HORMONAL REGULATION OF PLANT CELL GROWTH 



207 



As noted in the section on water uptake, al)o\ e, we liave attempted 

 to compare the growtli rate with the osmotic clianges in an effort to 

 estimate the turgor pressures of growing cells. However, we are 

 not vet confident enough about the quantitative interpretation of 

 the results to warrant any more definite statement than appears in 

 that section. 



A peculiar phenomenon which we first observed in experiments 

 with mannitol is illustrated in Fig. 7-4. The section was initially 



60 

 TIME MIN. 



Fig. 7-4. Effect of transfer from water to 0.25 M mannitol {af first arrow) 

 and back to water fsecond arrow). Coleoptile section 6 mm long. Broken line 

 is an extrapolation of the original growth rate, which may be compared with 

 the actual expansion which occurred after return to water, shown by the experi- 

 mental points. 



growing in water and was then transferred to 0.25 M mannitol, 

 whereupon it shrank osmotically and did not resume growth. When 

 it was transferred back to water, it exhibited a much greater ap- 

 parent growth rate than previously (probably augmented during 

 the first few minutes by reversal of the previous osmotic shrinkage ) , 

 which continued until the section had attained approximatelv the 

 length that it would have reached had the mannitol treatment not 

 been given and the previous growth rate remained unchanged; at 

 this point the rapid rate declined to about the previously prevailing 



