290 The Phenomena of Morphogenesis 



Structures much like calluses may be induced by other factors than 

 wounding, especially by various chemical substances. Among these sub- 

 stances are ether, chloroform, camphor, ethylene gas, liquid paraffin, and 

 especially various growth substances (p. 407). Many are nonspecific in 

 their action, and it is to be assumed that their effect is primarily one of 

 injury to the tissues, which produces changes in the cells and thus leads 

 rather indirectly to atypical growth reactions. Others, especially the 

 growth substances, produce rather specific reactions. 



One of the properties of the synthetic growth substances (such as in- 

 doleacetic, indolebutyric, and naphthaleneacetic acids and paraffin) is 

 their action in stimulating an increase in the number and the size of cells 

 to which they are applied. Sizable overgrowths and calluses may thus be 

 formed on various plant organs if sufficiently high concentrations of these 

 agents are applied; and if this treatment is repeated, tumor-like masses 

 are produced (Schilling, 1915; Brown and Gardner, 1936; Kisser, 1939; 

 Levine, 1940; and others ) . These often resemble the ones associated with 

 certain bacterial infections. 



Callus-like overgrowths are more readily obtained if the paste in which 

 the inducing substances are carried is applied to the more sensitive 

 regions such as those near the shoot tips, particularly after decapitation. 

 Many experiments have been performed, especially on herbaceous dicoty- 

 ledons, to test the effects of various growth substances on development 

 (see especially the publications from the Boyce Thompson Institute and 

 those of E. J. Kraus and his colleagues at the University of Chicago, 

 p. 405). Differences have been found among the growth substances in the 

 character and extent of abnormal growth they induce, and their effects 

 are also related to the kind of plant, its age, and the region treated. His- 

 tological study of these tumors shows that they resemble other calluses 

 and wound tissues, especially in the absence of a constant form or size, 

 the presence of some cellular differentiation as the tumor ages, and the 

 development on them of root and shoot primordia. 



Amorphous Calls Produced by Parasites. Amorphous structures which 

 in certain respects resemble intumescences, calluses, and chemically in- 

 duced tumors are caused by various parasites such as nematodes, mites, 

 insects, fungi, and especially bacteria. Even viruses are now known to be 

 involved in their production (Black, 1949). There is a great variety of 

 these, from small, simple structures to large and relatively complex forms. 

 Some of the huge burls on trees are due to parasites but others apparently 

 are caused by mechanical or other nonparasitic factors. Many amorphous 

 galls have a somewhat more highly organized character than others but 

 they show little constancy in size or form and their histology is less 

 regular and their differentiation simpler than in normal structures. Kiister 

 describes and figures many of these (1903a, 1911, 1925). He gave 



