What characteristic defines herbaceous plants?

Herbaceous plants are primarily characterized by having smooth and nonwoody stems. This trait distinguishes them from woody plants, which develop a hard, woody structure that supports greater heights and longevity. Herbaceous plants typically have stems that are flexible and softer, which allows them to thrive in various environments, often exhibiting rapid growth and seasonal cycles. This attribute is particularly important for their role in ecosystems, as it allows them to take advantage of sunlight during the growing season and to die back in unfavorable conditions while leaving behind roots to regrow in the next season.

The other characteristics offered, such as woody stems, thorny structures, and perennial growth forms, do not define herbaceous plants. Woody stems are typical of trees and shrubs, thorny structures are specific adaptations found in some plant species for protection, and perennial growth forms can be associated with a variety of plant types, both herbaceous and woody. Thus, the defining characteristic of having smooth and nonwoody stems is what classifies a plant as herbaceous.