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  Xylem and Phloem, Biology tutorial

Introduction:

Vascular elements are mainly made up of the Phloem, Xylem and the Cambium. These three components together make up the vascular elements of the plant or the strengthening tissues.

Xylem:

In vascular plants, xylem is one of the two kinds of transport tissue, phloem being the other. The term 'xylem' is derived from the classical Greek xylon means wood and in fact the best known xylem tissue is wood, although it is found all through the plant. Its fundamental function is to absorb and distribute water via the body of plants.

Anatomy of Xylem:

Xylem can be found:

1) In vascular bundles, present in non-woody plants and non-woody parts of plants having wood.

2) In secondary xylem, laid down through a meristem termed as the vascular cambium in the woody plants

3) As part of the stelar arrangement not splitted into bundles, as in most of the ferns.

Primary and secondary xylem:

Primary xylem is the xylem which is made all through primary growth from procambium. It comprises protoxylem and metaxylem. Metaxylem builds up after the protoxylem however before the secondary xylem. It is differentiated through broader vessels and tracheids. Developmentally, xylem can be endarch or exarch that is, grow internally or externally

Secondary xylem is the xylem which is made all through secondary growth from vascular cambium. However secondary xylem is as well found in the members of gymnosperm groups Gnetophyta and Ginkgophyta and to a lesser degree in members of the Cycadophyta, the two major groups in which secondary xylem can be found are as:

1) Conifers: There are around 600 species of conifers. All the species have secondary xylem that is relatively uniform in structure all through this group. Most of the conifers become tall trees: the secondary xylem of these trees is marketed as softwood.

2) Angiosperms: There are a few quarters of a million to four hundred thousand species of angiosperms. In this group secondary xylem has not been found in the monocots. In the remainder of angiosperms this secondary xylem might or may not be present, this might differ even in a species, based on growing conditions. In view of the size of group it will be no astonish that no absolutes apply to the structure of the secondary xylem in the angiosperms. Most of the non-monocot angiosperms become trees, and the secondary xylem of such is marketed as hardwood.

Phloem:

In vascular plants, phloem is the living tissue which carries organic nutrients (termed as photosynthate), specifically sucrose, a sugar, to all the parts of plant where required. In trees, the phloem is the innermost layer of the bark, therefore the name, derived from the Greek word phloos means bark. The phloem is mostly concerned by the transport of soluble organic material made throughout photosynthesis. This is termed as translocation.

Phloem Structure:

Phloem tissue comprises of less specialized and nucleate parenchyma cells, sieve-tube cells and companion cells.

1) Sieve tubes: The sieve-tube cells lack a nucleus, contain very few vacuoles and however contain other organelles like ribosomes. The endoplasmic reticulum is concentrated at the lateral walls. Sieve-tube members are connected end to end to make a tube which conducts food materials all through the plant. The end walls of such cells have lots of small pores and are termed as sieve plates and encompass enlarged plasmodesmata.

2) Companion cells: The survival of sieve-tube members based on a close relationship with the companion cells. All of the cellular functions of a sieve-tube element are taken out by the (much smaller) companion cell, a usual plant cell, apart from the companion cell generally consists of a huge number of mitochondria and ribosome. This is because the companion cell is more metabolically active than the typical plant cell. The cytoplasm of a companion cell is joined to the sieve-tube element through plasmodesmata.

Function of Phloem:

Dissimilar to xylem (that is mainly composed of dead cells), the phloem is composed of still-living cells which transport sap. The sap is a water-based solution; however rich in sugars build up by the photosynthetic regions. Such sugars are transported to non-photosynthetic portions of the plant, like the roots, or into storage structures, like bulbs or tubers.

The Pressure flow hypothesis was a theory stated by Ernst Munch in the year 1930 which described the method of phloem translocation. A high concentration of organic substance within cells of the phloem at a source, like leaf, creates a diffusion gradient which draws water into the cells. Movement takes place through bulk flow; phloem sap moves from sugar sources to sugar sinks by means of turgor pressure. A sugar source is any portion of the plant which is producing or releasing the sugar. Throughout the growth of plant period, generally during the spring, storage organs like the roots are sugar sources, and most of the growing areas of plants are sugar sinks. The movement in phloem is bidirectional, while, in xylem cells, it is unidirectional (upward).

After the growth period, if the meristems are dormant, the leaves are sources and storage organs are sinks. Developing seed-bearing organs (like fruit) are always sinks. Because of this multi-directional flow, coupled by the fact that sap can't move with easiness among adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing in the opposite directions.

As movement of minerals and water via the xylem is driven by negative pressures (tension) most of the time, movement via the phloem is driven by positive hydrostatic pressures. This method is known as translocation and is accomplished by a procedure termed as phloem loading and 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 through osmosis, making pressure which pushes the sap down the tube. In sugar sinks, cells actively transport solutes out of the sieve-tube elements, generating the precisely opposite effect.

Origin of the Phloem:

The phloem originates and grows up outwards from meristematic cells in the vascular cambium. Phloem is generated in phases. Primary phloem is laid down through the apical meristem. Secondary phloem is laid down through the vascular cambium to the inside of the established layer of the phloem.

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