412 Morphogenetic Factors 



and thus to gain a clue as to how these effects are produced. These sub- 

 stances vary considerably. Having studied many such compounds, 

 Koepfli, Thimann, and Went (1938) concluded that the minimal struc- 

 tural requirement for a substance to stimulate growth, at least in the 

 pea test for auxin, is to possess an unsaturated ring system with a side 

 chain adjacent to a double bond in the ring, and with a carboxyl group 

 in the chain separated from the ring by a carbon atom. Thimann (1957) 

 points out that there are a considerable number of biologically active 

 compounds which do not have this structure and at least one that does 

 not even contain a ring. It seems doubtful that an understanding of 

 the mechanism of action, either of auxin or similar synthetic substances, 

 will be gained by a knowledge of their chemical structure without an 

 equal knowledge of the reacting systems that they stimulate. The general 

 question of the chemistry and mode of action of plant growth substances 

 was discussed at the Wye College symposium (Wain and Wightman, 

 1956). 



Just how growth substances produce their morphogenetic effects is 

 not well understood. The first visible result of auxin action is a speeding 

 up of protoplasmic streaming, indicating that some aspect of metabolism 

 is being accelerated. The marked influence of auxin on growth also sug- 

 gests this, since growth requires the release of energy. Some physiologists 

 believe that auxin acts as a respiratory coenzyme and thus has an im- 

 portant share in the respiratory cycle. No enzyme has yet been found, 

 however, that can be activated by auxin in physiological concentrations. 



Since what appears to be the primary effect of auxin is cell enlarge- 

 ment, it seems plausible to conclude that water uptake is controlled by 

 it, and there is some evidence for this. The suggestion has been made 

 that auxin increases the osmotic concentration of the cell sap and thus 

 increases cell size. Cell growth may take place, however, even with a 

 decreasing osmotic concentration. Burstrom (p. 41) believes that cell 

 enlargement is not primarily caused by water uptake. 



Interest at first focused on auxin-induced changes in the cell wall as 

 related to growth, and Heyn (1940) has reviewed the evidence that 

 auxin directly increases the plasticity of the wall and thus its irre- 

 versible extensibility. Some workers believed that the effect of auxin 

 was indirect and only through its influence on the cytoplasm. Recent 

 studies, however, support Heyn's view. Thus Cleland and Bonner ( 1956 ) 

 present evidence that auxin directly induces a loosening of the cell-wall 

 structure and thus a relaxing of wall pressure, which makes possible an 

 expansion of the cell. The effect of auxin is independent of cell expan- 

 sion. Auxin may affect the wall by altering pectin metabolism. 



The relation of growth to protoplasmic viscosity and to the swelling 

 capacity of cell colloids suggests that auxin may have something to do 



