Galls by Gall- Flies. 103 



cambium produces the wood parench3^ma ; and in these 

 galls we have similarly a soft peripheral zone of sappy 

 parenchymatous cells, and a hard central zone of wood 

 parenchyma : and in all bark galls this woody base 

 penetrates more or less deeply into the ligneous tissue 

 of the tree, while the soft fleshy periphery always 

 projects from the bark. 



The new cell-growth is arranged in regular concentric 

 layers around the larva, and is accompanied by certain 

 changes in the cell-contents ; those cells lying next the 

 larva swell, the cell-contents become cloudy, and exhibit 

 a multitude of starch granules. 



The rudimentary gall draws its first nourishment 

 from the surrounding tissues ; later it acquires greater 

 independence, for a new element comes to assist in 

 its further development. From the spiral vessels lying 

 in the cambium-ring processes are driven into the 

 rudimentary gall ; and the entrance of these vessels 

 always occurs at a definite spot on the lower surface 

 of the gall, whether the connexion with the parent 

 tissue be by a broad base or by a small stalk. 



The gall has now become an independent structure, 

 and is practically withdrawn from the direct influence 

 of the cellular area which surrounds it, and from which 

 it sprang. 



Its individuality of organization is now displayed 

 by most complicated transmutations of cells originally 

 morphologically alike. This is especially seen in the 

 peripheral cells, which, by assuming peculiar pigments 

 or by developing hairs of various kinds, exhibit an 

 enormous variety of forms. It is still a moot point 



