Xylem Tissue:

Definition of Xylem:

Xylem can be defined as a complex tissue that is composed of four basic types of cell (tracheids, trachea, and xylem fibre and xylem parenchyma), remains in close association with phloem and has specialized functions like conduction of water and solutes, and mechanical strength.

Xylem and phloem together constitute the conducting tissues in plants. They occur both in primary —and secondary vascular tissues. Accordingly their origins differ. The procambial strands of apical meristem develop into primary xylem and primary phloem.

Secondary vascular tissues originate from cambium —the fascicular part of which originates from procambium strands. Xylem is heterogeneous tissue and the four basic cell types (Fig. 9.1 & 9.2) composing it are xylem parenchyma, xylem fibre, tracheids and trachea.

Cell Types of Xylem:

i. Xylem Parenchyma:

Parenchyma that forms one of the constituent of xylem is referred to as xylem parenchyma.

Xylem parenchyma is present in primary — and secondary xylem and respectively originates from procambium and cambium. In secondary xylem xylem-parenchyma is also termed as wood parenchyma and is present as axial — and radial parenchyma that accordingly occur as parallel and perpendicular to the long axis of organ where they lie.

Xylem parenchyma has no definite shape. It may be irregular, rectangular, round, oval and elongated etc. (9.1 A, B & C). The cell wall is usually thin when it is composed of cellulose only.

At later stage lignin may deposit and as a result cell wall becomes thick. Pits frequently occur on cell walls. It is simple when occurs between parenchyma cells. It may be simple, bordered or half bordered when occurs between parenchyma and tracheary elements (tracheids and tracheae).

Xylem parenchyma possesses living protoplast and contains starch, fat, crystals and tannins, and sometimes chlorophyll also.

Xylem parenchyma is mainly concerned with storage of starch, fat and ergastic substances etc. Transportation of minerals, solutes and water etc. occur through it. It gives mechanical support when turgid. The thick-walled parenchyma gives mechanical strength to the plant.

ii. Xylem Fibre:

Fibre that occurs as element of xylem is called xylem fibre. It is also referred to as xylary fibre and wood fibre.

Xylary fibres are elongated cells the length of which is many times longer than breadth. The two ends of a fibre are tapering to a wedge shape. The cell wall is usually thick. Due to lignin deposition the lumen of fibre becomes narrow.

Pits are present on walls and they may be simple and bordered. Fibres may be septate (Fig. 9.1G). Usually it is regarded that fibres lack living contents at maturity, but there are reports that wood fibres may retain living protoplasts as long as twenty years, e.g. Tamarix aphylla.

Fibres may occur in primary-and secondary xylem, but proportionately their occurrences are much less in primary xylem. In primary xylem fibre originates from procambium. In secondary xylem fibres originate from fusiform initials of cambium.

The following three types of xylary fibres are noted:

1. Libriform fibre (Fig. 9.1F):

These fibres are characterized in having elongated cell with thick cell wall, small number of simple pits on the wall and very narrow cell lumen in comparison to tracheids in which they are associated in the same plant. Bordered pit rarely occurs.

The inner pit aperture usually is slit like and pit canal is more elongated than that of tracheids. The inner apertures of the pit-pairs in libriform fibre are crossed with each other. Libriform fibre (liber = phloem) simulates phloem fibre, hence, the name libriform.

2. Fibre-tracheid (Fig. 9.1E):

This fibre has the characteristics that are intermediate between tracheids and libriform fibres. In contrast to tracheids fibre-tracheid is much elongated; the cell wall is thicker with reduced size of bordered pits. In comparison to libriform fibre fibre- tracheid is shorter in length and the cell wall is thinner with bordered pits. The inner apertures of a pit pair in fibre-tracheids are crossed with each other. The inner apertures are usually slit like.

3. Gelatinous or mucilaginous fibre:

This fibre is either libriform fibre or fibre-tracheid where the secondary wall lacks lignin or contains it in very small amount. The secondary wall has a-cellulose in its innermost layer termed G-layer. As a result the wall becomes highly hygroscopic. Gelatinous fibres are characteristic of the tension wood of angiosperm.

Tension wood is formed in response to gravity that causes lateral stress. Branches of angiosperm dicotyledonous plants exhibit tension wood that is formed to counteract the lateral stress caused due to gravity.

iii. Tracheids (Fig. 9.1H):

Tracheid is long, single-celled, non-living water conducting element of xylem with hard, thick (due to lignin deposition) and pitted wall, and the two ends of cell are chisel-like, oblique, imperforate and obtuse or tapering.

Tracheids occur in all groups of vascular plants. It is present exclusively in the xylem of pteridophytes and gymnosperms. Primitive angiosperms like Trochodendron, Tetracentron and Drimys etc. also have it. It occurs in primary— and secondary xylem. In primary xylem it originates from procambium whereas in secondary xylem it develops from cambium. Tracheids originate from a single cell.

Tracheid occurs parallel to long axis of an organ where it lies. It overlaps each other and communicates with neighbours by means of pits (Fig. 9.11). Pits are present on lateral and oblique end walls. In xylem the tracheids are situated one above the other and the end walls are in contact with that of others. Water diffuses through the pits (Fig. 9.3A).

Pits may be simple, bordered or half bordered. Bordered pit is observed in the pit pairs present between tracheids. The pit pair that occurs between tracheids and parenchyma exhibits simple pit on the parenchyma-cell wall and bordered pit on tracheid-cell wall. In ferns the bordered pit is transversely elongated termed scalariform pitting (Fig. 9.1 D). Tracheids are angular in cross-sectional view.

The cell wall of tracheids is thick due to the deposition of lignin. Lignin does not deposit uniformly and as a result different types of pit, annular and helical sculpturing are formed. Annular and spiral thickening are observed in protoxylem. Metaxylem exhibits bordered pit and scalariform pitting.

The function of tracheids is conduction of water and minerals in solution (Fig. 9.3A). It gives mechanical support to the organ where it lies. There is report that tracheids store water.

iv. Vessel:

Vessels (also termed tracheae; sing, trachea) are long, non-living thick walled (due to lignin deposition) elements of xylem and composed of vertical rows of single-cell units the end walls of which are perforated thus forming a continuous water-conducting tube.

Vessels are exclusively present in angiosperm. They also occur in gymnosperm, e.g. Ephedra, Welivitschia and Gnetum. In pteridophyta vessels are present in Selaginella, Equisetum and in the following four ferns: Actiniopteris, Pteridium, Regnellidium, and Marsilea. The occurrence of vessel in pteridophyte and gymnosperm is regarded as anomalous. In angiosperm, vessels occur both in primary — and secondary xylem and respectfully originate from procambium and cambium.

Each vessel unit is long and cylindrical in shape and lack living protoplast. Sometimes the length of a vessel unit may be shorter than breadth when it appears as drum shaped. The cell wall is thick due to deposition of lignin. Lignin does not deposit uniformly and as a result the following types of pitting (Fig. 10.6) are formed on the lateral walls: scalariform pitting, opposite pitting and alternate pitting.

The two ends of a vessel unit are perforated. Several perforate-units are aggregated longitudinally to form a chain of cells. The chain of cells forms a tube-like system and usually referred to as trachea or vessel.

The unit cells that make up the tube-like system are referred to as vessel members, vessel elements or vessel units. Vessel member that terminates in the tube-like system has tracheid- like terminating end, i.e. the terminating end is not perforated.

Apart from terminal position perforations may be sub-terminal or lateral. Part of a vessel member bearing perforation is known as perforation plate. A perforation plate may consist of a single or more than one perforation and accordingly the following perforation plates are noted (Fig. 9.2).

1. Simple:

Ex. Quercus. The perforation plate is composed of a single perforation that is present at each end of a vessel member. Usually vessel with transverse end wall has simple perforation plate.

Multiple:

More than one pore composes multiple perforation plate.

The pores are arranged in various ways and the following types are observed:

2. Scalariform:

Ex. Liriodendron and Phoenix dactylifera etc. The pores are laterally elongated and occur parallel to each other in the perforation plate. The intervening bars of thickenings appear ladder-like. Commonly vessel with oblique end walls has scalariform perforation plate.

3. Foraminate:

Ex. Ephedra. Several and almost circular pores are grouped together to compose the perforation plate. It is also referred to as ephedroid perforation plate.

4. Reticulate (Fig. 9.1J & K):

Ex. Rhoeo discolor, Hymenocallis. Many small pores compose the perforation plate. The arrangement of pores makes a mesh-like structure. The intervening regions between pores are in the form of network formed by secondary wall thickening materials. Reticulate perforation occurs less frequently.

Usually a particular type of perforation plate occurs in a plant. But sometimes the two ends of a vessel member may have different types of perforation, e.g. simple and scalariform.

The neighbouring vessel elements may also possess different types of perforation plates:

i. Function:

(a) Conduction of water and dissolved minerals occur from root to stem and leaves through vessels (Fig. 9.3B) and (b) vessels give mechanical rigidity to the organs where they lie due to the presence of hard Iignified cell wall.

The tracheary elements, i.e. the tracheids and vessels are like capillary tubes. The ability of these tubes to allow water transport through them is known as conductance. Conductance is related to vessel diametre. The conductance is greatly increased in the vessels with large diametre in comparison to those with small diametre.

One of the properties of water is that it tends to stick to many substances by adhesive force. Water sticks to inner sides of tracheary elements thus decreasing the diametre of them. So the movement of water through tracheary element is not uniform. Water present near the wall is stuck to the wall and so its movement is very slow.

The water in centre is free form adhesion and so its movement is faster. The vessels with wide diametre are advantageous because the friction caused by adhesion is minimized here. Though adhesion hinders water movement it is advantageous in the fact that it strengthens the water column present within tracheary elements.

It is important for the upward movement of water. The water column in tracheary elements is continuous from root to leaf via stems. All the water molecules are held together by cohesive force. As a result the water column tends to act as a unit. The continuity and erectness of the column are maintained by cohesion and adhesion.

There is also continuity of water among the mesophyll cells, guard cells and tracheary elements of a leaf. The leaves lose water by cuticular and stomatal transpiration. As a result a suction force is developed among the mesophyll cells, which are in direct contact with tracheary elements. This causes a tension in the water and a pull is established, and it is transmitted to the water present in root.

This happens, as the column is continuous. This pull, also known as transpiration pull, drags water column and the water moves upward. During movement water column is under tension, which is also transmitted to cell wall of the tracheary elements. This happens because water molecules and cell wall act as a unit due to their bonding by cohesive and adhesive forces.

During transpiration pull (Fig. 9.4B) the cell wall of tracheary element is also pulled inwardly. If the cell wall has not enough rigidity to resist it may buckle and in extreme case it may collapse. So for effective conductance the cell walls of tracheary elements must have enough strength to avoid collapse.

The other prerequisite of conductance is adhesion and cohesion. If somehow the adhesive and cohesive forces are overcome embolus (Fig. 9.4D) may appear in the water column —a phenomenon called embolism thus stopping water transport.

Embolus expands and stops when it comes in contact to solid surface only. Embolus cannot expand in tracheids, as they have no perforation plate. In vessels embolus spreads from one to other through perforations thus causing vessels functionless.

The movement of water is dependent upon its chemical potential. ‘The chemical potential of a substance in a system is a measure of the capacity of that substance to do work’-Kramer, 1969, p. 16. The chemical potential of pure water at one atmosphere of pressure or one bar of pressure is defined as water potential and its value is zero.

Addition of solutes dilutes water and reduces its activity. The potential of this system is less than that of pure water. Hence the water potential has negative value and is expressed as a negative number. Water moves from regions of higher to regions of lower potential. Transpiration, i.e. loss of water from leaves, lowers the water potential.

This is advantageous for transpiration pull and upward water transport. Water in the tracheary element has high cohesive force. It remains confined within the wet-table cell walls where adhesive forces operate. During upward movement tension develops, which may be as high as 300 or more atmospheres. This is also advantageous for upward movement of water and to keep water column erect.