THE MECHANISMS OF ORIENTATION AND GROWTH 197 



or some oligo-saccharide or pre-existing oriented chains. Even at the 

 time, of course, this was not a "first cause", but the periodic switches in 

 orientation now known to occur in so many cell walls show at least that 

 some other mechanism is from time to time invoked. This mechanism 

 can hardly lie anywhere except in the protoplasm. 



So much was clear in 1937, and since that time it has become pro- 

 gressively more probable that orientation in the cellulose envelope 

 implies orientation in the protein chains, both within a growing wall 



Fig. 67. Diagrammatic representation of the run of the fibrils in a primary wall 



(o) before, (/)) after a period of growth. The fibrils are supposed to maintain their 



orientation constant by slip at the points of crossing (after Frey-Wyssling). 



and in the cytoplasmic surface during secondary wall deposition. The 

 molecular configuration for normal and for supercontracted wool 

 keratin suggested by Astbury(65), made it possible even to hazard a 

 guess as to how protein chains might readily be responsible both for the 

 almost transverse orientation in growing walls and the almost longi- 

 tudinal orientation in adult walls (66). During periods when the 

 orientation of the cellulose chains in a wall layer remains constant, it is 

 still possible that the "crystallization" forces of the cellulose matrix 

 assume priority; but once the orientation changes then it is certain that 

 the protoplasmic mechanism has taken control. It is much too early 

 in the history of this most important aspect of the study to make any 

 definite pronouncement, but there is already in the literature a number 

 of interesting pointers. Much of this subsidiary evidence refers to the 

 animal rather than to the plant cell; we should not thereby be 



