LEAVES AND THEIR FUNCTIONS 



131 



Air necessary for Starch Formation. — The following experiment shows 

 not only that sunlight is necessary for the formation of starch, but also 

 that air must be present. 



Experiment. — Select any small plant the leaves of which contain stomata 

 on the lower surface (rose or lilac). Leave the plant in a dark room for 

 several days, so that the starch in the leaves may be completely used up. 

 Remove the plant from the dark room, and vaseline the lower surface of 

 two or three leaves, marking them so that you will know them. After a 

 few hours' exposure to direct sunlight, pick off the vaselined Icax'cs and also 

 some others from the same plant. Wash them in wood alcohol and then 

 place them in iodine. What 

 has happened in the case of 

 the vaselined leaves? In the 

 case of the other leaves? 



Air is necessary for the 

 process of starch making 

 in a leaf, not only because 

 carbon dioxide gas is ab- 

 sorbed (there are from 

 three to four parts in ten 

 thousand present in the 

 atmosphere), but also be- 

 cause the protoplasm of 

 the leaf is alive and must 

 have oxygen. This it takes 

 from the air around it. 

 It has been found that 

 plants can live in atmos- 



Diagram to Jllustrate the formation of starch. 



pheres containing as much as 10 per cent of carbon dioxide. If 

 more than 10 per cent is present, they may die of suffocation, 

 just as an animal would die if placed in air containing a large 

 amount of carbon dioxide. 



Comparison of Starch Making and Milling. — The manufacture 

 of starch by the green leaf is not well understood. The process 

 has been compared to the milling of grain. In this case the mill 

 is the green part of the leaf. Jhe sun furnishes the motive power, 

 the chloroplasts are the millstones, and soil water and carbon 

 dioxide are the raw products taken into the mills. The manufac- 

 tured product is starch, and a certain amount of waste (rei)resented 

 by the chaff in a mill) is also given out. This waste material is the 

 element oxygen. 



