GALLS 



143 



plants (e.g. those of the Finger and Toe disease, Fig. 135, or those 

 produced by many Rust Fungi), and externally they commonly 

 present an irregular and very variable form. Many animal 

 galls offer a striking contrast in these respects, not only ex- 

 hibiting a definite external structure, but, as in many of those 

 produced by gall-flies, a complex internal differentiation. In 



Fig. 70. — Pliotograph of galls (g.) on leaves of Dogwood 

 (Coruiis sanguinea). [Photo E. J. S.] 



some of these cases the central region, where the larva of the 

 gall -insect resides, consists of nutritive tissue composed of cells 

 with dense cytoplasm, rich in protein material, and serving for 

 the animal's nourishment. This region is in some cases sur- 

 rounded by a la3'er of sclerenchj'ma, whilst the outermost zone 

 consists of parenchymatous tissue. The latter is usually well 

 supplied with intercellular spaces, contains little or no chloro- 

 hyll, and is oiten traversed by vascular tissue. 



