320 FINE-STRUCTURE OF PROTOPLASMIC DERIVATIVES III 



residues. Consequently it is only in solution that they contract to a 

 pitch of 7.8 A (RuNDLE and Edwards, 1943) formed by six glucose 

 residues. It is obviously misleading when, in analogy to the proteins, 

 the extended amylose chain is called "denatured" amylose, because 

 its natural state in the starch grain seems to be the expanded modi- 

 fication. 



Fig. 158. Cross-sections of the two possible unit cells of crystallized starch (from Kreger, 



1951). 



The optical behaviour of the starch grains rules out contracted 

 chams in their structure (Frey-Wyssling, 1940 c). Since the con- 

 tracted amylose chains show their highest polarizability (corre- 

 sponding to n^) perpendicular to the helical axis which, according to 

 X-ray evidence, runs radially, the starch grains ought to be optically 

 negative. But, as mentioned above, they represent optically positive 

 spherites. Therefore, they must contain expanded chains which have 

 their highest polarizability (n^,) parallel to the fibre axis. The iodine 

 dichroism points in the same direction. Contracted amylose chains 

 (Fig. 1 5 3, p. 312) have their highest absorption coefficient (k^) parallel 

 to the fibre axis, which consequently runs perpendicular to n^. In 

 expanded chains, however, k^ and n,, coincide. Since this coincidence 

 is characteristic for starch grains, it must be taken for granted that 

 they consist of expanded chains. 



Further proof of radially orientated elements is provided by the 

 existence of rodlet birefringence in starch grains (Speich, 1941). This 

 raises the question as to what type of submicroscopic spaces permits 

 the penetration of imbibition liquids. They cannot have the same 

 character as in cellulose, because hitherto no submicroscopic struc- 



