VI CELL ENLARGEMENT 793 



slight elongation even up to concentrations of 0.5A/. Osmotic pressure measure- 

 ments, by the method of incipient plasmolysis, showed that much of the solute had 

 entered. This was evidently due to the provision of sucrose in the medium, since 

 the data show that external sucrose causes marked increase of internal osmotic 

 pressure. In the absence of sucrose only traces of mannitol enter, as the earlier 

 data (above) showed. Recently Bennet-Clark (ig56a) has done similar experi- 

 ments using KCl as solute; it is much less satisfactory, since it enters the tissues 

 even more rapidly, but the initial value of the suction force was found to be about 

 7 atmospheres, agreeing with the earlier data. 



In these experiments, MgClj behaves like other solutes. CaCl2, on the other 

 hand, inhibits much more strongly than other solutes. Concentrations as low as 

 2'io"^iV/ give definite inhibition of elongation, both of coleoptile sections and of 

 pea stems. It is evident that the calcium ion is acting as a specific inhibitor, and 

 its mode of action will be discussed in the next section. 



For pea stem sections placed in mannitol, the elongation is nearly proportional 

 to the log. of the mannitol concentration; elongation ceases at about 0.4 A/, or 

 some 9 atmospheres, and there is little or no evidence of recovery (Thimann et al., 

 1950). In tuber tissues similar behavior occurs, but the recovery, i.e. presumably 

 the entry of mannitol, is even more marked, and artichoke slices appear to come 

 to equilibrium with mannitol concentrations approaching i A/ (Burstrom, 1953b). 

 This entry of solute, as well as the uncertainty of the determinations of initial os- 

 motic pressure, invalidates some earlier claims as to the action of auxin on growth 

 and metabolism (Bonner et al., 1953). It remains true, however, that in mannitol, 

 of a concentration high enough to prevent elongation, auxin causes no increase 

 in respiration (Ordin et al., 1956). 



Changes of the opposite type occur in tuber tissues given no external solute; 

 these enlarge very well in auxin, and here it can be shown that the enlargement 

 is accompanied by marked decrease of internal solute concentration. The decrease 

 in osmotic content is indeed almost exactly proportional to the increase in volume 

 which has occurred (Hackett, 1952). Similar but much smaller decreases in 

 osmotic pressure, accompanying growth, were recorded earlier by Ruge as long 

 ago as 1937, in elongating Heliantlms hypocotyls. Some recent data for coleoptile 

 sections which appear to show a decrease in osmotic pressure more than propor- 

 tional to the volume increase in this tissue (Ordin et al., 1956) are probably due 

 to the metabolism of organic solutes. 



Taking into account the complications due to solute entry and to dilution of the 

 cell contents, it can be concluded that, provided energy is available, enlargement of 

 tissues is governed directly by the water gradient. There is no evidence that cells can 

 enlarge against a water gradient, i.e. that they can "secrete" water internally. But 

 we have seen that oxidative metabolism is essential for enlargement. If the water 

 enters only along its gradient, that is, it enters passively, then it must follow that the 

 metabolic energy is needed for something else. Discussion of this problem follows. 



(j) Concepts of the mode of auxin action and the nature of cell enlargement 



In the thirties, after the role of auxin in growth had been discovered, numerous 

 possible modes of its action were explored. It was shown, for example, that each 



Literature p. liiG 



