236 



G. E. Blackman 



0.4 



0.2 



32 



Fig. 2. The course of uptake of 2,4-D from a solution containing 1 mg/1 by the 

 roots of intact plants of P. sativum, H. annuus, G. hirsutum, and L. usitatissimum. 



It is apparent that the basic pattern is the same: The plants con- 

 tinue to accumulate 2,4-D over the 32 hrs. For each species absorp- 

 tion is most rapid in the first hour, but the subsequent fall in the 

 rate is dependent upon the species. With Hordeum and Avena there 

 is a second phase when uptake comes to a stop, followed by a third 

 phase when it recommences. There is some suggestion that this in- 

 terruption of absorption may also take place with Oryzn. 



Figure 2 contains the results for four dicotyledonous species: Pi- 

 sum sativum, 'Alaska'; Helianthus annuus, 'Pole Star'; Gossypium 

 hirsutum, 'Samarus 26J'; and Linum usitatissimum, 'Royal.' The 

 first three species are all well known to be killed or severely injured 

 by field applications of 2,4-D which are innocuous to the four species 

 of Figure 1, while 2,4-D, as long as the dose is limited, is widely em- 

 ployed in the United States for selective weed control in Linum. It is 

 therefore noteworthy that the course of uptake of the one resistant 

 species is quite different, following closely the pattern of Hordeum 

 and Oryza. 



The pattern of the three susceptible species only varies in the 

 time that elapses before uptake changes from a positive to a negative 

 rate. In parentheses it should be recorded that in the case of Brassica 

 alba, another susceptible species, uptake ceases within 4 hrs., but 

 there is no subsequent egress of 2,4-D into the external solution. 



