196 EXPERIMENTS WITH PLANTS 



burned, it may be considered to be equal to the heat of 

 combustion (or "fuel value" of starch) which is about 

 4.5 large Calories for every gram of dry starch. If we 

 estimate that a square meter of leaf surface produces 

 one gram of dry starch every hour in sunlight, it must 

 in so doing absorb the energy of the sunlight to the 

 extent of about 4.5 large Calories. But this amounts 

 to only about one -half of one per cent of the total 

 energy which falls upon it during that time in the form 

 of sunshine. It should be noted that it is principally 

 the red rays which bring about the chemical changes 

 here considered, just as in the eye they, more than 

 others, bring about the chemical changes which result 

 in the sensation of sight, and for that reason appear 

 much brighter to the eye than the blue rays. The 

 latter, as is well known, are the ones which bring 

 about the chemical changes in a photographic plate. 

 What is the nature of the openings through which the 

 air passes? With a sharp knife, make a shallow cut 

 and tear off a small piece of the delicate, semi-trans- 

 parent epidermis from the lower surface of the leaf,i 

 place it on a glass slide in a drop of water and cover 

 with a thin cover-glass. Examine first with the low 

 (one-half inch or two-third inch objective) and then 

 with the high power (one-sixth inch or one-eighth inch 

 objective) under a compound microscope. The epi- 



•The best leaves for this purpose are those of a Lily (or any plant of the 

 Lily family, e. g., Hyacinth, Narcissus, etc.), Iris, a Grass (especially the Oat), 

 Wandering Jew, Anemone, etc. 



