392 M or pho genetic Factors 



this was otherwise slow or difficult. The root-forming activity of 

 2,4-dichlorophyenoxyacetic acid, 2,4-D (Zimmerman and Hitchcock, 1942), 

 was especially conspicuous. By their application in paste or other means 

 under favorable conditions root initials may be produced almost any- 

 where on the plant. These substances are not equally active, and some of 

 them which have less value in root formation show other morphogenetic 

 effects. "Root-forming hormones" are now familiar aids in plant propa- 

 gation (Fig. 18-16). These substances are chiefly effective in the produc- 

 tion of root primordia and in most cases they actually check the later 

 growth of the roots. Because of their great theoretical and practical in- 

 terest, much work has been done on the root-forming effects of growth 



Fig. 18-16. Effect of "root hormones" on cuttings of holly. At left, controls. At right, 

 plants treated with mixture of indolebutyric and naphthaleneacetic acid. (Courtesy 

 Boyce Thompson Institute. ) 



substances. There are a number of general reviews of this work, among 

 them Pearse, 1939; Thimann and Behnke, 1947; and Avery and Johnson, 

 1947. 



The movement of synthetic substances, like that of natural auxin, is 

 polar except when their concentration is high. If applied at the apical 

 end of a cutting, they tend to pass downward and to stimulate root for- 

 mation at the base. If applied basally, they form roots there. The experi- 

 ments of Czaja and others have previously been described (p. 124) in 

 which regeneration in pieces of root is also polar in character, shoot buds 

 forming on the upper end and root primordia on the lower. This is evi- 

 dently owing to the accumulation of auxin at the lower (distal) end and 

 its consequent relatively low concentration at the upper one, a low con- 

 centration being related to shoot growth and a higher one to root growth 



