Final answer:
Stomata are openings for gas exchange and do not protect plants from herbivores, unlike trichomes, thorns, and spines which are defense mechanisms.
Step-by-step explanation:
The structure that does not protect stems and leaves from damage by herbivores is stomata. Stomata are small openings on the leaf surface for gas exchange, controlled by guard cells. They are not a defense mechanism. In contrast, trichomes, thorns, and spines are plant structures that provide defense against herbivores. Trichomes are hair-like structures that can deter herbivores, while thorns and spines are modified branches and leaves, respectively, that can cause physical harm to deter animals from feeding on them.