How does a systemic herbicide function?

A systemic herbicide functions by being absorbed into the plant and then translocated throughout its tissues. This means that once the herbicide enters the plant system, it can move through the vascular system, affecting various parts of the plant, including the roots, stems, and leaves. This movement allows the herbicide to target the entire plant, ultimately leading to its death.

In contrast, the other options describe different mechanisms of action for herbicides that do not accurately apply to systemic herbicides. For example, contact herbicides work by killing plant parts on contact, which contrasts with the internal action of systemic herbicides. The option about evaporation addresses a characteristic of some herbicides that can limit efficacy but does not pertain to the action mechanism of systemic types. Lastly, the idea that it works only on the leaves is misleading, as systemic herbicides impact the entirety of the plant rather than being limited to just one part.