2i6 CARBON-FIXATION 



Note that no starch is formed in the half to which access of carbon dioxide 

 is prevented. 



(ii) Smear the upper surface only of a pear or poplar leaf, and the lower 

 surface only of another similar leaf. Leave for three days as before, and then 

 test for starch. 



Find out which leaf possesses most starch ; then determine with micro- 

 scope on which surface stomala are most abundant. 



Ex. 125.— To show the effect of darkness on starch formation, tie up a leaf 

 of Tropesolum in a thick brown-paper bag so that no light can get at it. 

 Leave it covered up for two days, and then test for starch. 



Ex. 126. — Boil a quantity of young grass leaves for a minute or two and 

 then extract the chlorophyll by placing the leaves in strong alcohol in a 

 dark cupboard. 



Pour some of the solution into a beaker or large test-tube ; note the green 

 colour when held up to the light, and dark red colour when viewed by 

 light reflected from it. 



Note the effect on the colour when a few drops of hydrochloric acid are 

 added to the solution. 



Ex. 127. — Grow some seedlings of wheat, mustard, or peas in total dark- 

 ness, and note that the leaves are not green. Expose the plants to light 

 and observe when the first signs of a green colour are visible. 



Ex. 128. — Place a large can, bowl or basin upside down on a lawn or 

 grassy field so as to exclude light from the plants beneath it. Leave it for 

 one or two weeks and then examine the grass beneath ; note the loss of 

 green colour. 



