CARBOHYDRATES, CHITIN AND CUTIN 



315 



of the amylopectin. a-amylase is able to break down the amylopectin 

 into soluble fragments (dextrins) without at first generating maltose; 

 subsequently saccharification sets in by degrees (Meyer and Bern- 

 FELD, 1941c). The branching junctions of the amylopectin are in- 

 accessible to the /5-amylase, for, besides the usual 1-4 bond between 

 the glucose residues, there is an additional 6-1 bond passing into the 

 side chain, the splitting of which needs yet another enzyme. 



The microscopic structure of starch grains. The microscopic structure of 

 starch grains has been dealt with so often and so exhaustively (see 

 Badenhuizen's comprehensive review 1937) that, to avoid repetition, 

 I shall here touch only on a few points which appear to me of par- 

 ticular importance (Samec, 1942/43). 



The familiar arrangement in layers of starch grains is brought about, 

 in the unanimous opinion of the majority of investigators from 

 Nageli (1858) to our contemporaries, by alternate layers of stronger 

 and weaker refractive power, or containing a smaller or larger 

 percentage of water. Now if a weakly refractive, narrow layer were 

 bordered both inside and outside by neighbours 

 of higher refractivity, it would shine brightly in 

 the microscope at low adjustment and, when the 

 tube is raised, the bright Becke lines on both 

 sides should pass over into the optically denser 

 layers. This, however, does not take place (Frey- 

 Wyssling, 1936a, page 287). With pronounced 

 stratification, especially of eccentric starch grains 

 immersed in water (potato, Pellionia, etc.), it may 

 be seen distinctly how the Becke line at the edges 

 of all layers moves outivards only when the tube is 

 raised. True, there are cases when a pale lustre 

 can be seen to shift inwards, but on the outside 

 it is always incomparably stronger. From this we 

 may confidently conclude that every layer is more refractive on the 

 inside, the refractive power outwards as a rule diminishing quite 

 gradually, and then suddenly coming up against a layer of higher 

 refractive index. Thus, in a section through the grain the refractive 

 power in the various layers is not equally high or low, but there is 

 a continuous decrease towards the edges and discontinuous increase 

 at the outer edges of the layers, as represented in diagram bv the 



12 3 4 5 



Fig. 156. Microscopic 

 lamination of starch 

 grains. Diagram of 

 the refraction. Ab- 

 scissa: 1-5 layer edges. 

 Ordinate: Refractive 

 index «. 



