STARCH-GRAINS. 



57 



cellulose in its reactions \ At every point of a starch-grain both constituents occur 

 together ; if the granulose is extracted, the farinose remains behind as a skeleton ; 

 this skeleton presents the internal organisation of the whole grain, but is less 

 dense or poorer in substance, and its weight amounts to only from 2 to 6 p. c. 

 of the whole grain. Since the granulose greatly preponderates and is present at 

 every point, the starch-grain shows the blue granulose-colouring with iodine 

 throughout its whole extent. 



The starch-grains have always rounded forms organised around an internal 

 centre of formation ; when young and small the grains appear to be always spherical ; 

 but since their growth is scarcely ever uniform, their form changes into ovoid, lenti- 

 cular, rounded polyhedral, &c. 



The internal organisation of the starch-grain depends essentially on the dif- 

 ferent distribution of water in it {water of organisaiion). Every point of the grain 

 contains water in addition to granulose and farinose. Most usually the amount 

 of water increases from without inwards, and attains its maximum at a fixed point 

 in the interior. With the increase in the proportion of water, the cohesion and 

 density decrease, as also the index of refraction. This change in the proportion of 

 water is not, however, constant, but intermittent. -To the outermost least watery 

 layer succeeds a sharply defined watery layer, to this again a less watery one, and 

 so on, until the innermost less watery denser layer surrounds finally a very watery 

 part, the nucleus. All the layers of a grain are disposed round this nucleus as their 

 common centre, but every layer is not continuously developed round the whole 

 nucleus; in small spherical grains with few layers this is always the case, but 

 when their number increases, it does so most in the direction of most vigorous 

 growth, which is continuous in a straight or curved line with the direction of least 

 vigorous growth. This line is called the axis of the grain, and always passes 

 through the nucleus. 



The growth of the grains of starch is accomplished exclusively by Intussus- 

 ception ; new particles become intercalated between those already existing both in 

 a radial and tangential direction, by which means the proportion of water at 

 particular places is at the same time changed. The youngest visible globular 

 starch-grains consist of denser less watery substance; in this is formed subse- 

 quently the central watery nucleus; in the latter a central part may become 

 denser; and in this, when the increase in size has advanced sufficiently, a softer 

 nucleus may again arise. It may however also happen, after a softer nucleus 

 surrounded by a dense layer has arisen by diff"erentiation of the originally dense 

 grain, that in the dense layer a new soft one may arise, and it may thus become 

 split into two dense layers, the inner of which encloses the soft nucleus. The 

 layers increase in thickness and circumference by intercalation. When a layer 



i^ [The most recent researches seem to show that the supposed distinction between granulose 

 d farinose is one of mechanical or molecular condition only. The coloration of starch by iodine 

 pears not to depend on the formation of a definite chemical compound, but to be the consequence 

 the mechanical interposition of the iodine between the molecules of starch (see Miller's Chemistry, 



