170 PRACTICAL COURSE IN BOTANY 



formed in the tube of the funnel? Move back into the sunlight and 

 leave for a few hours ; what happens when you thrust a glowing splinter 

 into the tube ? 



EXPERIMENT 69. Is ANY FOOD PRODUCT FOUND IN LEAVES ? Crush 

 a few leaves of bean, sunflower, or tropseolum, and soak in alcohol until all 

 the chlorophyll is dissolved out. Rinse them in water, and soak the 

 leaves thus treated in a weak solution of iodine for a few minutes, then 

 wash them and hold them up to the light. If 

 there are any blue spots on the leaves, what are 

 you to conclude ? If a test for sugar is to be 

 made, use sap pressed from fresh leaves; for 

 oils and fats, leaves should be dried without 

 being placed in alcohol. 



EXPERIMENT 70. HAS THE PRESENCE OR 

 ABSENCE OF LIGHT ANYTHING TO DO WITH THE 



OCCURRENCE OF STARCH IN LEAVES ? Exclude 



the light from parts of healthy leaves on a grow- 

 ing plant of tropaeolum, bean, etc., by placing 

 patches of black cloth or paper over them. 

 FIG. 227. Leaf arranged Leave in a bright window, or preferably out of 

 with a piece of tin foil to ex- doors, for several hours, and then test for starch 

 elude light from a portion of as j n t h e j ast experiment : do you find any in 

 the surface. , . , , , 



the shaded spots r 



EXPERIMENT 71. Is THE PRESENCE OF AIR NECESSARY FOR THE 

 PRODUCTION OF STARCH ? Cover the blades and the petioles of several 

 leaves with vaseline or other oily substance so as to exclude the air, and 

 after a day or two test as before. 



185. Influence of plants on the atmosphere. These 

 experiments show that leaves cannot do their work without 

 light and air. The particular element of the atmosphere 

 used by them in the process of food making is carbon dioxide. 

 Their action in absorbing this gas and giving off oxygen 

 tends to counterbalance the opposite action of respiration, 

 decomposition, and combustion of all kinds, by which the 

 proportion of it in the atmosphere tends to be constantly 

 increased. In this way they help to regulate the quantity 

 of it present and have a beneficial effect in ridding the air of 

 one source of impurity. 



