Chap, ii Photosynthesis 311 



the intake to be directly proportional to the pressure of 

 the carbon dioxide, the light being constant. Their results 

 for Helianthus annuus were an absorption of 412 c.c. of 

 carbon dioxide and a construction of -8 gramme of carbo- 

 hydrate (sugar) per square metre per hour. 



The rapidity and extent of the formation of sugar, &c., 

 was thought to depend on the percentage of carbon 

 dioxide in the air. Observations made by Godlewski 

 in 1873 pointed to an optimum of 8-10 per cent. In 

 1885 Kreussler concluded it to be much lower, rinding 

 the greatest activity of construction at a concentration of 

 about 1 per cent. 



Comparisons of this nature have been proved futile unless 

 the standard of illumination is taken into account. This 

 was pointed out by Blackman in 1900, who explained the 

 discrepancies in the results of the older writers by showing 

 that the intensity of the light forms a most important 

 factor in the process, and that there is an optimum pressure 

 of carbon dioxide for every degree of illumination. 



Brown and Escombe at the end of the century made 

 a very important contribution to our knowledge on this 

 point. They showed that though, as Godlewski had 

 proved, a certain increase in photosynthetic activity could 

 be secured by increasing the proportion of carbon dioxide 

 supplied in the air, yet such a course is attended 

 with very grave dangers to the plants and results in 

 disaster to them. The increased photosynthesis does not 

 to any material extent contribute to the increase of the 

 dry weight of the plants, but exercises a serious disturbing 

 influence on the course of their nutritive processes. It 

 seems to throw them off their balance and results in 

 altogether abnormal development, one symptom of which 

 is a profound modification of the reproductive functions. 

 The extent to which this is seen is the greater as the 

 increased percentage is greater, but it is noticeable if the 



