STARCH GRAINS. 



57 



selves, and only undergo conversion later. It has already been shown in the pre- 

 ceding paragraphs how this happens with albuminous protoplasm-forming materials, 

 and with oily matter. In far larger quantities another substance, in the most eminent 

 sense organisable, Starch, is formed beforehand and stored up in an organised 

 form in anticipation of future use. The starch always appears in an organised form 

 as solid grains having a concentrically stratified structure, which arise at first as 

 minute masses in the protoplasm, and continue to grow while lying in it ; if at a 

 subsequent period they reach the cell-sap and cease to remain in contact with the 

 protoplasm which nourishes them, their growth stops ^ Every grain of starch con- 

 sists of starch, water, and of very small quantities of mineral substances (ash). 

 The first is a carbo-hydrate of the same percentage composition as cellulose, 

 to which it bears the greatest similarity of all known substances in chemical and 

 morphological properties. The starch, however, occurs in each grain in two 

 modifications : one more easily soluble, which assumes a beautiful blue colour with 

 solution of iodine and addition of water (Granulose), and the other less easily 

 soluble, which in its reactions comes nearer to cellulose (Starch-cellulose). At 

 every point of a grain of starch both materials occur together ; if the granulose 

 is extracted, the cellulose remains behind as a skeleton ; this skeleton shows the 

 internal organisation of the whole grain, but is less dense or poorer in substance, 

 and its weight amounts to only a small fraction of the whole grain (about 2-6 p. c). 

 Since, then, the granulose greatly preponderates, and is present at every point of 

 the grain, the grain shows, in the reaction with iodine, the blue granulose-colouring 

 throughout its whole extent. 



The starch-grains have always rounded forms, and their internal organisation 

 has reference to a centre of formation lying within themselves; the young small 

 bodies appear to he always spherical ; but since their growth is scarcely ever 

 uniform, their form changes into ovoid, lenticular, rounded polyhedral, &c. 



The internal organisation of the starch-grain is especially recognised by the 

 diff'erent distribution of water in it (water of organisation). Every point of the 

 grain contains water in addition to granulose and cellulose.- 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, on which partly 

 depends the power of perceiving these properties. 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, &c., 

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

 the nucleus. All the layers of a grain are disposed around this nucleus as their 

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

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

 when their number increases with growth, the number of layers increases most in 



* According to Hofmeister, the starch-grains in the milk-sap of Euphorbia appear to form an 

 exception ; nothing however is known about their development ; the milk-sap (latex) always 

 contains protoplasm-forming substances, albuminoids, which perhaps here also take part in the pro- 

 duction of the starch-grains. 



