ADJUSTMENT TO LIGHT 83 



(1) COa + HoO^CHoOg, carbonic acid; 



(2) CH203 = CH20 + 02, formaldehyde and free oxygen; 



(3) 6CH20 = C6Hi206, glucose; 



(4) C6Hi206-H20 = C6Hio05, starch. 



Experiment 24. Dependence of photosynthesis upon aeration and light. 

 Place two vigorous sunflower plants in darkness for twenty-four hours 

 in order to bring about the disappearance of the starch. Use wax or 

 vaseline to coat the upper surface of one leaf, the lower surface of a 

 second, both surfaces of a third, and allow a fourth leaf to remain un- 

 coated. Place this plant in full sunshine, leaving the other in the dark. 

 This experiment should be started as early in the morning as convenient. 

 During the afternoon of the same day, cut cross-sections from each of 

 the four leaves of the plant in sunlight, and from one of the leaves in 

 darkness. Note the relative amount of starch in each as well as its 

 distribution. 



104. Measurement of photosynthesis. The intensity of photo- 

 synthetic activity in different ph^nts, or in the same plant under 

 different conditions, is best measured by the quantity of starch 

 produced. This is not an exact measure, since much of the glu- 

 cose formed is removed at once instead of being converted into 

 starch. However, since the amount of glucose produced can not 

 be measured, the degree of activity can only be based upon the 

 starch formation. This can be done by treating entire leaves 

 with iodin, as in the method employed by Sachs. ^ A more accu- 

 rate method is to cut sections of the leaves to be studied, and 

 to count the starch grains, and in some cases the number of plas- 

 tids. The count is made for a segment 100 j wide across the 

 entire section. Two such segments are counted in different parts 

 of the leaf, and the result is multiplied by five to give the num- 

 ber of starch grains for a unit segment 1 mm. in width. 



Experiment 25. Relation of photosynthesis to sun and shade. Remove 

 the starch from two sunflowers by keeping them in darkness for twenty- 

 four hours. Place one plant in sunshine and one in a shade-tent of known 

 light intensity. After six or eight hours, cut sections of a leaf from 

 each, and determine the starch content for a unit segment. Compare 

 the results in the two leaves with respect to the light intensity for each. 



105. Translocation. A part of the glucose made by the chlo- 

 roplasts is used by the leaf itself, while a large amount is carried 



* Researcli Methods, 137. 



