HORMONAL REGULATION OF PLANT CELL GROWTH 197 



are concentrated in the case of the oat coleoptile, except that they 

 are found throughout the outer wall of the epidermis (cf. Heyn, 

 1933). If the effect of lAA (indoleacetic acid) noticed were really 

 on the middle lamella, it would seem unlikely to be able to account 

 for enlargement of the much more rigid cell wall proper, Northcote, 

 Goulding, and Home (1958) did not detect any uronic acids in 

 h\xlroh'sates of cell walls of Chlorella, where there is no middle 

 lamella. 



It would seem desirable not only to investigate what part hemi- 

 celluloses might play in expansion of the cell wall but also to con- 

 sider if cell wall protein might have a structural or enzymatic role. 

 Primary cell wall preparations have generally contained some pro- 

 tein, but the question of whether this was adhering cytoplasm tended 

 to obscure its significance. Lamport and Northcote (1960) reported 

 that the protein of sycamore tissue culture cell wall preparations, 

 unlike the bulk protein of the cells, contains a large proportion of 

 hydroxyproline (but cf. Pollard and Steward, 1959). This raises 

 the possibility that the primary wall may contain a distinct protein 

 and is also interesting in view of the structural role of hydroxypro- 

 line-containing proteins (collagen) in animals. Kessler and Nicker- 

 son (1959) obtained several polysaccharide-protein "complexes" 

 from yeast cell walls; these workers (Nickerson and Falcone, 1959) 

 are advocating the idea that disulfide cross-links between such pro- 

 teins may be structurally important and that splitting of the cross- 

 links by reduction to — SH could cause the cell wall expansion which 

 is involved in cell multiplication. This might be a good example of 

 a "molecular mosaic" growth mechanism. 



Cell Wall Plasticity 



In the previous section we indicated some doubt that the ability 

 of the cell v/all to grow under turgor is with certainty a plastic 

 stretching, as opposed to some biophysically distinct process of su- 

 perficially similar behavior in experiments. Long before the recent 

 revival of interest in wall plasticity as an explanation of growth, 

 Heyn's conception that growth was nothing but a physical stretch- 

 ing lost favor because of the demonstrated dependence of growth 

 upon metabolism. Some workers who attempt to measure "plas- 

 ticity" now qualify the term, indicating that they think this property 

 is not to be considered simply a passive physical deformation; for 



