190 CONTROL MECHANISMS IN CELLULAR PROCESSES 



wall (intussusception) expands its surface area, and water uptake 

 naturally follows osmoticallv. Such a mechanism would appear to 

 explain much better the strong dependence of growth upon respira- 

 tory metabolism, which has been widely studied, than would the 

 idea of passive plastic stretching. Auxin treatment does promote 

 synthesis of the cell wall, although to obtain large effects in stem or 

 coleoptile sections, it appears to be necessary to extend experiments 

 over many hours, whereas the effect of auxin on the growth rate 

 develops well within one hour. Several different views hold that 

 the promotive effect of auxin on cell wall synthesis is a secondary 

 consequence of its effect on growth. 



There are some cases in which cell wall synthesis is much less 

 than proportional to cell enlargement, so that the cell wall appears 

 to become thinner during growth. One example is the oat coleop- 

 tile section growing in auxin plus sugar, which was reported by 

 Bayley and Setterfield (1957) and has been confirmed repeatedly 

 by the author. Furthermore, when coleoptile sections grow in auxin 

 solution without added sugar, synthesis of new cell wall material is 

 usually negligible, even though 50 per cent elongation may occur; 

 this was noted by Bennet-Clark ( 1956 ) and has been found in many 

 of our own experiments. And long ago Bonner (1934) reported 

 that coleoptile sections grow at 2° C with negligible synthesis of 

 cell wall. It seems necessary to conclude that in this and other cases 

 growth is not caused by gross synthesis of the cell wall, but instead 

 results from an action on the existing cell wall, and may or may not 

 be accompanied by further synthesis. The rate relationships be- 

 tween growth and synthesis appear to fit better an apposition rather 

 than an intussusception process, which would be expected to be tied 

 closely to the rate of growth. The observations do not, however, 

 exclude the possibility that a minor amount of cell wall synthesis 

 may be critical to growth. 



About the actual site of wall synthesis during growth, there is as 

 yet very little evidence. These walls are usually less than 1 ^ thick, 

 which cannot be resolved by present radioautographic methods. 

 Setterfield and Bavley (1958) reported radioautographs of cross- 

 sections of the 4 fi thick outer wall of the epidermis of oat coleoptile 

 sections which had been grown in tritium-labeled sucrose, which 

 they felt indicated incorporation throughout the thickness of the 

 wall. Unfortunately in the critical experiment the section had more 

 than doubled its original size (and according to our data might 



