56 EXPERIMENTAL PLANT PHYSIOLOGY 



Observations. — Each afternoon for a week a leaf 

 from each plant is picked, put into a separate bottle of 

 methylated spirit, and carefully labelled with the date. 



At the end of the week the iodine test is applied : no 

 starch is found in the leaves of the plant deprived of 

 carbon-dioxide ; the leaves of the other plant show an 

 increase in the amount of starch each day. 



Inference. — Carbon-dioxide is essential to the forma- 

 tion of starch. 



If the potted plant used in the last experiment is 

 replaced by a seedling, growing in a culture-solution to 

 which has been added a small quantity of a carbonate, 

 it can be proved that the seedling is unable to avail 

 itself of the carbon from the carbonate in the solution 

 when it is deprived of carbon-dioxide from the air. No 

 starch is formed in this case. 



It has now been proved that starch, one of the 

 commonest substances found in plants and known to 

 contain carbon, is not formed when the air surrounding 

 the plant is deprived of carbon-dioxide. It is, however, 

 formed under certain conditions, if carbon-dioxide is 

 present. The conditions necessary are warmth and 

 sunlight and the presence of green colouring matter in 

 the plant. 



Hence this important conclusion is reached: A 

 green 'plant can, under the influence of sunlight, absorb 

 carbon-dioxide frcmi the air and use it as food for the 

 building v/p of starch. 



This process is termed carbon-assimilation. 



Experiment 36 



Aim. — To show that a plant when absorbing carbon- 

 dioxide also gives out oxygen. 



Method. — Some pieces are cut from a plant of Elodea 

 canadensis, put into a tumbler, and left for a time in 

 the sunshine. The tumbler is then placed in a large 



