Definition of Xylem
The plant tissue xylem is responsible for moving water and nutrients from the roots to other parts of the plant, including the stems and leaves. Xylem tissue is one of the characteristics that differentiate vascular plants from nonvascular plants. It supports additional soft tissues found in vascular plants.
Structure of Xylem
Xylem is composed of different types of cells. Long cells called tracheids help in the transportation of xylem sap and also offer structural support. Despite being shorter than tracheids, vessel components help in water conductivity. They are present in flowering plants but not in gymnosperms (e.g., pine trees). Perforation plates present on vessel elements link to create a single, continuous vessel.
Long fibres that help in plant support can also be found in the xylem, along with parenchyma, a tissue that comprises the majority of the soft portions of plants.
Functions of Xylem
Long tubes made of xylem cells, known as xylem saps, transmit materials and the fluid passing through the xylem cells. The primary function of the xylem is to transport water and soluble nutrients, like inorganic ions and minerals, upward from the roots to the entire plant. Passive transport is used to move these substances; hence, no energy is necessary. Capillary action enables xylem sap to move upward against gravity.
For photosynthesis to occur, plants need to absorb carbon dioxide from the atmosphere and water from the soil. However, much water evaporates when a plant’s stomata, or tiny pores in its leaves, evaporate much more than the CO2 taken. For plants to survive, water delivery systems to the photosynthetic areas on leaves needed to evolve; hence, xylem came into existence 400 million years ago.
Types of Xylem
The primary and secondary types of xylem serve the same purpose but are distinguished by the growth pattern under which they are formed.
Plant primary growth results in the formation of the primary xylem. The primary xylem is the growth that appears at the ends of roots, stems, and flower buds. Both primary and secondary xylem transport water and nutrients. They enable the plant’s roots and height to increase.
Secondary growth is the growth that enables a plant to enlarge over time; it results in the formation of secondary xylem. It arises every year after primary growth. Many secondary growths can be seen in wide tree trunks. The black rings on the interior of tree trunks, which are used to estimate the age of the tree, are caused by the secondary xylem.
Relationship between Herbs and Xylem
Like trees and other woody plants, perennial herbs have a vascular cambium growth zone between the root bark and the root xylem. Every year, the vascular cambium ring, active during the growing season, creates a fresh layer of xylem tissue or growth rings. So, what are herbs?
Herbs are delicate plants with short, green stems. They have fewer branches than others and are simple to pull from the ground. They have cellulose-based stems, less stiff than lignin-based stems. They degrade very fast, as opposed to the woody plants, providing a strong, rigid structure and protecting the branches from damage. Some examples of herbs are wheat, mint, and grass.
Herbs usually have annual growth rings that can only be seen under a microscope using a particular staining technique. “False rings” may be present in root cross-sections that resemble ring-like forms.
Stay tuned to BYJU’S Biology for more information.