sorption by the other plant tissues) to a thin, broken layer of shrunken, partially empty cells. Poorly developed in any but the very youngest galls of Acraspis. Possibly directly descended from phloem.
2. Protective layer. A sclerified tissue that is best developed in the European sub-genus of Cynips. The cell walls are thickened, and the cells may contain crystalline materials. The larval cell wall of most cynipid galls is largely made up of protective layer to which the remnants of the nutritive layer are attached on the inside and some spongy parenchyma tissue on the outside. The protective layer may be a direct development from sclerenchyma tissue in the vein. Apparently absent in Acraspis.
3. Spongy parenchyma. Occupying the central portion and constituting the major material in all the spongy and more hollow oak apple galls of this genus. Poorly developed in the subgenus Antron and absent, as far as I can see, from the galls of the subgenus Acraspis.
4. Collenchyma. Lying directly beneath the epidermis. A second layer in which the cells have thickened walls and usually crystalline contents. The layer appears hard and compact-crystalline to the naked eye. Practically absent (by an unfortunate coincidence) from the three species on which the first European studies were based, but present in most other species of that subgenus and in the other subgenera of Cynips. Constituting the bulk of the material in the galls of the subgenus Acraspis, and well-developed as the compact outer layer of Antron. Cook (1904) and Cosens (1914) treated this layer in certain species of Acraspis as modified parenchyma, but this seems to be an attempt to maintain the four layers of the European workers. This collenchyma layer in the gall may be developed from collenchyma to be found in a similar position in the normal leaf.
5. Epidermal layer. The outer covering of the gall, including the fairly normal epidermis and all of the abnormal developments from it. Largely naked or at the most with stellate hairs in most groups of Cynips. With a peculiarly faceted surface in many species of Acraspis, in some cases with each facet terminated by a unicellular process which may be spiny or long and wool-like. Obviously a modification of the normal leaf epidermis.
The precise homologies of these tissues must be made by some botanist using modern technic. It will be interesting to compare structures in galls of some of the species which occur either on upper and under surfaces of leaves, on veins, petioles, and (as in heldae) on young stems.
The distribution of the five types of gall tissues among the species of Cynips may be summarized as follows:
