112 THE NUTRITIVE FUNCTIONS. 



dumb-bell or two-headed club. By wounding the plant, 

 and placing a drop of the milky juice under the micro- 

 scope, these singularly-formed granules may be easily seen 

 by the addition of a little tincture of iodine, which gives 

 them a deep blue tinge. Starch granules are the most 

 easily observed in the cells of the potato where they are 

 very large. On the application of tincture of iodine, the 

 starch granules are readily distinguished by the deep blue 

 color which they immediately assume, whilst the walls of 

 the cells in which th*ey are contained remain colorless. 

 The mode of their formation is indicated by the peculiar 

 markings on their outer surface, each grain having a spot 

 at one end which is called the hilum, or ostiole, with fine 

 concentric lines drawn around it. Sometimes there are 



Fig. 23. 



Grains of starch, from the potato. 



two or three ostioles on the surface of the same granule. 

 Occasionally, crevices are seen radiating from the ostiole 

 in the form of a star, which open more or less profoundly 

 into the interior of the granule. When very young, the 

 grain of starch appears as a vesicle, with a perforation in 

 its wall. It is through this opening that the matter enters, 

 by a movement of intussusception, and is deposited in 

 successive beds. At each deposit, the vesicle dilates by a 

 kind of endosmotic phenomenon, until it finally acquires 

 a degree of solidity which arrests this movement, and it is 



