158 PHOTOSYNTHESIS 



to high illumination. This has been demonstrated by Ursprung ^^* with 

 the formation of starch in the chloroplasts. Starting with a starch-free 

 leaf, the formation of starch on illumination and the gradual accumulation 

 thereof with continued illumination of the leaf can be clearly followed by 

 the depth of color produced when the leaf is treated with iodine. If ex- 

 posure to bright sunlight is continued the amount of starch in the leaf 

 gradually decreases again. Thus a leaf of PJuiscolus after 5 hours of 

 illumination showed very deep coloration of the starch-iodine, while after 

 8.5 hours of illumination the reaction was faint. This phenomenon can 

 be produced by almost any source of light of sufficient intensity, namely, 

 sunlight, electric arcs and filament lamps, and the time required is pro- 

 portional to the intensity of the light. By using a spectrum the effect is 

 first brought about in the red-orange portion, the region showing, under 

 the circumstances, the highest photosynthetic rate. With higher intensity 

 the shorter wave-lengths bring it about in correspondingly shorter time. 

 It is therefore apparently proportional to the photosynthetic activity. 

 Ursprung has given the name of solarization to this phenomenon on 

 account of its analogy to the eft'ect produced in photographic plates under 

 similar circumstances. ^^^ 



W^e have here another case of the inactivation of the chloroplasts. 

 These organs after long exposure to intense light cease to function al- 

 though they are not killed, and, after a period of darkness, again produce 

 starch normally. It is possible that the phenomenon is in some manner 

 associated with the oxygen produced in photosynthesis.^"*' 



It must be borne in mind that solarization as observed by Ursprung, has 

 been confined to the presence of starch ; it would be highly desirable if simi- 

 lar experiments would be carried out in which more complete analyses of 

 the carbohydates in the leaf were made and consideration given to the 

 influence of temperature. 



The inhibiting effect on photosynthesis of long exposure to light of 

 high intensity has also been investigated by Ewart.^"^ From these results 

 it would appear that the inhibiting effect is due to destruction of the 

 chlorophyll. The question arises whether the inhibition of photosynthesis 

 under conditions of continued exposure to high illumination intensity is 

 due merely to the destruction of the chlorophyll or whether the chloroplast 

 ])lasma is also injured. Pantanelli ^'- inclines to the opinion that both 

 pigment and stroma are affected ; that the fatigue effects observed by him 

 in bright light are possibly due to a combination of chlorophyll destruction 

 and injury to the chloroplast plasma. It is evident that our limited knowl- 

 edge of the relation of pigment to stroma in the chloroplasts and of the 

 mechanism of these latter bodies prevents a more thorough understanding 



'"'Ursprung, Ber. hot. Ges., 35, 57 (1917). 



""■' Plotnikow, "Handbuch d. Photochemie," Berlin-Leipzig, 1920, p. 645. 



''" Pringsheim, Jahrb. unss. Bot., 12, 288 (1879). 



^^'Ewart, Ann. of Bot., 11, 439 (1897) ; 12, 379 (1898). 



'^■'Pantanelli, Jahrb. miss. Bot., 39, 167 (1903). 



