12 CHEMISTRY OF PLANTS. 



external ones are generally clearer than the internal, which, in fresii 

 starch, often appear almost gelatinous. The dark lines do not intersect the 

 line of the external circumference in any one of the granules ; and, how- 

 ever close they may lie to each other at the pointed end, every line per- 

 fectly returns to itself. On turning a single granule with deeply 

 blackened lines under the microscope, which may be easily done by the 

 addition of a drop of water, which will occasion a small current, we shall 

 see that the lines, when considered from all sides, remain equal, and 

 always encircle the nucleus in the same manner. From this it follows 

 that they cannot be mere markings upon the surface, but the surfaces of 

 contact of many hollowVvate scales laid around each other : from this the 

 whole granule is composed. Sometimes on making a fine section, with a 

 sharp razor, from a potato containing much starch, we may succeed in 

 seeing several granules of starch sharply intersected under the micro- 

 scope ; and we may thus perfectly convince ourselves that the layers to- 

 wards the interior are in general more aqueous and gelatinous, and that 

 those towards the exterior contain less water and are tougher. 



Perfectly dried granules exhibit a smaller number of lines, although 

 they are frequently more strongly marked ; and we may often perceive 

 that each broad black line corresponds to a thin layer of air. On suffering 

 starch to remain for any length of time in gum water, the lines gradually 

 will disappear more and more ; and on drying it with the gum, until the 

 whole forms a perfectly tough mass that may be cut with a knife, we 

 may easily obtain a great number of sections by cutting off small chips, 

 and even have several thin discs from a single granule. In the latter we 

 discover a tolerably homogeneous substance, having in the centre a some- 

 what irregular indentation, which has naturally been occasioned by the 

 drying up of the interior aqueous layers. 



On treating starch under the microscope with sulphuric acid, very dif- 

 ferent phenomena appear, according as to whether the acid is stronger or 

 weaker, and the action rapid or slow. On the rapid action of strong 

 acid, the granule is immediately affected from the point where it is 

 touched by the acid ; it becomes distended, and gradually dissolves, a pro- 

 cess that is quietly continued to the other end of the granule. We often 

 see granules which are quite dissolved at one end, while the opposite end 

 is still sharply defined, showing even a nucleus and layers. The whole 

 mass of the granule is quite uniformly affected, without the outer layers 

 being torn open, or the fluid contents escaping. In a slower action of 

 the acid, two different forms of solution occur alike frequently, depend- 

 ing probably upon the different degree of concentration of the acid. In 

 dilute acid the granule becomes gradually transparent and gelatinous ; 

 swells up, but in such a manner that it first exhibits an impression at 

 one side, and by degrees (swelling up less at the compressed side than ex- 

 ternalty) assumes a complete cup form, and is at last gradually dissolved 

 from the margins. The other form, exhibited by the slow action of the 

 very concentrated acid, consists in the nucleus passing over into a 

 decidedly recognisable air-bubble. This expands, causing one or two 

 jagged rents in the interior of the granule, which gradually inflates and 

 becomes gelatinous, whilst the lines disappear, as far as they are touched 

 by the rent, until the whole granule is rendered invisible (dissolved). 

 The first action of the sulphuric acid appears to be, that water is with- 

 drawn from the inner layers ; and this appears further confirmed by the 

 action of dry heat. 



On heating potato starch upon a small plate, to such a degree that only a 



