1) Plant part 2) Plant structure 
A domatium (plurial: domatia, from the Latin `domus`, meaning home) is a tiny chamber produced by plants that house arthropods. Domatia differ from galls in that they are produced by the plant rather than being induced by their inhabitants. Most domatia are either inhabited by mites or ants, in what can be a mutualist relationship, but other art.....
domatium A small structure developed in certain plants; especially on their leaves, serving as a shelter for insects, mites, or fungi.
noun a part of a plant (e.g., a leaf) that has been modified to provide protection for insects or mites or fungi