What defines contact herbicides?

Contact herbicides are defined by their mechanism of action, which involves killing any actively growing plant part they come into contact with. This method means that when the herbicide is applied directly to the foliage or stem of a plant, it disrupts cellular functions and causes damage that leads to the plant's death. The effectiveness of contact herbicides relies on direct contact with the plant tissues.

The other options describe different types of herbicidal actions. For example, some herbicides are absorbed by the plant’s roots and work systemically, meaning they travel through the plant's vascular system to exert their effects. Other types may prevent seed germination, which involves different modes of action that target the seeds rather than the foliage. Lastly, the effectiveness of herbicides in dry or moist conditions can vary, but contact herbicides do not have any specific requirement for dry conditions to operate effectively. Thus, A accurately captures the essence of what differentiates contact herbicides from their counterparts.