HORMONAL REGULATION OF PLANT CELL GROWTH 209 



peared when we inliibit sections already treated witli auxin. At the 

 moment we are inclined toward the following tentative interpreta- 

 tion of the stored growth effect: tlie growth rate is considered to 

 depend upon and be controlled by the rate of some chemical (en- 

 zymatic) reaction acting on the cell wall, which is sensitive to KCN 

 and requires turgor or some effect which turgor causes (such as 

 elastic expansion of the cell wall, or forcible juxtaposition of cell 

 membrane and cell wall site acted upon ) . This reaction uses some 

 substrate, to be called X, which is produced by general metabolism 

 vatliin the cell, at such a rate that, in sections untreated with auxin, 

 the growth reaction is rate-dependent upon X. When the growth 

 reaction is temporarily blocked, metabolism continues to produce 

 X which accumulates so that, when inhibition is relieved, the growth 

 reaction occurs at a more rapid rate, until the excess X has been used 

 up. At this point the amount of growth which has occurred should 

 be the same as if growth had continued uninterrupted, and the rate 

 should then fall until it again becomes equal to the rate of forma- 

 tion of X. We suppose that the effect of auxin is to promote some 

 reaction in the metabolic sequence leading to X; optimal auxin con- 

 centration accelerates the formation of X to such an extent that its 

 concentration builds up to a practically saturating value for the 

 "growth enzyme." Hence, further accumulation of X, as in inhibi- 

 tion of auxin-promoted growth, cannot increase the growth rate 

 further or lead to stored growth, whereas in the absence of added 

 auxin a period of inhibition sufficient to build up the saturating con- 

 centration of X will lead to the same rate of growth (temporarily) 

 as if auxin were added. 



We have recently tested the relation between auxin and stored 

 growth as follows: a section growing in water at the rate of 2.7 ^u. 

 per minute was transferred to 10~" M KCN, which quickly gave 

 nearly complete inhibition. After 20 minutes it was returned to 

 water and, after a lag of 9 minutes, began to grow and attained a 

 rate of 11.4 /jl per minute, which persisted until the section some- 

 what overshot the length to be expected from the previous growth 

 rate. Then its growth rate fell to 2.0 /x per minute (including a 

 decline in control growth rate?). It was then transferred to auxin, 

 and after 12 minutes its growth rate began to increase and quickly 

 reached 8.4 fx per minute. It was then treated with auxin plus 

 10~^ M KCN, again giving nearly complete inhibition; 17 minutes 



