In this article, we shall be looking at the vascular elements in seed plants. Vascular elements are basically made up of the Xylem and Phloem, and the Cambium.

These three components collectively make up the vascular elements of the plant or the strengthening tissues.

Xylem

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 absorb and distribute water throughout the body of plants

Fig 8.1: Multiple cross sections of a flowering plant stem showing primary and secondary xylem and phloem (Winterbone, 2005).

Anatomy of Xylem

Xylem can be found:

In vascular bundles, present in non-woody plants and non-woody parts of plants with wood.

In secondary xylem, laid down by a meristem called the vascular cambium in woody plants.

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 primaryxylemonly.

The moist 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, collenchyma, schlerenchyma in addition to those that serve to transport water. (McCulloh et al, 2003).

Primary and Secondary Xylem

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.

Developmentally, xylem can be endarch or exarch i.e. grow internally or externally.

Secondary xylem is the xylem that is formed during secondary growth from vascular cambium. Although 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:

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.

Angiosperms (Angiospermae): there are some quarters 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.

The xylem is responsible for the transport of water and soluble mineral nutrients from the roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. 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.

Phloem

In vascular plants, phloem is the living tissue that carries organic nutrients (known as photosynthate), particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word φλόος (phloos) “bark”.

The phloem is mainly concerned with the transport of soluble organic material made during photosynthesis. This is called translocation.

1. Pith, 2. Protoxylem, 3. Xylem I, 4. Phloem I, 5. Sclerenchyma (bast fibre), 6. Cortex, 7. Epidermis.

Phloem Structure

Phloem tissue consists of less specialized and nucleate parenchymacells, sieve-tubecells, and companioncells,fibres and sclereid.

Sieve tubes

The sieve-tube cells lack a nucleus, have very few vacuoles, but contain other organelles such as ribosomes. The endoplasmic reticulum is concentrated at the lateral walls.

Sieve- tube members are joined end to end to form a tube that conducts food materials throughout the plant.

The end walls of these cells have many small pores and are called sieve plates and have enlarged plasmodesmata.

Companion cells

The survival of sieve-tube members depends on a close association with the companioncells. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical plant cell, except the companion cell usually has a larger number of ribosome and mitochondria.

This is because the companion cell is more metabolically active than a ‘typical’ plant cell. The cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata.

There are three types of companion cell:

Ordinary companions cells – which have smooth walls and few or no plasmodesmata connections to cells other than the sieve tube.

Transfer cells – which have much folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer. They are specialized in scavenging solutes from those in the cell walls which are actively pumped requiring energy.

Intermediary cells – which have smooth walls and numerous plasmodesmata connecting them to other cells.

The first two types of cell collect solutes through apoplastic (cell wall) transfers, whilst the third type can collect solutes symplastically through the plasmodesmata connections.

Function of Phloem

Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport sap. The sap is a water-based solution, but rich in sugars made by the photosynthetic areas.

These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such as tubers or bulbs.

The Pressure flow hypothesis was a hypothesis proposed by Ernst Munch in 1930 that explained the mechanism of phloem translocation.

A high concentration of organic substance inside cells of the phloem at a source, such as a leaf, creates a diffusion gradient that draws water into the cells. Movement occurs by bulk flow; phloem sap moves from sugarsources to sugarsinksby means of turgor pressure.

A sugar source is any part of the plant that is producing or releasing sugar. During the plant’s growth period, usually during the spring, storage organs such as the roots are sugar sources, and the plant’s many growing areas are sugar sinks. The movement in phloem is bidirectional, whereas, in xylem cells, it is unidirectional (upward).

After the growth period, when the meristems are dormant, the leaves are sources, and storage organs are sinks. Developing seed-bearing organs (such as fruit) are always sinks. Because of this multi-directional flow, coupled with the fact that sap cannot move with ease between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in opposite directions.

While movement of water and minerals through the xylem is driven by negative pressures (tension) most of the time, movement through the phloem is driven by positive hydrostatic pressures.

This process is termed translocation, and is accomplished by a process called phloemloadingand unloading.

Cells in a sugar source “load” a sieve-tube element by actively transporting solute molecules into it. This causes water to move into the sieve-tube element by osmosis, creating pressure that pushes the sap down the tube. In sugar sinks, cells actively transport solutes outof the sieve-tube elements, producing the exactly opposite effect.

Some plants however appear not to load phloem by active transport. In these cases a mechanism known as the polymer trap mechanism was proposed by Robert Turgeon in 1931.

In this case small sugars such as sucrose move into intermediary cells through narrow plasmodesmata, where they are polymerised to raffinose and other larger oligosaccharides. Now they are unable to move back, but can proceed through wider plasmodesmata into the sieve tube element.

The symplastic phloem loading (polymer trap mechanism above) is confined mostly to plants in tropical rain forests and is seen as more primitive.

The actively-transported apoplastic phloem loading is viewed as more advanced, as it is found in the later-evolved plants, and particularly in those in temperate and arid conditions. This mechanism may therefore have allowed plants to colonise the cooler locations.

Organic molecules such as sugars, amino acids, certain hormones, and even messenger RNAs are transported in the phloem through sieve tube elements.

Origin of the Phloem

The phloem originates, and grows outwards from, meristematic cells in the vascular cambium. Phloem is produced in phases. Primaryphloem is laid down by the apical meristem. Secondaryphloem is laid down by the vascular cambium to the inside of the established layer(s) of phloem.

In conclusion, you have learnt the structure of the stem. You have also learnt that the xylem carries water up the plant while the phloem carries manufactured food down the plant. You further learnt the structures found in the xylem and the phloem that facilitate these functions

In summary, in a flowering plant, the following happens or takes place:

The Xylem and Phloem are the transporting tissues in vascular plants.

The Xylem Sap consists mainly of water and inorganic ions.

The Transpirational Pull and Root Pressure causes xylem sap to flow.

The Xylem can be found in vascular bundles; secondary xylem, and as part of a stellar arrangement.

The Primary xylem is formed during primary growth from procambium, while secondary xylem is formed during secondary growth from vascular cambium.

The Phloem structure consist of parenchyma cells, sieve-tube cells and companion cells.

The Phloem originates from meristematic cells in the vascular cambium.