Abnormal Growth 289 



characteristic appearance, and their presence often indicates the origin 

 of a meristem in the callus. 



The structure and arrangement of the cells in such wound tissue as 

 callus are often atypical, especially near the wound itself. The cells twist 

 and turn, and so-called "whorls" thus formed have been described by 

 Maule (1896), Neeff (1914), and others. Krieg (1908) observed several 

 concentric circles of cambium in the pith of ringed branches of Vitis 

 adjacent to a wound. In the outer ring, development was inverted, phloem 

 being formed toward the inside and xylem toward the outside. 



Fig. 11-9. Callus on cut stem of Cleome produced by application of growth substance. 

 ( Courtesy Boyce Thompson Institute. ) 



Save under particular conditions, a callus does not remain callus 

 indefinitely but tends to produce normally organized structures again. 

 This it does by means of new apical meristems, both of shoots and roots, 

 which frequently appear in it. Such meristems arise in several ways. Cells 

 abutting on nests of tracheids may produce a meristem in the form of a 

 hollow sphere. Others may appear elsewhere in the callus or where it is 

 in contact with the pith, cambium, or cortex. These meristems may form 

 secondary wood or phloem. Meristems of roots tend to arise well below 

 the surface and those of shoots either at or just below the surface. From 

 such meristems typical organs may be regenerated in any region of the 

 callus, thus showing that there have been no fundamental changes in 

 the genetic character of the callus cells themselves. Their potencies to 

 produce typically organized structures have been masked but not lost. 



