Page:The Gall Wasp Genus Cynips.pdf/121

, and now migrates by way of the canal into the developing gall. The empty egg is left in its original location in the xylem, the cavity in which it lies is gradually filled with a scar tissue (the development of which may be the means of forcing the larva to migrate thru the canal), the canal itself is closed at its base by the newly developing tissue, and the insect is thus enclosed in a larval cell in the heart of the gall.

Once having broken the leaf epidermis, the galls of folii remain attached to the leaf only by a very slender pedicel which, however, may contain a score of vascular bundles. The cytology of the mature gall has been described by Beyerinck (1883), by Hieronymus (1890), Fockeu (1889), and Weidel (1911), and its four zones of tissues are evident with a low power lens. These layers are: (1) An epidermis with few stomata, altho there are some stomata (contrary to the statements of some writers) on the basal two-thirds of the galls; (2) a parenchyma which has numerous, intercellular air spaces toward the outside and a much-branched, air-filled, and in the older gall a spongy parenchyma toward the center of the gall; (3) a protective zone of compacted tissue which, however, is not so well developed as it is in Neuroterus; and (4) a nutritive zone which is well developed, being formed of rounded cells that contain a granular protoplasm and droplets of oil. The nutritive zone lines the inside of the larval chamber and is absorbed, as with other Cynipidae, by the larval folii without the development of waste of any sort that can be detected within the larval cell. The collenchyma, which lies between the epidermal and parenchymal layers in most galls of Cynips, is absent in folii.

The mature galls of folii may weigh (acc. Trotter 1909) as much as 4.84 g. and average (acc. Kieffer 1901) between 2.0 and 3.0 g. This may be as much as twice the weight of the leaf which bears the gall, and Kieffer found that one leaf which bore 16 galls was burdened with 18 times its own weight. Most of the weight of these fresh galls is, of course, due to the water they contain, but because of the scarcity of stomata on the epidermis of the galls, they dry out very slowly. Trotter (1909) gives the following data on this point:

A fresh leaf weighing 0.70 g. dries in 9 days to 0.30 g.

A fresh gall weighing 4.84 g. requires 120 days to dry to 0.60 g.