270 FIRST YEAR COURSE IN GENERAL SCIENCE 



uses. They combine water chemically with carbon dioxide 

 from the air to form starch and sugar, which are foods for the 

 growing plant. The starch-making process is carried on by 

 the chlorophyll grains within the interior cells of the leaf by 

 the aid of the energy of sunlight. 



The presence of starch in leaves may be proved. First 

 extract the chlorophyll by heating the leaf in alcohol, until 

 the leaf has lost its color. Boil the leaf in water to cook the 

 starch, and then add a few drops of iodine. Iodine always 

 turns anything which contains cooked starch to a dark blue 

 color, and is usually employed as a test for the presence of 

 starch. 



303. Light and Chlorophyll Necessary in Starch Making. 

 It can easily be shown, from leaves which have grown in 

 the dark, that plants cannot make starch without light. Nei- 

 ther can leaves destitute of chlorophyll make starch. Toad- 

 stools and Indian pipe have no green color; the iodine test 

 shows no starch in these plants. 



Starch making is most rapid in direct sunlight, but it 

 probably goes on in all degrees of brightness, even in moon- 

 light. Light from other sources will also serve. For ex- 

 ample, if an arc light is in use in a greenhouse for half of 

 every night, lettuce plants grown there will be ready for 

 market a week before others which are not so treated. The 

 blooming of Easter lilies may in the same way be hastened 

 from four to ten days. 



304. The Proteid-making Process. It may be asked 

 why water is constantly carried up through the plant, and 

 then in large part evaporated. It is because the salts dis- 

 solved in the soil water are present in such small amounts 

 that the plant must absorb great quantities of water in order 

 to secure enough of these compounds for making proteid. 



These minerals are compounds containing several elements, 

 chief among which are nitrogen, sulphur, potassium, and 

 phosphorus. These elements the plant combines with the 



