HORMONAL REGULATION OF PLANT CELL GROWTH 201 



which we could measure its temperature to 0.1° C. As a precau- 

 tion we tie the fixed end of the section on securely witli fine thread. 

 The junction is mounted rigidly from the floor of a clear Plexiglas 

 trough which is held on the mechanical stage of a compound micro- 

 scope; the trough is so designed that from a reservoir a continuous 

 gentle stream of solution flows in one side and out the other, at a 

 level just covering the section, and by exchanging pinch clamps on 

 the inlet side hose we can suddenly change from solution of one 

 temperature (or composition) to another, getting the full tempera- 

 ture response within the tissue in about 5 seconds. With green light 

 as dim as possible, we observe continuously the end wall of a par- 

 ticular subepidermal cell at the free end of the section, with a 4 mm 

 objective lens, against an ocular micrometer scale witli 100 divisions, 

 each division being equivalent to 1.5 jx. We can read to 0.5 divi- 

 sion. In auxin at 23° C the sections normally grow about 6-10 

 divisions per minute. When the section approaches the end of the 

 ocular scale, we restore it to near zero by suddenly moving the 

 mechanical stage, and immediately reread. The most important 

 requirement for accuracy and clear vision is that the objective be 

 immersed in the growth solution. We shall describe the details of 

 the apparatus and technique fully elsewhere. As auxin we use a 

 solution of 3 mg per liter indoleacetic acid in distilled water. 



When the temperature of a rapidly growing section, in auxin, is 

 suddenly lowered from 23° to 5° C, growth immediately practically 

 ceases as far as intervals of time convenient for us to employ are 

 concerned. Since we should like to know whether the rate changes 

 subsequent to the initial effect, we have studied more extensively 

 the result of transferring between 23° C and 12° C, at which growth 

 is much depressed but not prevented. Fig. 7-1 shows such an ex- 

 periment, with arrows indicating the time of transfer between tem- 

 peratures. It is observed that there is a slight thermal contraction 

 of the apparatus during the first 20 seconds after the tempera- 

 ture transfer. It is clear tliat the entire effect of temperature on the 

 growth rate had developed within this interval and is registered in 

 our first measurements made thereafter 15 seconds apart within the 

 first minute. 



The effect of this 11° change in temperature is approximately 

 3.5-fold, which is of the magnitude one might expect if the effect 

 were due entirely to changes in rates of metabolic reactions. We 

 should allow, however, for the possibility that the observed change 



