STARCH. 



[ 611 ] 



STARCH. 



granules in the same plant, and other cha- 

 racters connected with the physical structure 

 confirm the conclusions from simple inspec- 

 tion. 



Starch is usually stated to be unaffected 

 by cold water, and this is generally the case ; 

 but if the granules 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 starch- 

 granules are heated {dry) 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 

 striae gradualh^ vanish, becoming melted into 

 a mass, as it were, the starch itself being 

 converted into dextrine. When starch-gra- 

 nules are heated in waterto 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 substance sometimes partly 

 escapes in the form of cloudy flocks, but is 

 not dissolved. Diluted 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 remark- 

 able appearances present themselves : the 

 acid touching certain parts of the granule 

 first, or acting most quickly on softer por- 

 tions, causes the softening internal layers to 

 expand and bulge out the external layers at 

 particular points (like hernice) until the entire 

 grain 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 sulphuric 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 nucleus. 

 The application of the polariscope to the 

 starch-granule also brings distinct evidence 

 on this point, showing that it is composed of a 

 substance increasing in tension or density 

 from the "nucleus" to the surface. With 

 polarized light, 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 (PI. 31. fig. 4). 



Pure starch is coloured blue by iodine, 

 whether in its natural state or softened by 

 hot water, the depth of the colour depending 

 on the quantity of iodine; where much is 

 added the colour is almost black. When 

 dilute sulphuric acid has been added pre- 

 viously, the colour is rather purple than 

 bhie, especially the faint tinge given at first 

 by weak solution of iodine. When the 

 starch-grains are heated dry, the colour 

 given by iodine changes proportionately to 

 the violence of the action from blue to 

 purple, red- wine colour, and finally brown. 

 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. 

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

 (in the latter case mostly with flat 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 apjjear like parenchymatous cells 

 (PI. 36. figs. 3 & 12), In the actively vege- 

 tating parts of plants starch-granules occur 

 very generally imbedded in the green glo- 

 bules called CHLOROPHYLL-granules, either 

 singly or in groups ; this is seen especially 

 well in the cells of the Confervacese, of the 

 Hepaticaceae, the prothallia of Ferns, in the 

 leaves of aquatic plants, such as Vallisneria, 

 in autumn, &c. The free granules occur 

 more particularly in the colourless organs of 

 ])lants, in tubers, rhizomes, roots and the 

 cambium region, in the season of rest, in the 

 endosperm of ovules, or the albumen or 

 cotyledons of seeds, &c. The parenchyma- 

 tously-grouped granules are found in the 

 albumen of seeds, especially of maize and 

 rice. The comparison of the states and of 

 the course of development of the crowded 

 granules of maize throws much light upon 

 the manner in which starch-granules are 

 formed. 



In the first place, two rival doctrines exist 

 as to the order of development of the parts 

 of the granule. Most authors assert that 



2r2 



