Why are the xylem vessels thicker than the phloem sieve tubes?

Xylem vessels are thicker than phloem sieve tubes primarily because they need to withstand the high pressure generated by the transpiration pull during water transport from roots to leaves. This requires strong, rigid cell walls made of lignin that provide structural support and prevent collapse. In contrast, phloem sieve tubes transport sugars and other organic nutrients at lower pressures, so they have thinner walls and are more flexible, allowing for efficient nutrient distribution. This difference in function necessitates the variation in thickness and structural composition between the two types of vascular tissues.