pores for water and gas exchange. Cork cells can be thin walled, hollow, air filled or thick walled filled with resins and tannins. Another name for the cork and cork cambium is periderm.