The phylogenetically ancient standing of the gall characters of these cynipids is evidenced in the internal structures of the deformations. The agamic galls of all of the species of the genus are produced on the veins of the leaves of white oak, with the single exception of Cynips heldae, which occurs either on leaf veins, petioles, or young stems of the oak. In most instances the galls appear on the under sides of the leaves. Beyerinck's figures of Cynips folii (re-drawn in our figs. 113-117) show the order of transformation of the normal fibrovascular tissue, and indicate something as to the plant elements involved. Other European students of gall histology have included species of Cynips in their investigations. There is the work of Lacaze-Duthiers (1850-1853), Fockeu (1889), Hieronymous (1890), Küster (1900, 1911) and Weidel (1911). Following the suggestion of Lacaze-Duthiers, all these workers have found four fundamental zones of tissue in most cynipid galls. These zones have been called the nutritive, protective, parenchyma, and epidermal layers, and in the degree and character of the development of each of these the European students have seen an essential uniformity of structure among the European species of Cynips.
Unfortunately, most of these histologic studies were made on three common European Cynips: folii, longiventris, and divisa. Only Weidel (1911) has given us a discerning study of Cynips disticha (fig. 123), and there he recognized five layers of tissue instead of the traditional four. My own studies of the gross anatomy of the galls of the entire genus Cynips, summarized in figures 117 to 124, lead me to believe that Weidel's five layers are the correct basis of homology in this genus.
If it is remembered that Beyerinck's studies (figs. 113-117) show that these leaf galls originate in the phloem of the fibrovascular bundle, from which they develop outwardly usually thru the lower epidermis of the leaf, the following interpretations will seem warranted:
