What is the primary mechanism of action for systemic herbicides?

Systemic herbicides operate primarily through the mechanism of translocation, which means they are absorbed by the plant and then moved throughout its various tissues. Once these herbicides enter the plant—typically through the leaves or roots—they travel through the vascular system, reaching areas where they can disrupt growth processes. This ability to move within the plant allows systemic herbicides to affect both the parts that were directly treated and those that were not, making them effective against perennial weeds and other difficult-to-control plants.

The choice regarding contact with foliage refers to how some herbicides require direct application to the plant's surface for effectiveness rather than penetrating into the plant system, which is not the case with systemic herbicides. Soil absorption could relate to some herbicides, but it does not convey their primary action, as systemic herbicides mainly rely on being taken up into the plant for translocation. Foliar runoff suggests a scenario where herbicide is lost after application rather than being effectively absorbed, which does not represent the action of systemic herbicides. Thus, translocation through plant tissues is the defining feature that distinguishes systemic herbicides from others, making it the correct answer.