424 • The Ohio Naturalist. [Vol. Ill, No. 7, 



of the cells b}- the parasite. Since the parasite continues its 

 attack upon different cells and the plant makes the repeated effort 

 to heal the wound, we have the ver}- active production of cells. 

 The parasite making its attack upon one side of the leaf, causes 

 the unequal growth resulting in a cavity. The increase in size 

 of the gall causes a different tension upon the inner and outer 

 surfaces and results in the elongation of cells near the outer sur- 

 face as described in Part I. 



When the galls first appear they are single, but in a very short 

 time others are formed just outside the first, thus forming a 

 cluster. 



In ErincuDi anoinoluni (Figs. 47, 48, a, b), occurring on leaves 

 and petioles of walnut, we find a condition similar to that of the 

 Phytoptus galls except that the parasite is on a free surface 

 instead of in a partly closed cavity. I was able to secure a very 

 complete series of this gall. The first indication of the gall on the 

 petiole or rib of a leaf is the increase in the amount of parenchyma 

 tissue between the epidermis and fibro-vascular bundles. The 

 physiological character of this tissue is also changed to some 

 degree, since the cells are not so easily stained with haematoxylin, 

 have rather thick walls, and contain a considerable quantity of 

 tannin. The epidermal cells now begin to form trichomes (Fig. 

 47). The parenchyma tissue and trichomes both increase in quan- 

 tity, the walls of the cells become thinner (Fig. 48, a, b), and the 

 deeper parenchyma tissue gradually loses its tannin, while the 

 outer cells retain it in great quantities. 



These galls always occur over a fibro-vascular bundle and are . 

 apparently closely associated with them. These bundles become 

 modified to some extent. 



The origin and development of these galls is the same as in the 

 Phytoptus galls except that the parasite works upon the exposed 

 surface instead of in a cavity. The fact that one produces a 

 cavity lined with trichomes while the other produces a protuber- 

 ance covered with trichomes, is probabl}^ due to the fact that the 

 latter is so closely associated with the fibro-vascular bundle which 

 prevents the curvature but causes the rapidly-formed cells to 

 swell outward into a protuberance. 



2. GALLS OF THE APHIDIDAE. 



In the Aphididae galls we have a condition ver}- similar to that 

 just described for the Acarina galls except that the shape of the 

 galls are far more definite and they show a higher degree of 

 development. Trichomes are not so numerous and masses of cells 

 projecting into the larval chamber as described for Phytoptus 

 galls are very rare. In the youngest galls the cell structure of 

 the leaf is modified, resulting in the formation of a large number 

 of small, irregular cells, the same as in the Acarina galls. As the 



