486 



PLANT GROWTH AND PLANT COMMUNITIES 



Figure 15. A. -J. Normal embryogeny in the carrot (reproduced from Ward- 

 law, 1955). K.-Q. Drawings of cells and cell colonies grown from free cells 

 of carrot phloem by the aid of coconut milk. 



logically similar materials, the free cells will tend to behave like 

 zygotes. These facts are reminiscent of certain cases of spontaneous 

 apomixis in which a given cell, despite its location in the plant body, 

 begins to grow and behave like the equivalent of a zygote. 



Thus the entire, organized, integrated living cell may have in- 

 herent but often undisclosed potentialities— i.e., totipotency. The way 

 in which these potentialities are suppressed ( as in much normal tissue 

 differentiation ) or evoked ( as in the responses of the free cells in cul- 

 ture) is presumably determined by the action of growth-regulating 

 mechanisms and compounds. These substances mediate their effect by 

 controlling growth, as it occurs by cell division and by cell enlarge- 

 ment, and thus determine by their interactions the patterns of develop- 

 ment that ensue {cf. Steward and Mohan Ram, 1960). Since these 

 controls or factors are essentially extra-nuclear, they are equivalent to 

 what Waddington ( 1957 ) has termed epigenetic factors or effects— 

 i.e., factors that override the fixed effects of normal genetic constitution. 

 To express these ideas we really need a term which is noncommittal as 

 to the mode of action and is free from the connotations of the existing 

 terms gene, hormone, and vitamin. This term should imply that the 

 substances or mechanisms in question can, within genetically fixed 

 limits, modulate the course or behavior of the cells or organisms, much 

 as the predetermined path of a vessel or a projectile may be modified 

 by suitable controls or instructions issued en route.* 



" A Latin word "moderamen" described the arrangements to steer or control, 

 as in a vessel or chariot. So one could call all these controls which are exercised 

 over the living system as "moderaminous" and the substances through which they 

 are mediated as "moderaminants." The scope of such a concept could embrace 

 effects otherwise somewhat arbitrarily assigned to a role as vitamins or hormones, 



