132 THE MICROSCOPIST. 



Diatoms have in this way been preserved in vast numbers 

 in the rocky strata of the earth. The markings on these 

 siliceous shells are so delicate as to be employed as a test 

 of microscopic power and definition. In a species of 

 Equisetum or Dutch rush, silica exists in such abundance 

 that the stems are sometimes employed by artisans as a 

 substitute for sand-paper. If such a stem is boiled and 

 macerated in nitric acid until all the softer parts are de- 

 stroyed, a cast of pure silica will exhibit not only the 

 forms of the epidermic cells, but details of the stomata or 

 pores. The same also is true of the husk of a grain of 

 wheat, etc., in which even the fibres of the spiral vessels 

 are silicified. The stellate hairs of the siliceous cuticle 

 from the leaf of Deutzia scabra forms a beautiful polari- 

 scope object. 



FORMED MATERIAL WITHIN VEGETABLE CELLS. 



1. Raphides. These are crystalline mineral substances, 

 principally oxalate, citrate, and phosphate of lime. They 

 occur in all parts of the plant, sometimes in the form of 

 bundles of delicate needles, sometimes in larger crystals, 

 and sometimes in stellate or conglomerate form. Mr. E. 

 Quekett produced such forms artificially by filling the 

 cells of rice-paper with lime water under an air-pump, and 

 then placing the paper in weak solutions of phosphoric or 

 oxalic acid. 



2. Starch. This performs in plants a similar function 

 to that of fat in animals, and is a most important ingre- 

 dient in human food, since two-thirds of mankind subsist 

 almost exclusively upon it. It is found in the cells of 

 plants in the form of granules or secondary cells. Each 

 granule under the microscope shows at one extremity a 

 circular spot or hilum, around which are a number of 

 curved lines, supposed to be wrinkles in the cell-membrane. 

 When starch is boiled in water, this membrane bursts and 



