In this article we will discuss about:- 1. Origin and Development of Vessel 2. Phylogeny of Vessel.

Origin and Development of Vessel:

Trachea (or vessel) originates from meristematic cells (Fig. 10.4) that are grouped together in a longitudinal file. The meristematic cells are known as xylem mother cell that develops from procambial cells in the primary xylem. In secondary xylem cambial derivatives give rise to xylem mother cell.

At the last stages of development the fusion of xylem mother cells end to end results in the formation of trachea. Due to fusion and subsequent loss of end walls the vessels are grouped together to form a continuous long tube.

Development of a Vessel Member with Helical thickening

Each xylem mother cell forms a vessel member. The primordial vessel member may or may not elongate in length during differentiation. But it usually widens laterally. Secondary wall materials deposit after the completion of longitudinal elongation and lateral expansion.

The deposition does not occur uniformly and forms specific pattern characteristic for the particular vessel element. The secondary wall materials do not cover certain portions of the primary wall.

These portions are the site of future pits and perforations. The cell wall at the future perforation region becomes thicker than the rest of primary walls. The thickening occurs due to the swelling of intercellular pectic substances and no secondary wall material is deposited on it. The swollen regions of primary wall later disintegrate to form perforations. This happens after the completion of deposition of secondary wall substances.

In longitudinal section the end walls of primordial vessel element appear as lens-shaped (e.g. Celery) or plate like (e.g. Robinia) after swelling. As many as three layers can be distinguished in this region. Middle lamella and the two primary walls of the two adjacent cells compose the three layers. The middle lamella consists of pectic substances whereas non-cellulosic polysaccharides compose the two adjacent, primary walls.

Xylem mother cells are thin walled, densely cytoplasmic, uninucleate and vacuolated. Cytoplasm remains active throughout the development of vessel elements. It slowly disintegrates as the vessel members mature.

As development progresses the nucleus becomes small and flattened, and lie either at the centre, on the side of lateral wall or against the end wall —the site of future perforation. Differentiating metaxylem becomes polyploid and contains more DNA than neighbouring cells.

In some members of Euphorbiaceae the developing vessel members become multinucleate. Commonly the cells remain uninucleate with large endopolyploid nuclei. At maturity nucleus disintegrates. The tonoplast- bound vacuolar sap contains hydrolytic enzymes. After the rupture of tonoplast cytoplasm and nucleus are exposed to hydrolytic enzymes. As a result autolysis of cytoplasm occurs.

After the introduction of electron microscope it is observed that xylem mother cell contains microtubules, endoplasmic reticulum, dictyosomes and mitochondria etc. Evidences suggest that microtubules direct the deposition of new secondary wall materials. It is observed that the concentration of microtubules increases at the region of developing thickening of wall. Cytoplasm present between the thickenings is devoid of microtubules.

In this region endoplasmic reticulum is present in close association with the plasmamembrane. In the cytoplasm active dictyosomes are also present. It is thought that vesicles produced from dictyosomes contribute wall components at the site of wall thickenings. The role of endoplasmic reticulum is to inhibit the process of wall thickenings. As the wall reaches its final state of maturity, the vacuoles present in the differentiating vessel member coalesce.

As a result a single large vacuole is formed. Later vacuolar membrane ruptures and this causes the degeneration of cytoplasmic contents. Sometimes mitochondria persist. The end walls of differentiating vessel members ultimately get dissolved and the remnants are swept away by transpiration stream. Thus the developing vessel members are grouped together and converted to a series of connected tubes.

The region of future perforation is clearly set off at an early stage from the secondary wall. It is the common partition wall between the two contiguous cells that are destined to form vessels. The cell wall at this region is composed of primary wall only. During development it swells along with the middle lamella. It becomes thicker than the neighbouring primary wall.

Then it along with the adjacent primary wall and middle lamella gets dissolved to form the simple perforation. In other cases small holes appear at the thickened regions, the walls gradually dissolve and form the characteristic perforation plate.

It occurs presumably by the action of still living protoplasts. Thus cell-to-cell continuity is established and a continuous tube is formed. Now the vessels become functional in conduction. Around the simple perforation the secondary wall appears as a rim.

Phylogeny of Vessel:

The evolutionary specialization of vessel is discussed within the vessel itself along with other features related to it. These include length, end wall pitting, thickening of cell wall and the outline in transverse section of a vessel while the other features comprise the abundance, groupings and ring porousness of vessels.

The following discussion is based on Metcalfe and Chalk (1950), and Carlquist:

a. Length:

Vessels have arisen from no other cells than tracheids that are usually long and narrow spindle shaped. So it is assumed that long vessels are primitive. The advanced forms are short and wide.

As specialization increases the length of fusiform initial that forms vessels also reduces. The length of a vessel may be as short as a few centimetres or as long as several metres. The size is not random. It is to be noted that certain organs have only long vessels and others have only short ones.

b. End wall:

Vessel with long slanted end is considered as primitive as it is present in tracheid like vessels. Evolution proceeded towards the formation of transverse end walls.

c. Perforation plate:

Vessels with scalariform perforation plate with many bars in a very slanted end wall is considered as primitive. This type of perforation is associated to long vessel, which is also considered as primitive. During evolutionary specialization the number of bars reduced.

Accordingly the number of pores also reduced. The final stage in specialization is the loss of all bars and simple perforation is the result at the end walls. Simple perforation with circular rim is more advanced than long oval rim (Fig. 10.5).

Different Types of Perforation Plate

The specialized vessels are short with large diameter. The perforation plate is simple with circular rim in an almost transverse end-wall. This type of vessel is advantageous for good conduction of water.

In a wide vessel the adhesive interaction between the molecules of water and cellulose of cell wall is decreased and this minimizes the contact between water and cell wall. As a result water flows easily. Moreover the simple perforation plate with circular rim allows easy flow of water. In scalariform perforation plates the bars may offer resistance to water flow.

The long vessels require extra reinforcement in addition to that provided by the secondary wall. Scalariform bars and the rim of perforation plate provide the extra means of strengthening the long vessel. In short vessels with simple perforation the mechanical strength is obtained from the rim of perforation plates in addition to the secondary wall.

d. Lateral wall pitting:

It is suggested that scalariform pitting is the most primitive. Evolutionary sequence advanced through transitional types to opposite type. The final stage in the evolution is alternate pitting. Scalariform pitting is associated with long vessel, whereas alternate pitting is associated with short vessel. Vessels with medium length are associated with transitional and opposite pitting (Fig. 10.6).

Different Types of Lateral Wall pitting of Vessel Member Diagram illustrating different types of lateral wall pitting of vessel member.

e. Thickening of cell wall:

Spiral thickening on the secondary wall is an advanced character. This type of thickening is closely linked with ring porousness of wood. Metcalfe and Chalk is of opinion that probably ecological factors associated with ring-porousness are responsible for the correlation.

f. Outline in transverse section:

Vessel, as seen in transverse sections, ranges from angular in outline to circular in outline. It is suggested that transition occurred from angular to circular in outline. This type of transition is observed both in dicots and monocots.

g. Abundance:

The abundance of vessel offers characteristics of taxonomic significance. It is measured as the number of vessels present per square millimetre of areas in a transverse section of wood.

h. Grouping:

In the transverse section of wood it is observed that vessels may be present as single or in groups. The groupings may be of multiples of two or three. In a group the vessels may be arranged in radial, oblique, or tangential lines.

This type of distribution and character are considered as most valuable taxonomic character. These characters can be used to trace the trends of evolutionary specialization and to indicate affinity. It is suggested that solitary vessel represents the primitive condition. During specialization vessels are arranged in groups.

i. Ring porousness:

Ring porous wood has two marked zones based upon differential distribution of large and small vessels. In this wood numerous large vessels occur at the beginning of growth ring and fewer small vessels are situated at the end. This type of wood is formed due to either different seasonal climatic variation, cold winters or very dry season followed by wet climate.

In contrast, diffuse porous wood has no marked zones of small and large vessels and their distribution is at random throughout the growth ring. Gilbert (1940), after an extensive study on North Temperate woods opines that ring porosity is an advanced feature. But Carlquist does not consider ring porosity as one of the trend of vessel evolution and showed the occurrence of ring porosity in the South Temperate Zone.

Moreover the data obtained by Metcalfe and Chalk are not in correlation with features of vessel advancement. The ring porous feature is present in the vessels of scalariform (primitive) and simple perforation (advanced) with almost equal percentage.

Therefore it is concluded that ring porosity represents an ecological specialization of a wood. This is an evolutionary adjustment to seasonal climatic condition. Ring porous wood occurs in widely separated taxonomic groups.

It was previously mentioned that vessels evolved from tracheids. Tracheids are reported from the oldest fossil Cooksonia from Upper Silurian deposits.

Cooksonia is a leafless, rootless vascular plant and consists of a horizontal rhizome and erect stems. The xylem contains annularly thickened tracheids only. Baragwanathwia, another early vascular plant, also had tracheids with annular thickenings in the xylem, but in contrast to Cooksonia it had leaves.

Vessels are polyphyletic in origin. Vessels occur in Pteridophyta (Selagmella, Equisetum and the four ferns- Aetna op tens, Pteridium, Marsilea and Regnellidium), Gymnosperm (Ephedra, Gnetum and Welzvitschia) and angiosperms.

Aside from angiosperms, vessels of the above mentioned genera of Pteridophyta and Gymnosperm are considered as anomalies. Vessels appeared first in the secondary xylem of dicots and later in the primary xylem. In monocots vessels first arose in roots and then later in the stems and leaves.