What type of plants does not experience annual enlargement of the stem diameter?

The correct answer is that monocots do not experience annual enlargement of the stem diameter. This is primarily due to the way their vascular tissue is organized. Monocots, such as grasses, lilies, and palm trees, have scattered vascular bundles throughout the stem. This configuration does not allow for the formation of secondary growth (increase in thickness), which is typical in dicots, where the vascular cambium is active and contributes to the annual growth rings.

In contrast, dicots, such as oak or maple trees, undergo secondary growth as they typically have a well-defined vascular cambium that produces new layers of xylem and phloem each growing season, leading to an increase in stem diameter. Plants like cacti, while they do have unique adaptations to survive in arid environments, can also exhibit secondary growth but not in the same manner as traditional dicots.

Fibrous plants, which may refer to plants with fibrous root systems or those that have a significant amount of fiber in their tissues, do not inherently imply a lack of growth in diameter. As such, they can include both monocots and dicots. Therefore, monocots stand out in this context for their anatomical structure and lack of annual stem thickening.