Mature phloem is a live tissue,whereas xylem is dead when mature.Explain why it is necessary for phloem to be alive to be functional,whereas xylem can function as dead tissue:
To understand why phloem needs to be alive, while xylem can function as dead tissue, let's first explain the roles of phloem and xylem in plants. Phloem and xylem are both types of plant tissues responsible for the transportation of water, minerals, and nutrients throughout the plant.
Phloem, also known as the food-conducting tissue, is responsible for transporting sugars (mainly in the form of sucrose), amino acids, hormones, and other organic molecules from the leaves (source) to the growing parts of the plant such as roots, shoots, and fruits (sinks). It consists of living cells called sieve elements and companion cells. The sieve elements form long tubes, known as sieve tubes, through which the sap flows.
On the other hand, xylem is the water-conducting tissue responsible for the transport of water and minerals from the roots to the rest of the plant. It consists of dead cells called tracheary elements, including tracheids and vessel elements. These cells are hollow and form a network of interconnected tubes that allow water to move vertically through the plant.
Now, the reason why phloem needs to be alive and functional is because the transport of sugars, hormones, and other organic molecules requires active processes such as metabolism, energy production, and the ability to respond to stimuli. These processes can only occur in living cells. The living companion cells associated with the sieve elements also play a crucial role in maintaining the function of phloem by providing metabolic support to sieve elements.
In contrast, the function of xylem is primarily to transport water and minerals through a passive process called transpiration. Transpiration occurs as water evaporates from the leaves, creating a negative pressure that pulls water up through the xylem. This process does not require metabolic activity or energy production, which is why xylem cells can function as dead tissues. The hollow structure of the dead tracheary elements allows for efficient water transport while offering structural support to the plant.
In summary, phloem needs to be alive and metabolically active to transport sugars and other organic molecules from source to sink. Xylem, on the other hand, can function as dead tissue because its primary role is the passive transport of water through transpiration.