Abnormal Growth 287 



the tissues that compose them are really to be considered as "new" 

 structures, morphologically different from the familiar categories. Cer- 

 tainly they have cells and tissues unlike any normally found in their host 

 plants. Such are the various hairy projections on the surface of certain galls, 

 the opening mechanisms, the mechanical tissues, and others concerned 

 with nutrition and aeration. As to whether these are "new" or not depends 

 on our definition of that term. A given morphological category, such as 

 the leaf, presumably has a continuous evolutionary history beginning 

 with early vascular plants. It is part of the norm of plant structure. In 

 this sense such galls are certainly new. They have arisen, however, be- 

 cause of a novel factor in the plant's environment-the gall wasp. It seems 

 probable that if such wasps had existed in the Paleozoic they would have 

 induced galls in the vascular plants of that era. Gall-making ability on 

 the part of the wasp is advantageous to it and doubtless has been devel- 

 oped by selection, but the gall response by the plant to the parasite is not. 

 Presumably the capacity for producing these galls has long been among 

 the developmental potencies of plants. It can be regarded as new histori- 

 cally but not morphologically. 



From the morphogenetic point of view the most important fact about 

 these prosoplasmatic galls is that they are highly organized and specifi- 

 cally formed structures induced by an outside stimulus. Here we can see 

 the process of form determination manifest in a simpler fashion than in 

 normal development, for here the inducing agent is not a part of the 

 developmental mechanism but is introduced into the organism. If we 

 understood exactly how these galls are formed, we should doubtless gain 

 some important clues as to the morphogenetic process generally. Some- 

 thing more is involved here than partial loss of organization, the change 

 that takes place in most other galls. Here is operating a constructive, not 

 a degenerative, process. 



AMORPHOUS STRUCTURES 



In the two previously discussed categories of abnormal growth the 

 original organization was either present in altered form or something 

 entirely different from the normal was produced. There is a third group 

 in which organization at its highest level, with the production of specifi- 

 cally formed structures, is inoperative to a large degree, and only form- 

 less, or amorphous, structures are developed. Within these structures 

 there may be some histological differentiation but it is much less than in 

 normal tissue. The cells remain alive, however, and perform many normal 

 physiological activities. They must still possess a certain amount of organ- 

 ization, evident in regulatory action. If this were not so, death would 

 ensue. 



