Question

______ on stems help protect plants from herbivores.

Explanation:

In biology (a subfield of Natural Science), structures like thorns, spines, or prickles on plant stems are adaptations that help protect plants from being eaten by herbivores. The most common and relevant term here is "Thorns" (or "Spines", but thorns are more associated with stems for defense against herbivores).

Answer:

Thorns