What primarily causes the formation of galls on plants?

The formation of galls on plants is primarily caused by gall-making insects or mites. These organisms induce galls by laying eggs on or within plant tissues. The plant's response to the presence of the insect or mite leads to abnormal growth or swelling of the plant's cells, resulting in the formation of these galls.

Gall-making insects and mites, such as certain types of aphids, wasps, and thrips, have evolved mechanisms to manipulate the plant's physiology, causing it to produce these growths. The galls provide a protective habitat and a nutrient source for the developing larvae of these pests, illustrating a complex interaction between the host plant and the gall-inducing organism.

While external injury, soil contamination, and excessive water can negatively affect plant health and lead to other types of stress or disease, they do not result in gall formation as directly influenced by specific insects or mites. Galls are a unique response specifically triggered by these gall-making species, making that answer the most accurate in understanding the biological interaction involved.