In the sentence "The stems measure a foot in diameter," the verb "measure" is being used transitively. In this context, "measure" is an action that is being performed on the object "a foot in diameter," which provides more information about the stems.
Intransitive verbs, on the other hand, do not take a direct object. For example, in "The stems grow," "grow" does not require an object. Since "measure" here has a direct object, it is transitive.