16 THE STORAGE OF ENERGY 



iodine is applied. The part exposed to light will appear blue- 

 black rich in starch, while the screened portion is starch free. 

 It has been shown that during the day the starch content of 

 leaves may rise to 6-44 per cent, of the dry leaf weight. At 

 night the starch value may drop as low as 0-38 per cent. Timi- 

 riazeff has devised a very neat experiment which demonstrates 

 that starch formation is greatest where there is greatest absorp- 

 tion of light. Living hydrangea leaves, previously deprived of 

 starch by retention in the dark, have then projected on them a 

 solar spectrum for 5 or 6 hours. The leaf is decolorised and 

 treated with iodine, and then the absorption bands of the chloro- 

 plast pigment complex are found mapped out in blue, showing 

 that starch has been formed only where light has been absorbed. 

 Other carbohydrates are also found. Cane sugar is formed and 

 can be detected before starch can be found, and it is generally 

 present in greater amounts, 7-63 per cent, to 2-63 per cent, of 

 the dried-leaf weight. Other sugars are present in small variable 

 quantities. 



(2) It seems that CO 2 is absorbed beyond the needs of respira- 

 tion, and that O 2 is evolved. Engelmann has provided in a 

 striking manner a demonstration of the fact that the maximum 

 evolution of oxygen takes place where there is the maximum 

 absorption of light and, as stated above, the maximum formation 

 of starch. He placed a filament of cladophora in water, to which 

 he added some motile bacteria having an avidity for oxygen. 

 On the thread of alga he projected a minute solar spectrum and 

 kept it under the microscope. It was seen that the bacteria 

 gathered just at those places (red and violet) where light was 

 absorbed. 



Kniep and Minder have determined the carbon assimilation, 

 and they find it directly proportional to the amount of energy 

 absorbed as light. Further, Willstatter and Stoll have estimated 



(a) The CO 2 taken up by a leaf area in the dark, i.e. respira- 



tory CO 2 . 



(b) The CO 2 absorbed in light of a definite intensity. 

 (ft) _ (a) assimilated CO 2 . 



(c) The O 2 evolved in the dark - respiratory O 2 . 



(d) The O 2 evolved in the light -total O 2 . 

 (d) - (c)=non-respiratory O 2 . 



TTT- -^ - 

 (d)-(c) O 2 



(Cf. respiratory quotient, see Chap. III.). 



(e) TTT- -^ -^ assimilation coefficient. 

 ' (d)-(c) O 2 



