THE BIOLOGY OF PLANT GROWTH 447 



genes pass out their information and cause production of the required 

 enzymes only in a few specific spots within the plant. In all other 

 tissues the genetic information concerning indoleacetic acid synthesis 

 is inert— tinned off. 



That information about how to produce enzymes suitable for the 

 synthesis of indoleacetic acid is in fact contained within normal stem 

 cells is quite clear from the fact that we know how to cause stem 

 cells to start producing enzymes needed for indoleacetic-acid synthesis. 

 This can be done most elegantly by transformation of normal stem 

 cells into crown-gall tumor cells. In the course of this transformation, 

 stem cells commence production of the enzymes required for indolaec- 

 tic-acid synthesis and thus become autonomous with respect to this 

 growth substance ( Henderson and Bonner, 1952 ) . 



In any case, however, the mystery of why indoleacetic acid is pro- 

 duced only in particular places and not in others is the mystery of dif- 

 ferentiations. Students of plant growth substanceology need not be 

 unduly ashamed that they have not solved this matter, since no one 

 else has solved it either. Students of plant growth substancology may 

 be more concerned with the fact that they do not yet really understand 

 in detail how indoleacetic acid causes plant cells to grow more rapidly. 

 True, the mode of action of indoleacetic acid is slowly being chased to 

 its lair. The growing plant cell consists of living material surrounded 

 by a carbohydratey cell wall. The cell wall is vmder tension, and the 

 tension is due to the osmotic uptake of water by the cell contents. The 

 factors that immediately control the rate of cell expansion are, then, the 

 osmotically induced load, or tension, to which the wall is subject and 

 the resistance of the wall to deformation under this load ( Bonner, 

 1961 ) . Appropriate mechanical analyses have shown unambiguously 

 that the more rapid elongation of auxin-treated plant material is due to 

 softening of the cell wall— to changes \vithin the wall which cause it to 

 yield more rapidh' to a given load ( Tagawa and Bonner, 1957 ) . Since 

 indoleacetic acid increases the rate of cell extension by causing in- 

 creased cell wall deformability, it is evident that the hormone in some 

 way alters cell wall chemistry. 



Such alterations have been sought since the beginnings of auxinol- 

 ogy. Only in recent years, however, after the advent of appropriate 

 methodology, have they been found. With such methodology it has 

 been shown that the application of indoleacetic acid to an appropriate 

 plant tissue results in increased rate of synthesis of a particular cell 

 wall component— namely, pectic material (Ordin ct ah, 1955; Alber- 

 sheim and Bonner, 1959 ) . The new pectic material produced under the 

 influence of indoleacetic acid is of short chain length, in contrast to the 

 long-chain protopectin that characterizes the bulk of cell wall pectic 



