HORMONAL REGULATION OF PLANT CELL GROWTH 191 



nearly have doubled its total cell wall material) so that, had all syn- 

 thesis occurred at the inner surface of the cell wall, label should be 

 spread throughout the inner half of the thickness of the wall. This 

 is exactly the impression given by the clearest part of Fig. 4, of Set- 

 terfield and Bavlev (1958), representing the cellulose residue. The 

 unextracted preparation of their Fig. 2 does seem to show label 

 throughout the wall, which might indicate that non-cellulosic mate- 

 rials are introduced throuojhout its thickness. 



By an elegant self-absorption method Green ( 1958 ) studied the 

 site of incorporation of tritium into the cell walls, about 1 /x thick, 

 of single growing cells of the alga Nitella, and obtained results con- 

 sistent with the assumption that all new cell wall material was being 

 added at the inner surface. He subsequenty (1960) showed that 

 the orientation of the microfibrils changes, as one traverses the thick- 

 ness of this cell wall, in a manner which agrees qualitatively with 

 what would be expected from the reorientation hypothesis of 

 Roelofsen, mentioned above. Green (1960) felt that these results 

 indicated that the cell wall is an "inert accumulation," which simply 

 yields passively to turgor forces rather than participating actively 

 in growth. It is hard to reconcile this with the view, arrived at 

 above, that the mechanism of growth must involve some action on 

 the existing cell wall, and it seems most unlikely that growth even 

 in Nitella could be nothing but a passive stretching of inert material, 

 independent of any regulation by the cell. Conceivably growth 

 could involve some action on the cell wall materials which would 

 not result in incorporation of tritium, but it also should be noted that 

 the self-absorption method was probably not precise enough to 

 detect incorporation throughout the wall of a small proportion, say 

 a few per cent, of the total new material synthesized. It is inter- 

 esting that Roelofsen ( 1959 ) found it helpful to postulate an intro- 

 duction of non-cellulosic matrix materials throughout or across the 

 thickness of the cell wall, in order to explain certain observations, 

 even though he held that the microfibrils behave passively and are 

 added only or mostly at the inner surface. 



The author has conducted rather extensive, as yet unpublished 

 experiments, with oat coleoptile sections, comparing effects on growth 

 with effects on cell wall synthesis, under various treatments. While 

 the quantitative relation between the two processes is such as to 

 suggest, as noted earlier, that the gross aspects of wall synthesis are 

 not the basis of growth, yet the parallelism between effects on growth 



