Xylem

Xylem is mainly a type of tissue in vascular plants that help in the transport of water and some nutrients from the roots to the leaves. On the contrary, phloem is the type of transport tissue; that helps in the transport of sucrose and other nutrients throughout the plant. Xylem and phloem give vascular plants their characteristics; both of these are the vascular tissues that transport substances throughout the plant. The presence of xylem tissue is one of the main distinguishing features that help in separating vascular plants from nonvascular plants. The xylem provides structural support to other soft tissues that are present in the vascular plants. In the year 1858, Carl Negali coined the term xylem. The term xylem is derived from the Greek xylon (which means “wood”). Wood is a very popular example of a xylem.

Structure of Xylem

Xylem is composed of several different types of cells. Tracheids are generally the long cells that help in the transport of xylem sap and also provide structural support. Vessel elements are mainly shorter than tracheids, but they also help to conduct water. They are mainly found in flowering plants, but not in gymnosperms such as pine trees. Vessel elements possess perforation plates that remain connected to each vessel element to form one continuous vessel. Xylem also comprises parenchyma, it is a type of tissue that forms most of the soft parts of plants, and long fibres that help support the plant. In a cross-sectional view of a plant, under a microscope, the xylem appears to be star-shaped.

The function of the Xylem

The important function of the xylem is to transport water, and some soluble nutrients together with minerals and inorganic ions, upwards from the roots to the rest of the plant. Xylem cells form long tubes that help in the transport of materials, and the mixture of water and nutrients that flows through the xylem cells is known as xylem sap. 

These substances are mainly transported via passive transport, so the process doesn’t require any energy. The phenomenon that allows xylem sap to transport upwards against gravity is known as the capillary action. 

This happens when surface tension allows the liquid to move upwards. Water also helps in moving up through the xylem by adhering to the xylem cells. Although, it gets harder to work against the gravity to transport materials as a plant grows taller, so xylem has set an upper limit on the growth of tall trees. 

Xylem evolved in plants around 400 million years ago. To prepare food via photosynthesis, plants are required to absorb carbon dioxide from the atmosphere and water from the soil. Although, when the stomata (they are small holes in a plant’s leaves) are wide open to allow CO2 in, a lot of water evaporates, even greater than the amount of CO2 taken in. Plants that have a developed system to transport water to the sites of photosynthesis on leaves possess a better chance of survival.

Types of Xylem

The two distinct types of xylem are primary and secondary, both of them perform the same function but are mainly categorized based on the type of growth that they are formed of.

Primary Xylem

Primary xylem is formed with the primary growth of a plant. This is the growth that mainly occurs at the tips of stems, roots, and flower buds. It allows the plants to grow taller and the roots to grow longer. This growth is known as primary since it occurs first during the growing season, before the secondary growth. Both primary and secondary xylem help transport water and nutrients.

Secondary Xylem

Secondary xylem is formed with a plant’s secondary growth; this is the type of growth that allows the plant to get wider with time. Wide tree trunks, for example, have a lot of secondary growth. This happens each year after the growth of the primary xylem. The secondary xylem is the one that gives the inside of tree trunks dark rings that can be used to determine the age of the tree.

Development of Xylem

The first xylem that can be seen to develop in a growing plant is known as protoxylem, and it comprises narrow vessels as the plant is not that big. Metaxylem develops later on and it possesses larger vessels and cells. There are mainly four ways by which protoxylem and metaxylem are arranged in a plant: centrarchid, exarch, endarch, and research.

Centrarch: here, the xylem forms only one chamber in the middle of the stem, with the metaxylem surrounding the protoxylem. This formation is no more seen in any living plants present today.

Exarch: in exarch condition xylem develops in multiple strands, and each strand further develops inward towards the centre of the root. Xylem in the roots of vascular plants develops in this way only.

Endarch: here, the xylem develops as multiple strands, and each strand further develops outward towards the periphery of the stem. Xylem in the stems of vascular plants develops in this way.

Mesarch: xylem occurs in multiple strands, and each strand further develops from its middle both lie towards the centre of the stem and in the opposite direction i.e. towards the periphery. Xylem in leaves and stems of ferns are seen to develop this way.

Differences Between Xylem and Phloem

Conclusion

Xylem formation begins once the actively dividing cells of growing roots and shoot tips (i.e. the apical meristems) give rise to the primary xylem. When this occurs, the primary xylem cells die and lose their conducting function, and form a hard skeleton that serves only to support the plant. Therefore, in the trunk and older branches of a large tree, only the outer secondary xylem (sapwood) helps in the conduction of water, whereas the inner part (i.e. heartwood) is made up of dead but structurally strong primary xylem. In temperate or cold climates, the age of a tree can be determined by counting the number of annual xylem rings that are formed at the base of the trunk. Here, we come to an end of this topic, we hope that you were able to grasp a clear concept of xylem.