Eriophyes sp.
Chadwick's No. 65

Host Fagus grandifolia Ehrh.

This gall occurs on the under side of the leaves of the host plant. It is an erineum, white and frost-like at first, turning to a brown colour at a later stage. The anatomical structure of the interior of the leaf remains perfectly normal, the effect of the stimulation being confined to the epidermis. This produces a large number of unicellular, capitate hairs, resembling miniature mushrooms, the resemblance to which is the more striking since the stalks of the hairs are bulbous at the base, as shown in Fig. 3. The hairs on the normal leaf are long, acicular and unicellular. As this gall is produced with no increase in the number of cells in the leaf, anatomically it stands as a simple type of "hypertrophy", the remaining forms all exhibit the more advanced phenomenon of cell proliferation.

- A Cosens: (1912) A contribution to the morphology and biology of insect galls ^©

Reference: https://www.biodiversitylibrary.org/item/99818#page/13/mode/1up