Physiological and Anatomical Effects 125 



of roots are located over the vascular strands, presumably because of 

 concentration of the auxin in the strands. 



Auxins were thought only to inhibit root growth until it was 

 found tliat the optimum auxin level for root grow'th is very low in- 

 deed (Boysen-Jensen, 1936). Promotions of root growth with auxin 

 have been obtained in two ways: (1) The exposure of roots of some 

 species to very low concentrations of auxins can sometimes promote 



Fig. 56. Root abnormalities after treatment of bean roots with 2,4-D showing prolifer- 

 ation of lateral root primordia (redrawn from Wilde, 1951). 



growth (Thimann, 1936; Moewus, 1949; Naylor and Rappaport, 1950). 

 (2) The exposure of roots to auxin solutions for only a brief period 

 can sometimes promote growth (Thimann and Lane, 1938). The basic 

 requirement for stimulation by auxins seems to be that roots be used 

 which do not already have optimal or super-optimal auxin concentra- 

 tions. This can be done by selecting roots which are either very young 

 or somewhat senescent (Pilet, 1951; Moewtis and Moewus, 1952). 



Flowers and Fruits 



Auxins can exert strong influences on the process of floral initia- 

 tion, on setting of fruits, and on subsequent fruit growth. 



In the preceding chapter it was pointed out that increasing the 

 auxin content of a plant at the time of flower initiation can either 

 promote or inhibit the process. The addition of auxins to pineapple 

 plants can actually cause floral initiation. The application of these 

 influences upon initiation are discussed in chapter XV. 



