54 GENERAL BIOLOGY 



partially covered leaf to the sunshine for a while, 

 we find, when we treat it with iodine, that, whereas 

 the strip covered by cork gives a negative test as 

 before, the rest of the leaf which was exposed to the 

 sunshine turns a deep blue with the iodine. This 

 demonstrates that starch has been actually formed 

 in that part of the leaf which has received the sun's 

 rays. This conclusion we can confirm with the 

 microscope, for the starch granules may be found 

 in the chlorophyll bodies of the cells after such ex- 

 posure to sunlight. 



Out of the water absorbed by the plant, and the 

 CO 2 always present in the air, in the presence of 

 chlorophyll and the sunshine, the plant can synthe- 

 size starch, 1 which may be conveyed (in the form of 

 sugar) to other parts of the plant-body to be stored 

 up as reserve food or utilized at once as a source of 

 energy for the vital processes of the plant. The 

 CO 2 in the air is the source of the bulk of the carbon 

 compounds in the substance of the plant. This is 

 shown by the fact that most plants grown in an 

 atmosphere free from CO 2 die (of starvation) even 

 if the soil in which they are rooted be richly supplied 

 with carbon compounds. Conversely, they will 

 thrive in a substratum free from carbon compounds 

 if they have access to ordinary air. 



Although starch is the first evident product of 

 this process (photosynthesis), yet it is probably 



1 6 C0 2 + 6 H 2 = (C 6 H 12 6 ) + 6 O 2 ... (C 6 H 12 O 6 )n (- n HzO 

 glucose 



= (C 6 H 10 O f ,) n . 



starch 



