abrupt swelling - A significant increase in the diameter of a stem, petiole, etc, emerging directly from unaffected tissue. Sometimes encircling the stem, other times emerging only from one side.
hidden cell - A gall making no externally visible change to the host (typically in a stem or fruit) until the inducer chews its egress hole.
leaf blister - Localized distortions of the leaf lamina, typically creating a cup opening toward the lower side of the leaf.
leaf curl - Broad deformation of the lamina of a leaf, pulling the edges in. Typically irregular and sometimes causing entire leaves to roll up. Often accompanied by discoloration.
leaf edge fold - A single layer of the leaf edge folded back against the leaf.
leaf edge roll - A tight roll of tissue only at the edge of a leaf, of varying thickness.
leaf snap - Two distinct, otherwise typical host leaves are joined together around a gall cell
leaf spot - A flat (never more than slightly thicker than the normal leaf), typically circular spot on the lamina of the leaf, sometimes with distinct rings of darker and lighter coloration (eye spots). Fungal leaf spots often have small dots above; midge spots have an exposed larva below.
modified capitulum - Flowers in Asteraceae are clustered into tight structures called "capitula", which themselves resemble a flower at first glance. Developing capitula have the overall appearance of flower buds, and are sometimes called "flower buds" or "inflorescence buds" in technical and popular literature. Several gall-formers develop within developing capitula. These occupied capitula are referred to as galls, even when there is not an obvious external difference. There are usually at least some external clues (arrested development vs. nearby capitula, color change, change in texture, swelling, etc.), however.
non-gall - Any gall-adjacent plant symptom or other structure that doesn’t meet the definition of a gall: a novel element of a plant caused by an organism living within the plant. Examples of non-galls include scale insects; leaf curl, spots, or blisters caused by pathogens or external herbivores; and stem swellings caused by miners or borers lacking internal cells.
oak apple - A spherical or near-spherical gall with thin outer walls, a single central larval cell surrounded by either spongy tissue or fine radiating fibers.
pocket - A structure formed by pinching the leaf lamina together into a narrow opening (a point or line) and stretching it into various forms, from beads to sacks to spindles to long purses. The walls may or may not be thickened relative to the normal leaf.
rust - Plant deformations caused by fungi in the order Pucciniales. They cause swelling and curling of stems and petioles and blisters on leaves, easily recognizable for their bright orange coloration, seen in characteristic rings.
scale - An herbivorous insect of the superfamily Coccoidea. The post-reproductive females of the family Kermesidae have thin, globular, hollow shells fixed in place on their host.
stem club - A substantial enlargement of the growing tip of a woody plant, tapering more or less gradually from normal stem width below it, blunt or rounded above.
tapered swelling - An increase in the diameter of a stem, petiole, etc, gradual from either side of the gall.
witches broom - A dense profusion of buds or shoots on woody plants.