angiosperm: xylem and phloem

angiosperm: root structure

xylem

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Macroscopic characteristics

	Recent wood can often be identified by macroscopic characteristics, particularly by colour, gloss, odour, weight and structure. As such characteristics are generally modified or destroyed in fossil, historic or carbonized wood, only a few species or species groups of the indigenous flora can be identified with the naked eye or only with the aid of a magnifier (5 to 20x).
	Coniferous wood In coniferous wood it is possible to distinguish the species which have resin canals from those which do not. The transition from earlywood to latewood can be sharp or continuous.		Ring porous dicotyledonous wood The diameter of the pores in the earlywood is much greater than the diameter of the pores in latewood. Arrangement and size of pores in the latewood, fine and large rays can be differentiated.		Semi-ring to diffuse porous dicotyledonous wood In semi-ring porous woods, the pores are more numerous in earlywood, in diffuse porous woods the size of pores and distribution is more regularely. Arrangement and size of pores, fine and large rays differ from species to species.

	An identification key based on macroscopis characters is not given here as it exits e.g. In Bosshard (1974/75) or Gottwald (1958).

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Xylem

The Xylem

The xylem is the principal water-conducting tissue of vascular plants. It consists of tracheary elements, tracheids and wood vessels and of additional xylem fibres. All of them are elongated cells with secondary cell walls that lack protoplasts at maturity. Bordered pits are typical for tracheids, while wood vessels are marked by perforated or completely dissolved final walls.

The xylem takes also part in food storage, support and the conduction of minerals. Xylem and phloem together form a continuous system of vascular tissue extending throughout the plant. The principal conductive cells of the xylem are tracheary elements, of which there are two types, tracheids and wood vessels. Both are elongated cells with secondary cell walls that lack protoplasts at maturity. They are completed by the xylem fibres and parenchyma cells. Much speaks on behalf of the origin of xylem fibres and wood vessels from the tracheids.

Since 1851, the isolation and depiction of lignified cells is done according to the maceration procedure of SCHULZE. Small pieces of wood are covered with a mixture of potassium perchlorate and concentrated nitric acid. The complete volume should not be larger than 1/10 of the reaction container, because a lot of gas develops very quickly at cautious heating. The surfaces of the wooden pieces are strongly attacked. The single cells can be scraped off after washing of the preparation and examined under the microscope.

Let us start with a portrait of the different cell types.:Tracheids are the chief water-conducting elements in gymnosperms and seedless vascular plants. They can also be found in angiosperms. Tracheids are elongated cells, closed at both ends. They are 1 mm on average. Tracheids are regarded as the prototype of prosenchymatic cells, since the cell's ends are pointed and true final walls are missing. Tracheids look often square in cross-section, the lignified secondary wall is relatively thin. Their entire cell surface is evenly coated. The walls are opened by numerous pits that are, depending on their origin, either round, oval, gap- or groove-shaped. They occur solitarily, statistically scattered, arranged in turns around the middle axis or grouped together. Such groups can often be found at the cell's ends. If gap-like pits lay on top of one another, a ladder- or stair-like perforation may be the result. It is commonly called scalariform. We will meet this structure again, when talking about vessel elements. The pits are often surrounded by a halo and are then called bordered pits. Bordered pits are especially common in the tracheids of some gymnosperms. Their structure can be discerned best in a cross-section through neighbouring cells. The middle lamina between the cells is preserved within the pits. Their centre is made up by a disc of primary cell wall material, called torus. No secondary walls exists in the pit's structure. The area between torus and wall (the former middle lamina) is called margo and is very porous, allowing the movement of water and ions from tracheid to tracheid. Bordered pits exist only in cells with secondary walls.

Botanists think of wood vessels (tracheae) as the water-filled tubes of the xylem. M. MALPIGHI, who thought that he had found an important common element in the anatomy of animals and plants introduced the term trachea in the 17th century. Wood vessels are the chief water-conducting elements of angiosperms.

In contrast to the tracheids the final walls of the single vessels are perforated or, much more so, completely resolved. Wood vessels are therefore generally thought to be more efficient water conductors than tracheids. The length of the single tube (composed of numerous cells) makes it difficult to isolate a vessel as a whole. It can be as long as several meters. It is commonly assumed that at least in some species the wood vessels are as long as the whole shoot.

During ontogenesis the wood vessels increase strongly in width.They are usually round in cross-section and have a larger diameter than the tracheids, a feature that enhances their capacity for water-conduct. Exceptionally wide-lumened elements can be seen with deciduous trees, that are known to lose particularly large amounts of water due to transpiration. The water-loss of a fully developed birch tree with an estimated number of 200 000 leaves can be up to 400 litres per day. Even wider are the vessel elements of many lianas. But the oldest living trees, the redwoods and other sequoias at the pacific coast of California have without exception tracheids with very narrow lumina. Vessels are marked by characteristically structured secondary wall coatings (lignin) at the inner surface of the primary walls. Deposits in the form of screws, rings or nets exist. These strengthenings make it possible for tracheary elements to be stretched or extended, although the cells are frequently destroyed during the overall elongation of the organ.

Beside this vessels with pits or scalariform openings exist, whose walls are nearly completely lined with secondary wall material that is only opened by round or gap-like pits. Many transitions between the two pit types can be found. Often some or even all of the types are members of the same vascular bundle. But there are species, that lack one or the other type. Wood vessels develop -just like tracheids- during primary growth from the cells of the procambium. Where secondary growth occurs wood vessels are produced by the cells of the cambium.

Wood growth and structure

Wood growth and structure



Wood is a complex, natural product. Between different tree species, its density, stability, durability, strength, burning properties, electrical resistance, reaction during drying, impact resistance, bending properties, acoustic properties, pulping qualities, workability and appearance varies markedly.

Hardwoods and softwoods

Trees are classified into two groups: hardwoods and softwoods. This can be confusing because not all hardwoods are hard and not all softwoods are soft. Balsa wood, for example, is a hardwood, while some of Australia's hardest timbers—for example, Callitris pine, are softwoods. The difference is in the cellular structure of the wood.

Hardwood timbers are made up of four cell types. Small wood fibres make up the bulk of the wood. Large cells, called vessels, function as pipes that move sap up the tree through the mass of fibres. Other cells are largely used to store food. With some experience it is possible to distinguish different hardwood timbers based on the number, size and location of the vessels.

Softwoods do not have vessels. Softwoods have a simpler fibrous structure based on only two cell types.

The easiest way to distinguish between the two tree groups is to remember that flowering trees are all hardwoods while cone-bearing trees are softwoods. Hardwoods include eucalypts, wattles and oaks while pines and cypresses are softwoods.



Growth rings and the pith

As the tree trunk and branches thicken, a series of concentric layers of wood cells are laid down around a central core called the pith. These appear as growth rings. The pith is the remnant of the growing shoot that gives the tree its height. Because the pith has a cell structure different to the rest of the tree, it is often easily seen as a corky pipe in the log’s centre.

Growth rings often, but not always, represent annual growth. In temperate areas, tree growth during spring is represented by the presence of large cells with thin walls that appear lighter in colour. The darkness is due to the presence of smaller, thick-walled cells that are laid down towards the end of the growing season. In some hardwoods, the vessels form a ring in the early wood (ring-porous species) making the rings easier to see.



Sapwood and heartwood

The living tree’s stem is composed of different layers, illustrated in the cross-section diagram above. The cambium is a very thin layer of cells that divide to produce the bark cells (which protect the tree) and the wood cells. Sugars produced in the leaves travel down the inner bark, or phloem, feeding the cambium and, ultimately, the roots.

Only a very small proportion of the wood cells produced by the cambium are actually alive. Most of the newly formed wood cells hollow out to form the sapwood through which water and dissolved minerals travel from the roots to the leaves. Other cells are used to store food provided by photosynthesis, including starch. The sapwood is almost always creamy white or yellow in colour and is usually two to five centimetres thick.

In most tree species the sapwood band can be easily distinguished from the inner heartwood. The heartwood supports the tree’s stem but doesn't otherwise contribute to its growth. Heartwood’s colour comes from resins, minerals and other compounds being deposited in the cells as they are decommissioned from their role as sapwood. These deposits, and the lack of food, add durability, colour and strength to the timber

Xylem

Multiple cross sections of a stem showing xylem and companion cells^[1]

In vascular plants, xylem is one of the two types of transport tissue, phloem being the other. The word "xylem" is derived from classical Greek ξυλον (xylon), "wood", and indeed the best known xylem tissue is wood, though it is found throughout the plant. Its basic function is to transport water.

The xylem transports water from the root up the plant. The xylem is mainly responsible for the transportation of water and mineral nutrients throughout the plant. Xylem sap consists mainly of water and inorganic ions, although it can contain a number of organic chemicals as well. This transport is not powered by energy spent by the tracheary elements themselves, which are dead at maturity and no longer have living contents. Two phenomena cause xylem sap to flow:

• Transpirational pull: the most important cause of xylem sap flow, is caused by the evaporation of water from the surface mesophyll cells to the atmosphere. This transpiration causes millions of minute menisci to form in the cell wall of the mesophyll. The resulting surface tension causes a negative pressure in the xylem that pulls the water from the roots and soil.
• Root pressure: If the water potential of the root cells is more negative than the soil, usually due to high concentrations of solute, water can move by osmosis into the root. This may cause a positive pressure that will force sap up the xylem towards the leaves. In extreme circumstances the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation. Root pressure is most common in the morning before the stomata open and cause transpiration to begin. Different plant species can have different root pressures even in a similar environment; examples include up to 145 kPa in Vitis riparia but around zero in Celastrus orbiculatus^[2].

Xylem can be found:

• in vascular bundles, present in non-woody plants and non-woody plant parts
• in secondary xylem, laid down by a meristem called the vascular cambium
• as part of a stelar arrangement not divided into bundles, as in many ferns.

Note that, in transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only.

The most distinctive cells found in xylem are the tracheary elements: tracheids and vessel elements. However, the xylem is a complex tissue of plants, which means that it includes more than one type of cell. In fact, xylem contains other kinds of cells, such as parenchyma, in addition to those that serve to transport water.

Primary xylem is the xylem that is formed during primary growth from procambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. It is distinguished by wider vessels and tracheids.

Secondary xylem is the xylem that is formed during secondary growth from vascular cambium. Secondary xylem is also found in members of the "gymnosperm" groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta. The two main groups in which secondary xylem can be found are:

1. conifers (Coniferae): there are some six hundred species of conifers. All species have secondary xylem, which is relatively uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is marketed as softwood.
2. angiosperms (Angiospermae): there are some quarter of a million to four hundred thousand species of angiosperms. Within this group secondary xylem has not been found in the monocots. In the remainder of the angiosperms this secondary xylem may or may not be present, this may vary even within a species, depending on growing circumstances. In view of the size of this group it will be no surprise that no absolutes apply to the structure of secondary xylem within the angiosperms. Many non-monocot angiosperms become trees, and the secondary xylem of these is marketed as hardwood.

Photos showing xylem elements in the shoot of a fig tree (Ficus alba): crushed in hydrochloric acid, between slides and cover slips.

Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from the Silurian (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier Ordovician rocks. The earliest true and recognizable xylem consists of tracheids with a helical-annular reinforcing layer added to the cell wall. This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the protoxylem (first-formed xylem) of all living groups of plants. Several groups of plants later developed pitted tracheid cells, apparently through convergent evolution. In living plants, pitted tracheids do not appear in development until the maturation of the metaxylem (following the protoxylem).

In most plants, pitted tracheids function as the primary transport cells. The other type of tracheary element, besides the tracheid, is the vessel element. Vessel elements are joined by perforations into vessels. In vessels, water travels by bulk flow, like in a pipe, rather than by diffusion through cell membranes. The presence of vessels in xylem has been considered to be one of the key innovations that led to the success of the angiosperms^[3]. However, the occurrence of vessel elements is not restricted to angiosperms, and they are absent in some archaic or "basal" lineages of the angiosperms: (e.g., Amborellaceae, Tetracentraceae, Trochodendraceae, and Winteraceae), and their secondary xylem is described by Arthur Cronquist as "primitively vesselless". Cronquist considered the vessels of Gnetum to be convergent with those of angiosperms^[4]. Whether the absence of vessels in basal angiosperms is a primitive condition is contested, the alternative hypothesis being that vessel elements originated in a precursor to the angiosperms and were subsequently lost.