What is a swelling found on a leaf or stem caused by an insect or pathogen?

Galls are abnormal growths that form on leaves, stems, or other plant tissues in response to the activity of insects or pathogens, such as bacteria and fungi. When an insect lays eggs in or feeds on a plant, the plant may respond by producing a gall, which provides nourishment and protection for the developing insect. This response can also occur as a result of infection by pathogens, where the plant's immune system reacts by creating a mass of tissue in an attempt to impede the spread of the pathogen.

By contrast, cysts refer to a different type of structure, often associated with parasitic organisms, while nodes are points on a stem where leaves or branches are attached and do not indicate any abnormal growth or swelling. Fungal masses may refer to aggregates of fungal mycelium or fruiting bodies, but they do not represent the specific concept of galls being associated with a reaction to insect or pathogen activity on a plant. Thus, galls accurately describe the swelling that forms as a direct response to external biological stimuli affecting the plant structure.