278 The Phenomena of Morphogenesis 



evolution. Heslop-Harrison lists three causes for such phenomena: ab- 

 normalities in growth, of little morphological significance; abnormalities 

 in development, owing to failure of hormonal systems or other form- 



Fig. 11-1. Sepal phyllody. Sepal 

 of a rose flower showing abnor- 

 mal development into a structure 

 much like a foliage leaf. (From 

 T. E. T. Bond.) 



determining factors; and minor abnormalities arising from genetic or 

 environmental causes. Some teratological changes may be reversionary 

 and some progressive. 



Fig. 11-2. Left, normal tendril of Cucurbita. Right, abnormal structure intermediate 

 between a tendril and a foliage leaf. ( From Worsdell. ) 



There are various categories of these abnormal structures, or tera- 

 tomata (Figs. 11-2, 11-3), which are only loosely related to each other. 

 About certain of them a considerable literature has grown up. In many 

 cases the causal agent is known, but often it is not. Little developmental 

 work has been done on most of them. 



