16 THE STORAGE OF ENERGY 



of a leaf from the light, leaving a certain portion exposed, and 

 then observe any differences between the normal and the darkened 

 portions. If a leaf, like sunflower or fuchsia, is chosen and 

 previously kept overnight in the dark, exposure to light for 

 15 minutes is sufficient, and one hour is more than ample for our 

 purpose. The leaf is then bleached with warm alcohol, and 

 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, is absorbed beyond the needs of respira- 

 tion, and that Og 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 CO2 taken up by a leaf area in the dark (respiratory CO2). 



(b) The CO2 absorbed in light of a definite intensity (total COg). 

 Total COo — respiratory COo, i.e. (b — a) = assimilated QO^. 



(c) The O2 evolved in the dark (respiratory O2). 



(d) The O2 evolved in the light (total O^). 



Total O2 — respiratory Og, i.e. (d — c) = non-respiratory Oo. 



