67 



EXPERIMENT TO SHOW THAT THE ABSENCE OE 

 LIGHT ALONE WILL PREVENT THE PROC- 

 ESS OF PHOTOSYNTHESIS 



By Cyrus A. King 



In the Botanical Gazette of November, 1903, Bernice L. Haug 

 discusses the question as to whetlicr or not Detmer's experi- 

 ment to show that H^^ht is essential for pliotosynthesis is rehable, 

 and concludes that it is not. 



By means of melted paraffine, she shows that the leaves of 

 Primula obconica, even though the plant be in good sunlight, can- 

 not produce starch when the stomata, which are found only on 

 the under surface, are closed. This experiment shows also, as 

 she has pointed out, that CO2 is not readily diffused through the 

 intercellular spaces of the leaf. 



To determine the effect of the cork disks of Detmer's experi- 

 ments, she cut a circular opening in the upper disk and then 

 fastened the cork ring through the leaf to the disk below. This 

 allowed the light to reach the leaf from above and, at the same 

 time, held the disk belovv precisely as if the upper disk had been 

 entire. No starch was formed under the cork ring, as one would 

 expect ; neither was starch formed in the central portion which 

 was exposed to light. The absence of starch in the latter posi- 

 tion must have been due to the fact that CO., was cut off by the 

 close-fitting disk on the under surface. 



In performing some physiological experiments two of the 

 writer's students, Messrs. R. C. Paris and J. H. Tilley, tried this 

 experiment, using narrow strips of black cloth about as coarse- 

 meshed as cheese-cloth. Through the kindness of Mr. Olsen. 

 Superintendent of the Central Park green houses, the experi- 

 ments were tried there on several genera. The most pronounced 

 results were obtained from the experiments on hydrangea and 

 rose. The leaf in the accompanying photograph was removed 

 from a hydrangea plant after it had been exposed to the sunlight 

 during the entire day. The black cloth strips used were cut 

 more than twice as long as the width of the leaves and one was 

 wrapped around each leaf near the middle. One pin was used 



