56o SCIENCE PROGRESS 



The energy relations of the photosynthetic process have 

 formed the subject of a series of investigations by A. Ursprung. 

 In the first place, the absorption curve of the green leaf was 

 determined by means of a large spectrometer with a glass prism, 

 a Hilger linear thermopile, and moving coil galvanometer. 

 (" t)ber die Absorptionskurve des griinen Farbstoffes lebender 

 Blatter," Ber. deut. bot. Ges.] 36, 73-85, 191 8.) A Nernst lamp 

 was used as source of light and the leaves of Phalaris arundinacea 

 var. picta used as experimental object. This variety has 

 variegated leaves, and the amount of light absorbed by the 

 green pigments of the leaf was determined by comparison of 

 the absorption of green and white parts of the leaf otherwise 

 similar. The absorbing power of a weak alcoholic extract of 

 the leaf pigments was also examined. Both in the extract 

 and in the living leaf absorption was found to take place 

 throughout the green region of the spectrum. The maximum 

 absorption in the red end of the spectrum was found between 

 the B and C lines, but it appears that infra red rays can also 

 be absorbed by chlorophyll. In the second paper (" Uber die 

 Bedeutung der Wellenlange fiir die Starkebildung," Ber. deut. 

 bot. Ges., 36, 86-100, 191 8) the influence of light of different 

 wave-lengths, but of the same intensity, on assimilation as 

 indicated by starch formation, was examined. The curve repre- 

 senting the relation between wave-length and power to form 

 starch runs roughly parallel to the curve of absorption obtained 

 by the author and described in his previous paper, but by no 

 means absolutely so. The lower assimilation in the violet end 

 of the spectrum, where, nevertheless, there is a high degree of 

 absorption, may perhaps be explained as due to the fact that 

 the stomata are less widely open in light of this end of the 

 spectrum than in the red. 



The discovery of the steps in the assimilatory process is 

 clearly one of the most difficult problems of plant physiology ; 

 but this has not deterred many writers from putting forward 

 theories of the chemistry of photosynthesis, and fresh theories, 

 or rather variants of the existing theories, are still being pro- 

 pounded. It will suffice here to give references to papers by 

 G. Woker (" Zum Assimilationsproblem," Pfluger's Archiv f. 

 ges. Physiologie, 176, 11-38, 1919) and P. R. Kogel (" Uber die 

 Photosynthese des Formaldehyds und des Zuckers," Biochem. 

 Zeitsch., 95, 313-16, 1919), in which hypotheses on the chemistry 

 of the assimilatory processes are put forward. Of much greater 

 interest, because they are the results of actual scientific in- 

 vestigation, are the conclusions of H. A. Spoehr with regard 

 to the carbohydrate equilibrium in plants. From work on cacti 

 Spoehr (" The Carbohydrate Economy of Cacti, Washington," 

 Carnegie Institution, 1919) shows that low temperature and 



