PLANT LEAVES 197 



Experiment 102. Under an inverted funnel in a battery jar, place 

 some pond scum or hornwort. Fill the jar with fresh water and over 

 the neck of the funnel place an inverted test tube filled with water. 

 When placed in the sunlight, bubbles of oxygen will 

 rise into the test tube and collect. The oxygen can be 

 tested by turning the test tube right side up and quickly 

 inserting a glowing splinter. If the splinter bursts into 

 a flame, oxygen is present. (A freshly picked leaf 

 covered with water and put in the sunlight will be seen 

 to give off these bubbles.) After a small amount of gas 

 has been collected in the test tube, mark the height of 

 the water column and place the battery jar in the dark, 

 allowing it to remain there for ten or twelve hours. No oxygen is 

 given off in the dark. Place the jar in the light again. Oxygen is 

 given off. Is the sun's energy needed to enable the plant to give off 

 oxygen ? 



The starch manufactured is insoluble in water and 

 is stored in the leaf during the day. But at night, when 

 the leaf is not manufacturing starch, it is able to digest 

 the starch by means of a special substance, leaf diastase, 

 which it forms. This changes it into sugar, which is 

 soluble and which flows to other parts of the plant. Com- 

 pounds such as starch and sugar, in which there is only 

 carbon, hydrogen and oxygen, are called carbohydrates. 



The cells in the leaf and in other parts of the plant 

 have the power to change the sugar and combine it with 

 other substances contained in the sap, thus forming more 

 complex chemical compounds. These contain nitrogen 

 and sulphur, besides the elements of the sugar. Such 

 compounds are called proteins. They are essential to the 

 formation of plant protoplasm and are very important as 

 animal foods. 



The digested and soluble substances which are prepared 

 by the leaves are transported to other parts of the plant, 

 where they are combined by the protoplasm of the living 

 cell with other substances contained in the cell sap. Thus 



