Abnormal Growth 277 



dents of plant morphogenesis since by the study of these unusual struc- 

 tures development may be examined at various levels and degrees of or- 

 ganization. Relatively little work has been done in this field, however, 

 and most of the results are descriptive and relate to mature structures. 

 In the few cases, such as crown gall, where many experimental and 

 developmental studies have been made, these have proved to be very 

 rewarding. 



There is some confusion between the concepts of "pathological" and 

 "abnormal" growth. Pathology is concerned with questions about the health 

 and survival of the organism when it is attacked by parasites or sub- 

 jected to unfavorable conditions. Abnormal growth is often produced 

 by this means though here, again, it is difficult to draw the line. One would 

 hardly call the aecium of wheat rust an abnormal growth, but a crown 

 gall certainly is one. There are many cases of abnormal growth, on the 

 other hand, which clearly are not pathological, such as inherited fascia- 

 tions or the root tubercles of legumes. A student of abnormal growth is 

 not concerned with the health of the plant, nor does a pathologist ex- 

 amine primarily the ways in which the plants with which he deals 

 diverge from the norm. Historically, however, the two fields have been 

 close together, and Kuster's (1925) classic book on abnormal growth is 

 entitled "Pathological Plant Anatomy." 



There is no very obvious way in which to organize the widely various 

 phenomena of abnormal growth. It will be most logical, perhaps, to 

 proceed from cases where divergence from the norm is relatively slight 

 and move to those where it is more extreme. In the present chapter 

 there will be discussed (1) the abnormal development of organs be- 

 longing to the usual categories, (2) the production of new types of or- 

 ganized structures, and (3) the production of amorphous structures. 

 The whole field has been briefly reviewed by Bloch ( 1954 ) . 



ABNORMAL DEVELOPMENT OF ORGANS 



In many cases structures still recognizable as leaves, stems, roots, flow- 

 ers, or other organs have been modified in many ways, sometimes very 

 radically. This is the field of teratology, the study of malformations, 

 freaks, and monstrosities, which has long excited the curiosity of bot- 

 anists (Moquin-Tandon, 1841; Masters, 1869; Worsdell, 1915; Penzig, 

 1921; and Heslop-Harrison, 1952). Little but descriptive work has been 

 done on most of them. For a long time their scientific value was chiefly 

 to morphologists, who looked to malformations for evidence as to the 

 morphological nature of certain organs. Thus the "metamorphosis" of 

 petals and sepals into leaf -like structures (phyllody; Fig. 11-1) suggests 

 that they are really leaves but have been modified in function during 



