MORPHOLOGY OF HIGHER PLANTS. 335 



brooms." Galls exhibiting strange forms are also produced by 

 the Gymnosporangia on the stems of the common juniper, the 

 leaves of the mountain ash, etc. In this group would also be 

 included the CROWN-GALLS occurring on a large number of plants, 

 as grapes, peach, juniper, and field crops. At one time it was 

 thought that these galls were due to frosts or mechanical injuries. 

 The extended researches of Smith (Bulletin No. 213, Bureau of 

 Plant Industry, U. S. Department of Agriculture) have -shown 

 that crown-galls are in the nature of bacterial diseases. He 

 showed that crown-galls not only resembled malignant animal 

 tumors, especially sarcoma, but demonstrated that this resemblance 

 was more than superficial. II. ZOO-CECIDIEN, or those galls which 

 are formed within the body of the plant and due to attacks by 

 insects. This group includes by far the larger number of galls, 

 and is further subdivided according to the various animal-groups 

 causing them. 



Galls differ in structure, but the general nature of the anat- 

 omy may be seen in a study of the common " ink ball " or " ink 

 gall," formed on Quercus coccinea by Cynips aciculata. These 

 galls are produced during the summer months on the young 

 branches and sometimes on the acorns. When mature they fall 

 from the trees and are nearly globular in shape, varying from 20 

 to 30 mm. in diameter. They are solid throughout and of the 

 consistency of the pulp of a green apple. Externally they are 

 smooth, and are colored a mottled green, yellow, or brownish- 

 red. At this stage they are made up of three distinct zones: 

 (i) A central area, made up of nearly isodiametric starch-bear- 

 ing, parenchymatous cells. (2) The middle zone is composed 

 of radially elongated parenchymatous cells, possessing thick, 

 porous cellulose walls containing a lining of protoplasm and a few 

 starch grains. With the development of the egg of the insect 

 there also appear in the cells of this middle zone numerous starch 

 grains closely resembling those found in the central zone. (3) 

 An external layer made up of irregular parenchymatous cells, 

 somewhat collenchymatic in character, with a lining of protoplasm 

 as in the cells of the middle zone. 



In some studies on the origin of tannin in galls Kraemer (Bot. 

 Gas., 1900, p. 275) showed that in the " ink gall " there are three 



