STRUCTURE AND COMPOSITION OF STARCH-GRAINS. 



339 



layers lie on the growing side and thin off towards the hilum, as seen in Fig. 234. 

 Naegeli first showed that the whole of the visible internal structure of the starch- 

 grain depends upon an unequal distribution of water and starch-substance. The 

 hilum always consists of a watery portion poor in substance : the layers of the grain 

 are formed of starch-substance containing much and little water alternately, and this 

 in such a manner that the outermost layer of a grain is always one poor in water 

 and rich in substance, and dense. Moreover, the proportion of water of all the 

 layers increases from the circumference towards the hilum ; whence is to be 

 explained the fact that, on the desiccation of the starch-grain, fissures arise in the 

 hilum, and radiate from this towards the periphery. 



The chemical substance of a starch-grain, however, apart from this varying 

 diff'erence in the proportion of water, is composed of two substances, chiefly distin- 

 guished by their solubility: these are termed granulöse and starch-cellulose. The 

 first predominates by far in quantity, and is at the same time the one which effects 

 the blue colouration of the starch-grain with iodine. Being the more easily soluble, 

 it may be removed by various solvents from the starch-grain, the starch -cellulose alone 

 at length remaining behind, in such a manner, however, that it still forms a skeleton 

 exhibiting the essential structure of the starch-grain. If large starch-grains are 

 macerated in fresh saliva at 35-55° C, or, according to Franz Schulze, digested at 

 60° in a saturated solution of common salt containing io/q of hydrochloric acid, the 

 whole of the granulöse is removed sooner or later : i. e. it is chemically altered and 

 converted into sugar, while the starch-cellulose remains behind in the form of a 

 laminated skeleton. This latter is coloured copper-red with iodine, or not at all, 

 and represents only a small proportion of the mass of the entire starch-grain. The 

 different solubility of the two substances is also made evident on germination by 

 the behaviour of the starch-grains in the reservoirs of reserve-materials. The starch- 

 grains in the living cell, however, may be dissolved in a very different way. In 

 some cases the solution begins with the removal of the granulöse, while the cellulose 

 remains behind. This however often takes place only here and there : the extraction 

 proceeds at isolated spots from without inwards. The extracted places are coloured 

 copper-red with aqueous solutions of iodine, the rest of the mass blue. This I found 

 to be the case in the endosperm of germinating Wheat (Fig. 235 ^). In other cases the 

 solution begins likewise at single spots on the circumference, but the entire substance 

 is progressively dissolved : cavities are produced, and the grain here also finally breaks 

 up into pieces, as seen in Fig. 235 a-/. In the cotyledons of the germinating Bean, 

 the solution of the ellipsoidal grains begins from within ; before they break up into 

 pieces, the granulöse is often so completely extracted that the grains assume a copper- 

 red colour with iodine, bluish here and there. Subsequently all is dissolved. In the 

 germinating Potato, and in the root-stock of Canna, on the other hand, the solution 

 of the grain proceeds from without inwards, layer after layer being removed. It evi- 

 dently depends here, as with the action of saliva, upon whether the solvent acts slowly 

 and first extracts the granulöse only, or energetically attacks and dissolves the sub- 

 stance as a whole. 



When the solution of starch or granulöse is here spoken of, a chemical con- 

 version into sugar at the same time is always to be supposed. Starch-grains as such 

 are not soluble simply in water. If they are crushed in cold water, however, a small 



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