MICROSCOPIC OBJECTS. 28 



Fig. 55. Starch from the Potato (Solanum tuberosum), 



x200. 



This starch is drawn as viewed under the polariscope, and 

 exhibits the black cross which is seen in all large starches 

 when viewed under the polariscope. This makes it easy to 

 distinguish starch from all other substances when it is pre- 

 sent in any adulterated article. 



Potato- starch forms one of the largest of the starch-gra- 

 nules, and is often used in the adulteration of arrowroot &c. 

 The granules, when mounted in fluid, exhibit the rings or 

 concentric layers of growth, as seen in fig. 51 ; but when 

 required for the polariscope, they show best when mounted in 

 balsam, or, what is better, in balsam dissolved in chloroform. 



N.B. No heat must on any account be used in the mount- 

 ing of starches, as it destroys their structure. 



Fig. 56. Starch from the Spanish Chestnut (Castanea 

 vulgaris) , x 350. 



Most starches are found of various sizes, which arises from 

 their being in different stages of growth : the structural centre 

 is called the hilum ; and around this is deposited, in the course 

 of the growth of the granules, a series of concentric rings, 

 increasing in number according to its age (see fig. 51). A 

 great quantity of starch is found in nuts, and also in most 

 roots and seeds, it being the chief source of support to the 

 young plant. Starch is insoluble in cold water; but upon 

 the application of heat, sulphuric acid, &c., it loses its cha- 

 racteristic structure and becomes converted into a soluble 

 substance called dextrine. 



