188 THE MOLECULAR ARCHITECTURE OF PLANT CELL WALLS 



H3C.OCHN 



HOH 



CH OH 



twist is right handed and continues to be so for anything up to two and a 

 half hours, varying with the particular sporangiophore observed. Dur- 

 ing this time the rate of twisting is slowly diminishing; it finally comes 

 momentarily to a halt and then left-handed twisting again develops, 

 reaching a constant rate which thereafter continues for several hours. 

 Now the sporangiophore is simply a hollow tube of chitin filled with 

 liquid, or semi-liquid, cytoplasm containing a vacuole, and we can 



hardly escape the conclusion that these 

 pecuUar effects of growth must be due 

 to some feature of the wall. This view 

 was indeed expressed very early in the 

 study, both by Castle himself and by 

 Heyn. We may think of the organiza- 

 tion of a chitinous wall as resembling 

 very closely that of the cellulosic walls 

 we have been considering up to now, 

 with molecular chains arranged parallel 

 to each other in "micelles", and so on, 

 except that the unit of structure is not 

 now /3-glucose, but a derivative, acetyl 

 glucosamine (Fig. 63). 



Castle suggested that the twisting 

 might be due to the anisotropic res- 

 ponse of such a wall to the hydrostatic 

 pressure within the cell resulting from 

 its turgor. This is undoubtedly along 

 the right lines, but naturally cannot 

 lead to any quantitative check until 

 much more consideration is given to 

 the various factors which may be in- 

 volved. The suggestion of Heyn that 

 twisting could be due to failure of the 

 wall under stress, along slip planes predetermined by the crystalline 

 structure of the chitin, can hardly be accepted since in that case the rate 

 of twisting should be rigidly constant — which is by no means what is 

 observed. 



In attempting to derive an interpretation which may be satisfactory 

 in a quantitative as well as a qualitative sense, it is obviously necessary 

 to determine accurately the rate of rotation and of elongation under a 

 variety of conditions. This has been done in a series of classic papers 

 by Castle. Next, it is imperative to define the orientation of the chitin 



HOH^C 



N'H.COCH, 



HOH 



Fig. 63. Two glucosamine residues 

 in a molecular chain of chitin. 



