INTERDEPENDENCE OF ORGANISMS 



41 



growth. Atypical pocket gall is shown on the witch hazel 

 leaf in fig. 3 2 . It is formed by the descent of the tissue 

 attacked to form a pocket upon the leaf blade: 

 the attacking insect is carried into the pocket, which 

 usually dilates, and forms a spacious chamber. The 

 covering gall, on the contrary, rises up around 

 the point of attack, and covers the insect over, leaving only 

 a small aperture at the top. The removal of a pocket gall 

 leaves a hole through the leaf: the removal of a covering 

 gall leaves only a superficial scar. 



Closed galls, as already stated, re- 

 sult from internal attack : the cavities 

 they contain lie wholly within the 

 plant substance. They likewise differ 

 among themselves in the degree of 

 their development. The simpler ones 

 (fig. 32c) have thin walls, of the ord- 

 inary tissues of the part bearing 

 them. The nucleated galls (fig. 32a) 

 show often a high degree of differen- 

 tiation of parts. There are often 

 three well defined layers in their 

 walls: an inner (when mature) very 

 hard layer forming the "nucleus" 

 whose cavity contains the gall maker, 

 an intermediate softer and more or less spongy layer, 

 and an outer hard layer, often protected with spines 

 and hairs and ornamented with beautiful colors. The 

 stone-like nucleus in the middle and the form and color of 

 the exterior greatly enhance the superficial resemblance of 

 the gall to a fruit.* 



Fig. 33. Compound gall 

 on the root of wild let- 

 tuce {Lactuca spf) 



*It is to be noted in passing, that the gall when fruit-Hke almost 

 invariably resembles the fruit of some kind of plant other than the 

 one that bears it. 



