Final answer:
Xylem vessels have the largest diameter when compared to phloem vessels. This allows for efficient water transport throughout the plant. Phloem cells, being alive, are structured differently and generally have a smaller diameter.
Step-by-step explanation:
The largest diameter among the two types of vascular tissues in vascular plants, xylem and phloem, belongs to the xylem vessel cells. Xylem tissue comprises cells like xylem parenchyma, tracheids, and vessel elements. Tracheids and vessel elements are specialized for water conduction and are dead at maturity, with vessel elements having a significantly larger diameter to allow efficient water transport through perforation plates.
The phloem tissue, responsible for transporting organic compounds, consists of living cells such as sieve cells, companion cells, phloem parenchyma, and phloem fibers. Due to their structure and the requirement of life for functionality, phloem cells tend to have smaller diameters than xylem cells.
It is important to highlight that having a larger diameter in xylem vessels increases the flow rate of water, which is advantageous for the plant's productivity, especially when water is abundant. Conversely, in phloem, transport is facilitated by bulk flow through living cells, which necessitates a different structure than the dead xylem cells used for water transport.