STARCH. 



[ 732 ] 



STARCH. 



starch-pranules ; but their dependence on 

 the existence of the concentric layers is 

 beyond doubt. They are seen in the proper 

 relative positions when the g-ranides are 

 rolled over in all directions beneath the 

 microscope ; their relative numbers and 

 forms correspond to the size and stage of 

 development of the granules in the same 

 plant ; and other characters connected with 

 the physical structure confirm the conclu- 

 sions from simple inspection. 



Starch is not chemically an individual 

 substance, but consists of two independent 

 isomeric substances : one of which, granu- 

 lose, is soluble in saliva, is coloured blue by 

 iodine, and is dissolved by weak solutions of 

 chromic acid ; while the other, cellulose, 

 which composes from 2 to 4 p. c. only of the 

 grains, is not affected by saliva or iodine, and 

 is soluble in solution of cuprate of ammonia, 

 but not in chromic acid solution. 



Starch is usually stated to be miaffected 

 by cold icater, and this is generally the case ; 

 but if the granides of Tous-les-mois are 

 crushed before placing them in water, so as 

 to expose the internal substance, the water 

 is sometimes absorbed by the inner layers, 

 and these swell up considerably without the 

 outer layers being affected. When dry 

 stare li-granules are heated gradually upon a 

 slide, until some of them assume a yellowish 

 colour, either the air-bubble above-men- 

 tioned appears — occasionally with a partial 

 separation of the concentric layers through 

 expansion of the films of air existing between 

 them, while other parts become fused, — or 

 the general shape remains unchanged, and 

 the strife gradually vanish, becoming melted 

 into a mass, as it were, the starch itself 

 being converted into dextrine. When 

 starch-granides are heated in water to the 

 boiling-point, they usually soften and 

 " blow up " into a large sac, the inner part 

 softening first, and pushing out the more 

 superficial ; if the sac bursts, the inner siib- 

 stance sometimes partly escapes in the form 

 of cloudy fiocks, but is not dissolved. Di- 

 luted sulphuric acid acts somewhat in the 

 same manner as hot water ; but if stronger 

 acid is allowed to attack the granules locally 

 or partially by flowing in from one side 

 upon the object, very remarkable appear- 

 ances present themselves : the acid touching 

 certain parts of the granule first, or acting 

 most quickly on softer portions, causes the 

 softening internal layers to expand and 

 bulge out tlie external layers at particular 

 points (like hemice) until the entire gi-ain is 



softened, when these coalesce and the whole 

 expands into a thin sac. Gradual action of 

 the acid causes a more uniform expansion, 

 which is usually accompanied by a sudden 

 crack running out from the nucleus into the 

 substance (indicating the abstraction of 

 water ?), followed almost immediately by a 

 collapse of the wall above this crack, and 

 a sudden expansion of the whole into a sac 

 or an irregular gelatinous film. Solution of 

 potash produces much the same effect as 

 dilute sidphuiic acid. 



All the above appearances indicate that 

 the starch-granule is composed of concentric 

 " shells " of a substance of the same nature, 

 but less dense and more rich in water in the 

 interior layers, firmer, less hydrated, and 

 more resisting in proportion to the distance 

 from the starting-point of growth or nu- 

 cleus ; and, according to modern views, the 

 layers are alternately more and less hydrated. 

 With polarized light, moreover, the starch- 

 granule exhibits a black cross, and with a 

 plate of selenite a beautiful coloured system, 

 especially well seen in large grains like those 

 of the potato or Tous-les-mois. 



Pure starch is coloured blue by iodine, 

 whether in its natiu'al state or softened by 

 hot water, the depth of the colour depend- 

 ing on the quantity of iodine ; where much 

 is added, the colour is almost black. When 

 dilute sulphuric acid has been added pre- 

 viously, the coloiu' is rather purple than 

 blue, especially the faint tinge given at first 

 by weak solution of iodine. When the 

 starch grains are heated di-y, the colour 

 given by iodine changes, proportionately to 

 the violence of the action, from blue to 

 purple, red-wine colour, and finally brown ; 

 the starch being converted into dextrine. 

 The best application is the solution of iodine 

 in iodide of potassium ; and this should be 

 used very weak in investigation of starch. 



Starch-granules occur either isolated (PI. 

 46. figs. 8 & 21), or in groups (figs. 7, 10, 11) 

 (in the latter case mostly with fiat faces, so 

 as to fit together into round, oval, or similar 

 forms), or packed closely in the parent cell 

 in such numbers that they press upon each 

 other and appear like parenchymatous cells 

 (PI. 46. figs. 3 & 12). In the actively vege- 

 tating parts of plants, starch-granules occur 

 A'ery generally imbedded in the green glo- 

 bules called CHLOROPHVi.L-graiuiles, either 

 singly or in groups ; this is seen especially 

 well in the cells of the Confervacepe, of the 

 Hepaticse, the prothallia of Ferns, in the 

 leaves of aquatic plants, such as Vallisneria, 



