192 CONTROL MECHANISMS IN CELLULAR PROCESSES 



and on synthesis in different types of experiments is truly striking. 

 We are not yet prepared to reject the possibiUty tliat a small amomit 

 of cell wall synthesis, possibly not of a qualitatively different nature 

 from the l^ulk apposition synthesis, may go on within the already 

 formed cell wall and be involved in the mechanism of growth. 



The fact that the predominant direction of orientation of the 

 microfibrils is the direction in which the cell grows least has long 

 suggested that the properties which determine cell enlargement 

 should be sought in the non-cellulosic matrix of the cell wall, the 

 hemicelluloses and pectins. Van Overbeek stated this view clearly 

 in 1939. Roelofsen (1959) insists instead that the "connections" 

 between cellulose microfibrils must play the critical role, but this 

 idea would not in actuality be a different one if the microfibrils are, 

 as seems probable, separated from one another by these other sub- 

 stances. Although much has been written in the belief that some 

 kind of action on the microfibrillar system itself is the basis of 

 growth, this view rests almost entirely on the (possibly equivocal) 

 interpretation of electron micrographs and is not supported by direct 

 evidence. Some reports have, however, indicated the occurrence 

 of a chemical action on the matrix which is affected by auxin; these 

 we shall now discuss. 



The Pectin Hypothesis 



The view is generally prevalent in the botanical literature that 

 primary cell walls (the walls of growing cells) contain a large pro- 

 portion of pectin ( polygalacturonic acid partially esterified with 

 methanol). Kerr (1951) has often been cited as having indicated 

 that pectin makes up the "continuous phase" or matrix of the cell 

 wall, although this was an assumption on his part, and he was aware 

 that the "pectin" which is extracted from cell walls is actually a 

 mixture containing pentosans and hexosans as well as pectin itself. 

 Since that time, however, authors regularly refer to the non-fibrillar 

 part of the cell wall in terms such as the "pectin matrix," and con- 

 sider that this matrix is responsible for the "plasticity" of growing 

 cell walls. 



Ordin, Cleland, and Bonner (1955, 1957) and Bennet-Clark (1956) 

 suggested that the inhibitory effect of Ca^^ on growth of stem and 

 coleoptile sections could be explained by the formation of Ca+ + 

 double salts involving two anhydrogalacturonic acid carboxyl groups, 



