Factors Altering Effectiveness 147 



classical polar transport of auxin. As in the polar system, velocity is 

 independent of auxin concentration in the carbohydrate-linked trans- 

 port (Day, 1952). It is different from the polar transport in that (a) it 

 is dependent upon the concurrent translocation of carbohydrates, (b) 

 it is at least ten times more rapid than polar transport, and (c) it is 

 not polar in direction. Indigenous growth hormones are translocated 

 principally by the polar system, whereas auxins applied at high con- 

 centrations are translocated principally by the carbohydrate system, 

 unless they are applied in an oil carrier. 



THE STATUS OF THE PLANT 



Stage of Development 



A clear demonstration that the age of the plant at the time of 

 treatment has a very strong bearing on the effectiveness of an auxin 

 application has been brought forward by Blackman (1950). In a 

 series of studies using 2,4-D and 2-methyl-4-chlorophenoxyacetic acid, 

 he found that there are strong differences in herbicidal toxicity asso- 

 ciated with the stage of development of the plant and the auxin 

 applied. In the discussion of the anatomical effects of auxins in chap- 

 ter V, we pointed out that cells which are less differentiated in de- 

 velopment are the most responsive to auxin applications. On this 

 basis, one would expect that plants which naturally contain a high 

 proportion of cells in a weakly differentiated condition would be most 

 susceptible to auxin applications. This is in fact the case. It seems 

 generally true that germinating seedlings, which are almost entirely 

 made up of weakly differentiated cells, are more sensitive than at later 

 stages of growth. This factor contributes to the selectivity of auxins for 

 germinating weed seedlings in a standing crop. 



Experiments on the toxicity of 2,4-D for crop plants established 

 that young plants of cabbages, soybean and tomato could be killed 

 by concentrations that had little or no detrimental effect on older 

 plants of the same species (Weaver et al, 1946). 



Blackman (1950) has found that other stages in the development 

 of the plant may show renewed sensitivity to auxins. The boot stage 

 of cereal grains is a particularly susceptible stage (Olson et al, 1950). 

 The flowering stage of Hoary Pepperwort is a particularly susceptible 

 stage to 2,4-D (Blackman et al, 1949). The excess of weakly differen- 

 tiated cells in the plant is not entirely responsible for the degree 

 of susceptibility, however, for in some instances a slow-growing stage, 

 such as the vegetative rosette of dandelion which occurs in the fall, 

 is a particularly susceptible stage to 2,4-D applications. 



