114 PHYSIOLOGY OF NUTRITION. 



The starch grains from the cotyledons of seeds of Phaseolus 

 vulgaris are centric. If we examine them in a drop of water, we 

 observe in the middle of each grain a cavity, which, however, is 

 due to the action of the water, for if the starch grains of the bean 

 are observed in glycerine, they do not exhibit this cavity (see 



FIG. 37. Starch grains from the cotyledons of Phaseolus vulgaris. Magn. 540. (After 

 Strasburger.) 



Fig. 37). The starch grains from the endosperm of Triticum 

 present themselves as round structures of very different sizes. 

 They are centric. Indications of lamination are very difficult to 

 make out. 



44. The Behaviour of Starch towards Iodine. 



A little potato starch is treated with distilled water in a test- 

 tube. We put it aside for a few hours, shaking frequently, and 

 then filter off the fluid. If, by means of a glass rod, we add a 

 little alcoholic Iodine solution to the filtrate, the blue colour does 

 not appear. Distilled water is unable to take up starch from 

 uninjured starch grains. 



A little potato starch is treated with water in a test-tube. On 

 warming, a turbid mixture of starch substance and water (starch 

 paste) is formed. On adding to the starch paste, after it has 

 cooled down, some alcoholic Iodine solution, the fluid assumes 

 a very characteristic beautiful blue coloration. This reaction is 

 very delicate, as we can easily prove by pouring a smalt quantity 

 of the starch paste into a very large quantity of water, and then 

 adding Iodine solution. The fluid still becomes blue. On warm- 

 ing starch paste coloured blue by Iodine, the characteristic colour 

 quickly disappears, since water at a high temperature can dissolve 

 a fairly large quantity of Iodine, and hence, under the conditions 

 described, can withdraw it from the starch substance. When the 

 starch paste cools, the blue colour reappears. 



A very small quantity of potato starch, or a small quantity of 

 some other kind of starch, is mounted in a drop of water on a 



