228 NUTRITIVE METABOLISM 



Measurement of photosynthetic action may be accomplished by 

 the estimation of the amounts of carbon dioxide absorbed, and 

 oxygen given off, or by the determination of the amount of the 

 products of photosynthesis. 



294. Demonstration of the Accumulated Product of Photosyn- 

 thesis : Iodine Test. Place two plants of Tropceolum in the dark 

 room for a day and then bring one of them int6 strong light in 

 the morning. Near the close of the afternoon take a leaf from 

 the illuminated specimen, and boil it for a minute in water.. Place 

 the boiled leaf in a beaker containing alcohol and warm to 50 or 

 60° C. until the green color is extracted. Prepare a saturated 

 solution of chloral hydrate, and color it slightly by the addition 

 of a solution of iodine. Pour some of this solution into a shallow 

 glass tray, and put into it the bleached leaf, which has been rinsed 

 in water. The chloral hydrate is a cleaning reagent and will 

 render the leaf transparent, and the iodine will color the starch. 

 The density of the coloring will denote the amount of starch 

 present. If a leaf from the darkened plant is treated at the same 

 time, the difference in the amounts of starch present may be seen 

 at a glance, by the different color of the stained leaves. 



295. Accurate Estimation of the Amount of Carbohydrates in 

 Leaves in Darkness and in Light. Take all of the leaves from 

 two plants treated as in the last experiment and find the total 

 amount of carbohydrates in the one exposed to light, and in the 

 one kept in darkness for 48 hours, according to the methods 

 described in 223, 224 and 225. 



296. Growth of Plants in Darkness, and in Air Lacking Carbon 

 Dioxide. It has been found that plants grown in darkness are 

 unable to absorb and make use of carbon dioxide since the 

 supply of energy necessary to carry on photosynthesis is not 

 furnished. If a green plant is compelled to live in an atmos- 

 phere lacking carbon dioxide it is unable to carry on photosyn- 

 thesis, and pathological phenomena ensue which result in the 

 disintegration of the chlorophyl and finally in the destruction of 

 the plant. It will be of interest to compare the behavior of the 



