Herbicides 279 



entiated are much the most responsive to auxin applications. The 

 changes in susceptibihty of the plant from one stage of development 

 to another may be due in large part to differences in growth rate 

 and the associated proportion of young, weakly differentiated tissues 

 in the plant. Thus seedlings, which are almost universally highly 

 susceptible, are made up primarily of meristematic and young expand- 

 ing tissues with a rapid growth rate. When flowering has been com- 

 pleted the plant is generally quite resistant to auxins and is pri- 

 marily made up of mature, well differentiated tissues in a static 

 condition with respect to growth. 



Changes in growth rate forced upon the plant by environmental 

 factors may greatly alter the susceptibility to auxin herbicides. Der- 

 scheid (1952) has pointed out that differences in susceptibility in some 

 instances may be more accurately correlated with growth rate than 

 with stage of development. It has commonly been observed that plants 

 experiencing a condition of very low moisture availability are much 

 less susceptible to auxin herbicides than plants with normal moisture 

 and experiencing normal growth (Erickson and Gault, 1950). It has 

 also been observed in several instances that plants suffering from 

 nutrient deficiencies are likewise less susceptible than normal plants 

 (Freiberg and Clark, 1952). These and other factors in the environ- 

 ment which alter growth rate may likewise alter susceptibility to 

 herbicides. 



Environmental Factors 



The effects of various environmental factors on auxin effective- 

 ness have been discussed in chapter VI, bringing out the particular 

 importance of light, temperature and nutritive status of the plant. 



Morphological Factors 



A fourth category of factors of susceptibility relates to the 

 morphology of the plant. The greater susceptibility of dicots compared 

 to monocots has been asserted to be partly attributable to the fact 

 that the buds of dicots are in a more exposed position morphologically 

 and hence more easily damaged by the spray. Roberts and Blackman 

 (1950) have pointed out that leaf shape can have a considerable in- 

 fluence on susceptibility, particularly with respect to leaves which are 

 cupshaped at the base permitting accumulation of the auxin solution 

 over the tender buds at the axils of the leaves. Experiments with 

 several varieties of peas are cited in support of this suggestion. This 

 has been at least partly discredited by Willard (1947) who states that 

 he has filled the cup-like leaf sheaths around the apical bud of a corn 



