196 PLANT GROWTH SUBSTANCES 



enough. This conception is unquestionably true. Nevertheless, the dif- 

 ferences in response of plants within certain limits of dosage, time of 

 application, and environment are large enough to make possible the 

 killing or severe injury of certain undesirable species in the presence of 

 crop plants. Generally the monocotyledons have been more resistant 

 to 2,4-D than the dicotyledons, but there are sevefal notable exceptions 

 in each group. Among the monocotyledons certain strains of the bent 

 grasses and buffalo grass have proved susceptible. Among the leguminous 

 crops Buchholtz (4) found red clover more tolerant than alfalfa, and 

 Willard and Shaw (48) found Ladino white clover and common white 

 clover more tolerant than alfalfa, red clover, sweet clover, alsike clover, 

 and lespedeza although all species could be classed as relatively susceptible. 

 Because of their susceptibility to 2,4-D, the spraying of legumes with 

 this herbicide has commonly been considered too hazardous, but thou- 

 sands of acres of flax, also a dicotyledon, have been sprayed successfully. 



Marked varietal differences in response to 2,4-D both in mono- 

 cotyledons and dicotyledons have been observed by many investigators. 

 Dunham and Tandon (16) found large varietal differences in tolerance of 

 flax to 2,4-D. An application of four times the amount that reduced the 

 yield of Crystal and B5128 did not injure Redwing. Similar or even 

 larger differences exist in corn. Buckley (7), Holden et al. (26), Jugen- 

 heimer et al. (27), Lee (31), Leng and Slife (33), Miller (37), Rossman 

 and Staniforth (41), and Viehmeyer (46) found wide differences in re- 

 sponse to 2,4-D among inbreds, among single crosses, and among double 

 crosses. Elder and Davies (18) reported varietal differences in sorghum. 

 Derscheid et al. (12) found no significant reduction in yield among nine 

 oat varieties but did find differences among barley varieties. Seven 

 varieties of spring wheat responded in a similar manner to 2,4-D accord- 

 ing to Helgeson et al. (25). Sexsmith (42) noted no differences among 

 six varieties. Albrecht (2) concluded that bent grass strains vary con- 

 siderably in their tolerance to injury from 2,4-D. 



Some environmental effects. — Differences in reaction to 2,4-D may be 

 very marked among varieties and strains but they are influenced by the 

 dosage, the time of application, and environmental factors. Dunham 

 and Robinson (15) sprayed ten varieties of flax with 4 oz. of triethanola- 

 mine and 1.3 oz. of butyl ester per acre and reported that all varieties 

 responded aUke at these low dosages. Dunham (13) states that varieties 

 of flax differ most widely in their response to 2,4-D: (a) when the ester 



