CHAPTER V. 

 THE MICROSCOPY OF THE COMMON STARCHES. 



I. Origin and Nature of Starch. — ^AU living things 

 depend for existence upon the potential energy of organic 

 compounds which they take in as foods and break down 

 to simpler substances; and to make good this constant 

 drain upon the limited stock of organic matter in the 

 -world there is only one source of supply, the chlorophyll 

 bodies of green plants. Here rays of sunlight are absorbed, 

 and by their energy carbon dioxid and water are united 

 to form starch, oxygen being set free; the kinetic energy 

 of the sun is thus transformed and stored up in potential 

 form. Ultimately the whole structure of organic life 

 depends on this process, as the universe, of mythology, 

 rested upon the back of the fabled tortoise. 



Starch is an insoluble carbohydrate substance belong- 

 ing to the cellulose group and having probably the formula 

 (CsHioOs)^. It occurs as a white powder made up of 

 small grains a few hundredths, or rarely a tenth, of a milli- 

 meter in diameter, of a shape varying with the plant by 

 which it is formed, but more or less characteristic for 

 each species. The grains are in general built up of con- 

 centric layers, the outer one resembling cellulose, while 



5« 



