PLANT STUDY 183 



Plants furnish to men and many other animals a very 

 large part of their food supply. Perhaps the most impor- 

 tant foods which man obtains directly from 

 Food plants are the starches, sugars, and oils. 



Formed by ^ ^ e starc ^ producing plants the follow- 



Plants ing are well known: potato, sago palm, 



arrow-root, the manioc from which tapioca 

 is derived, rice, and the common cereals. Mix a few drops 

 of dilute iodine with a little flour paste, and note that the 

 mixture changes immediately to a bright blue. A thin sec- 

 tion of potato treated with very dilute iodine and examined 

 under the low power of the microscope will show the blue 

 starch grains in the colorless potato cells. 



Sugar is derived mainly from three sources, sugar cane, 

 beets, and the evaporated sap of the maple tree. It is also 

 found in many ripe fruits; as, grapes, figs, prunes, apples, 

 pears; it is also present in small quantities in the sap of 

 other trees than the maple, notably in the sugar pine of the 

 Pacific slope. Pupils in the higher grades will be inter- 

 ested in knowing that sugar, wood fibre, and starch are 

 very much alike chemically. Each is a chemical union of 

 carbon (charcoal is a familiar example) and water in 

 slightly different proportions. The following experiment 

 demonstrates this for sugar: 



Fill a test-tube about one-sixth full of thick sugar syrup. 

 Add the same amount of sulphuric acid. Set the tube in 

 a glass and put the glass on a piece of heavy cardboard. 

 Note that the mixture slowly turns brown and then very 

 black. The mixture rises in the tube and considerable heat 

 is generated. The acid has decomposed the sugar by tak- 

 ing out the water, leaving the almost pure carbon. If the 



