R. S. DUNHAM 197 



is used, (b) when rates of amine or sodium salts are heavier than recom- 

 mended, and (c) when apphcations are made in bud and bloom stage. 

 There is evidence also that both temperature and sunlight are influencing 

 factors (28,34,47). Arakeri and Dunham (3) studied the relation of some 

 environmental factors to the pre-emergence treatment of corn with 

 2,4-D and concluded that out of all the factors studied, the most 

 important were water, soil type, pH, and organic matter content of the 

 soil; less important was time of application, and least important were 

 depth of planting and dosages. 



Stage of growth and selectivity. — The stage of growth of a plant at the 

 time it is sprayed has been recognized as an important factor by many 

 investigators. Commonly it has been reported in terms of height. More 

 recently an attempt has been made by Dunham et al. (14) to describe 

 the stage of small grains and flax in terms of morphological development. 

 The "tiller," "shooting," and "boot" stages of wheat and the cotyledon, 

 true leaf, and stem elongation stages of flax are illustrated. 



Derscheid (11), in summarizing the abstracts of 27 investigators who 

 worked with 2,4-D on spring wheat, oats and barley, states: "These 

 data indicate that all three crops are less tolerant at early 3-leaf and 

 5-leaf stages of growth than after they have become fully tillered. 

 The most susceptible period, however, is when heads are emerging from 

 the boot." Elder (17) reviewed the data on winter wheat and states that 

 it "is more resistant to 2,4-D in early spring when fully tillered or in 

 early joint stage and more susceptible when treated soon after planting 

 in the fall months. The boot to heading stage is a susceptible period." 

 Dunham et al. (14) state that information relative to flax is not so 

 clearly established as for the small grains, but apparently the most 

 susceptible time is from bud formation to bloom. They also advise the 

 farmer to avoid spraying when the stem is rapidly lengthening. 



There is increasing evidence that susceptibility is closely related to 

 periods of rapid growth in the plant. Conflicting results with corn when 

 described in terms of the height of the plant can be explained on the 

 basis of rapidity of growth. Lee (30) reports that: "Experiments con- 

 ducted in both 1947 and 1948 indicate that small corn is more susceptible 

 to damage than larger corn. This year (1948) in Indiana large corn, 

 12 inches or more in height, was more easily damaged than small 

 corn. The reason was apparently because of the diff"erence in growing 

 conditions." 



