ELEMENTARY STRUCTURE. 



Fig. 41. 



Fig. 41. Compound starch granules of WestTlncUa Arrow-root. After 



Schleiden Fig. 42. Wheat stareli: magnified 250 diameters. Fig. 



43. Rice starcli: magnified 250^diameters. 



Starch granules, when fully formed, usually present a small 

 rounded spot, which is commonly situated at one end, and 

 which represents the original nucleus upon which after- 

 development has taken place; this is called the hilum or 

 nucleus. Surrounding this spot a number of fine lines may 

 be also commonly observed, which completely encircle it so 

 as to present the appearance of a succession of irregular con- 

 centric shells placed around a common point. The cause 

 of these appearances has given rise to much discussion, and 

 cannot be said even at present, to be completely under- 

 stood. By some observers, as Nageh, Martin, and Busk, 

 the starch granule is supposed to be a cell, having a Avail of a 

 different nature to that of its contents; the appearance of the 

 concentric striee being then supposed to be due, either to succes- 

 sive layers of deposit in its interior, the boundaries being thus 

 visible as concentric lines, as is supposed by Niigeli; or to the 

 inroUing or involution of the starch cell, as maintained by ^Martin ; 

 or to the doubling inwards of the wall, so as to form rugae or 

 folds, as believed by Busk. By those who thus maintain the cel- 

 lular nature of the starch granule, the nucleus or hilum is 

 supposed to be a cavity in the cell, or a pore or funnel-shaped 

 aperture leading into it. The more commonly received opinion 

 as to the structure of the starch granule, and that which seems 

 to me to be the con-ect one, is as follows : — the starch gra- 

 nule appears originally in the form of a minute rounded body, 

 which constitutes the nucleus or hilum; whether this be solid or 

 hollow cannot be positively stated: — around this nucleus, as a 

 starting-point, there is deposited in the course of growth a suc- 

 cession of concentric shells or layers of a like nature as regards 

 their chemical composition, but varying in the amount of water 

 they contain, the outermost being harder, firmer, and containing 

 less water than those in the interior, Avhich are nearer to the 

 nucleus, from Avhich all growth commences. That the different 

 layers of the starch granule vary in density may be at once proved 

 c 4 



