206 CONTROL MECHANISMS IN CELLULAR PROCESSES 



Complete inhibition is usually first obtained in 0.2 M mannitol, in 

 agreement with results in the literature. This is true both with and 

 without auxin. 



It seems significant that complete inhibition of expansion occurs 

 at a mannitol concentration substantially below that equivalent to 

 the osmotic potential of the cells, and therefore at a positive value 

 of turgor pressure. With each section we continue to higher man- 

 nitol concentrations above the totallv inhibitorv one until we ob- 

 serve plasmolvsis in the subepidermal parenchyma cells at the end 

 of the section; we then return to successively lower concentrations 

 and find to within 0.1 or 0.5 M the one which first completely re- 

 verses plasmolvsis, which sets limits to O. We encountered values 

 below 0.3 M onlv in occasional sections (whose growth was in- 

 hibited in 0.1 M mannitol). Since non-growing sections are in fact 

 in osmotic equilibrium with the medium (unless they have addi- 

 tional "active" P ) , it is justifiable to compute P from the difference 

 in n inside and outside the cells, which indicates that turgor pres- 

 sures of 2 atm or more still exist in the cells at the point of complete 

 osmotic inhibition of cell enlargement. 



Since mannitol inhibits cell wall synthesis ( see for example Bay- 

 ley and Setterfield, 1957), it seemed desirable to check this result 

 with some chemically unrelated substance, to which the cells might 

 be presumed to be at least as impermeable as to mannitol. We have 

 employed Carbowax 4000, a polyethylene glycol. With it also we 

 observe complete inhibition of the growth rate at a concentration 

 definitely below that equivalent to the osmotic potential of the cells,, 

 and it may be noted that with either this or mannitol the completely 

 inhibited sections feel turgid to the touch. It seems certain that the 

 statements in the literature referring to treatments of oat coleoptile 

 sections in 0.2 to 0.28 M mannitol as "isotonic" or "slightly hyper- 

 tonic" are an error, possiblv terminological, but they convey the im- 

 pression that to inhibit growth completelv it is necessary to reduce 

 turgor pressure to zero, which seems not to be the case with the oat 

 coleoptile section. 



With this method one can also follow closely the time course of 

 osmotic swelling and sin-inking of the sections when transferred into 

 or between non-plasmolyzing mannitol concentrations. An example 

 of such an osmotic shrinking is shown in the first part of Fig. 7-4. 

 The half-times of these osmotic changes are generally about 5-6 

 minutes, and we have noticed no striking effect of auxin on them. 



