7g6 PLANT GROWTH lO 



millimole of O,, which could at most correspond to only 2 milliequivalents of OH- 

 ions (Hackett and Thimann, 1953). 



The remaining possibility is that the metabolic energy is used to modify the cell 

 wall, reducing its resistance to enlargement. This would imply that the primary 

 wall is not an inert excretion, but is under continuous metabolic control. There is 

 some direct evidence for this mode of action : if auxin is applied to potato disks in 

 presence of mannitol or other inert solutes, growth is reduced accordingly (as 

 described above), but when the disks are afterwards transferred to water or plain 

 auxin solution they enlarge rapidly, and essentially catch up with the controls 

 which have been in plain auxin throughout (Thimann, ig54a). Evidently the 

 auxin has been acting in presence of the mannitol to modify the cell-walls, even 

 though water uptake was osmotically limited; on removal of the limitation, 

 growth can then take place at maximum rate because the walls have already been 

 modified. 



In 1951 Kerr pointed out that the elastic properties of the cell-wall could best 

 be explained by assuming that the protopectin forms a continuous phase in which 

 the cellulose micelles are embedded. Growth would in this case be controlled by 

 the pectin. Increases in pectin methylesterase after auxin treatment were mention- 

 ed above (p. 790). Operating along this line of thought, Ordin et al. (1955) have 

 found that '"'C labeled methyl groups, from methionine added to coleoptile sec- 

 tions, yield '"^C activity in the hot-water-soluble fraction of the coleoptile cell wall 

 after 4 h. of auxin treatment, to a greater extent than in controls. The radioactivity 

 incorporated into other fractions was much greater but showed no consistent 

 response to auxin treatment, so that the results must be regarded as preliminary. 

 The transfer of ^"^C into the cell-wall from acetate or sucrose is not increased by 

 auxin (Boroughs and Bonner, 1953). The role of pectins for intercellular cement- 

 ing, long known from cytological studies, has more recently been clearly shown 

 by treating onion root-tips with pectinase (Brown, 1951). Not only did the 

 cells separate, but the walls broke down enovigh so that free nuclei could be ob- 

 tained. When whole barley roots were grown in solutions of pectinase (Cormack, 

 1955, 1956), the epidermal and other outer layers of cells separated completely 

 from one another and grew in rows, attached only at their ends, and in various 

 bizarre forms; on returning to water, or especially to solutions of calcium ions, the 

 tissues slowly returned to normal. Growth was, however, not accelerated by the 

 pectinase. 



Bennet-Clark (1956a) has proposed that the growth-inhibiting effect of calcium 

 (see p. 782) is due to its binding together in pairs the free COOH-groups 

 of pectic acids. Praseodymium, which is trivalent, was found to inhibit more 

 strongly than calcium, while ethylenediaminetetracetic acid (EDTA, Sequestrene, 

 Versene) which chelates with calcium, and other metals, actually promotes growth 

 (Heath and Clark, 1956), though its effects are small. In the writer's own experi- 

 ments the effect of EDTA is shown on coleoptile sections but not on mesocotyl 

 sections from the same plants, which makes it unlikely that it exerts a general 

 effect on cell walls. Its effect is greatly increased in the presence of lAA. 



In favor of an action of auxin on the cell wall is also the general observation that 

 auxin operates only on plants with walls of the "higher plant" type, namely those 



