USE OF THE MICROSCOPE IN BOTANY. 21 



tained therein. Starch is the most generally diffused 

 substance, except protoplasm,* met with in vegetable 

 cells ; it occurs in all classes of plants, except funguses. 

 It can always be detected by the application of iodine, 

 which immediately turns the granules blue. I should 

 recommend you to make yourself acquainted with 



Fig. 4. Section of Potato, showing Cells and Starch Granules. 



various forms of starch-granules of several common 

 plants, such as wheat, rice, Indian corn, and arrow- 

 root. It is supposed by some microscopists that the 

 structure of a starch-granule is composed of a series 

 of concentric shells or layers, which are firm as they 

 approach the outside wall, but are less dense and 

 more full of water as they approach the centre or 

 nucleus. The granules may be isolated from the 

 cells by macerating slices in water for a few days. 



* The name is applied to the nearly colourless granular viscid 

 substance, nitrogenous in nature, which constitutes the formative 

 matter in the cells. From Trpwror and 7r\tto-/ua, " form." 



