ELEMENTAEY STRrCTUEE. 31 



The largest granules known appear to be those of Canna starch, 

 or, as it is commonly termed, ' Tous-les-mois,' where they are 

 sometimes as much as the 3^ of an inch in length {fig. 00); 

 while the smallest granules, among which may be mentioned 

 those of Eice starch {fig, 69), are frequently under ^^^ of an 

 inch in length. 



Starch granules, when fully formed, usually present a small 

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

 is generally regarded as the original nucleus upon which after- 

 development has taken place ; this is called the hilum or nucleus. 

 Some observers, however, as Carpenter, regard the hilum as 

 marking the point at which the granule in its early state is 

 attached to the cell-wall. 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 concentric 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 understood. 

 By some observers, as Nageli, Martin, and Busk, the starch 

 granule is supposed to be a cell, having a wall of a different 

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

 stride being then supposed to be due, either to successive layers 

 of deposit in its interior, the boundaries being thus visible as 

 concentric lines, as is supposed by Nageli ; or to the inrolling or 

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

 doubling inwards of the wall, so as to form rugse or folds, as 

 believed by Busk. By those who thus maintain the cellular 

 nature of the starch granule, the 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 of the development and 

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

 me to be the correct one, is as follows : — the starch granule 

 appears at first in the form of a minute rounded body, which 

 constitutes the hilum, but whether this be solid or hollow cannot 

 be positively stated : — around this hilum, as a starting-point, 

 there is deposited in the course of growth a succession of con- 

 centric shells or layers, so that the innermost layers are the 

 first, and the outermost the last layers which have been formed. 

 These layers are of a like chemical nature, but vary in thickness 

 at different parts of their circumference, showing that deposit 

 of starchy matter has been more rapid at one part than another ; 

 they also vary in the amount of water they contain, the outer- 

 most layers being harder, firmer, and containing less water than 

 those in the interior. Other observers again maintain that each 

 layer is deposited i7iside its predecessor. That the different 

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

 proved by the action of polarised light, when each gi'anule 

 usually exhibits a black cross. Those who adopt this view of 



